Caudal Glands in Nematodes: Morphology, Evolutionary Shifts and Functional Implications
The presence of three caudal glands one dorsal and two subventral with distinct terminal outlets is an apomorphic feature supporting the monophyly of Nematoda. These glans primarily secrete adhesive substances for attachment but have evolved additional roles in locomotion and feeding in certain lineages. Comparative analysis reveals three or two separate gland outlets in 77 species across six ordinal taxa, while vestigial glands without external openings suggest possible internal functions. A single, common gland opening (spinneret), likely arising through independent tail-end invagination, has evolved approximately 32 times across nematode lineages. Additionally, a reversal in gland arrangement (one ventral and two subdorsal) occurred independently in about four instances. Variation in gland number, position (incaudal, suprarectal, and precaudal), and structure is documented, with shifts from three to four or six gland cells and reductions to two in some taxa. Approximately 80 independent losses of caudal glands across nematode phylogeny are detected, including 30 in Mononchida and around 40% in marine species. In clades with terrestrial and limnic representatives (e.g. Dorylaimida, Mononchida) it appears that the glands were lost in the terrestrial environment and aquatic habitats were invaded secondarily. Potential homologies between caudal glands and phasmids are discussed, along with implications for tail morphology to gland structure and function.
Introduction
Aquatic nematodes characteristically possess three large caudal gland cells that occupy the posterior body region and discharge secretions terminally. These glands play a critical ecological role by enabling temporary adhesion to solid substrates, thus facilitating foraging in flowing-water environments. In live specimens, the glands appear as a dark mass and are stainable with various dyes [1], however, they are often indistinct in permanent mounts. Despite their inclusion in the basic adenophorean body plan, caudal glands have been historically underreported and poorly illustrated in species descriptions. Hence, dedicated morphological studies focusing on the caudal gland complex and the associated spinneret apparatus remain scarce. Also, the existing data on their number, arrangement, and outlet morphology are fragmentary, imprecise, and frequently inconsistent across publications and authors’ interpretations. Nevertheless, these structures hold diagnostic value and potential phylogenetic significance, warranting renewed attention in nematode systematics.
Motivated by Steiner’s [2] research proposal “Of special necessity is a comparative study of the distribution, morphology, and function of the foot glands and phasmids, as well as their obliteration and disappearance in distinct worm groups” we undertake a comprehensive analysis of caudal gland morphology across Nematoda. This article synthesizes scattered observations into a cohesive phylogenetic framework, documenting structural diversity and evolutionary modifications of the caudal gland complex. We systematically reviewed hundreds of species descriptions and examined information derived from both light and electron microscopy. While efforts were made to ensure broad coverage, there is the possibility of overlooked literature and misinterpretations stemming from terminological inconsistencies, particularly regarding the term “spinneret,” which is used variably across studies.
In our analysis, we uncovered numerous unresolved questions that merit further investigation and presented testable hypotheses on the evolutionary pathways and transformation sequences of the caudal gland apparatus. The preliminary phylogenetic trees derived from a synthesis of studies by numerous authors [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17], bearing several polytomies, serve as a foundation for future refinement. To resolve the polytomies, sequences of all taxa mentioned in the phylogenetic trees must be analysed together. This work re-establishes the caudal gland complex as a significant morphological and phylogenetic character system in nematode biology.
A Brief Outline of Nematode Bauplan Evolution
The ancestral lineage and stem species of Nematoda likely inhabited marine environments during the Precambrian period. It is widely accepted that the phylum Nematomorpha represents the sister group to Nematoda [18, 19, 20]. Nematomorpha, which are parasitic in arthropods (primarily Crustacea), have undergone significant morphological modifications to adapt to parasitism, including a free-living but non-feeding adult phase. Consequently, only a few traits can be interpreted as synapomorphies supporting the sister- group relationship between Nematomorpha and Nematoda.
Despite extensive structural reductions, some novel features evolved in their common ancestor (Figure 1), including a vermiform, cylindrical body, a flexible cuticle, undulatory locomotion by means of exclusively longitudinal muscles, hexaradial symmetry in anterior sensory structures, non-flagellate sperm transferred during copula, a cylindrical muscular sucking pharynx formed by myoepithelial cells and lined with a secreted cuticle, and a cuticle-lined cloaca. In the evolutionary trajectory of Nematoda, the cloaca was lost in females with the separation of the vulva and anus [21, 22, 23], and a tail developed dorsal to the terminal gut opening, serving as a container for adhesive glands possibly supporting a hemi-sessile lifestyle [21].
Several additional traits shared by Nematomorpha and Nematoda such as a terminal rather than subventral stoma, food intake by suction, a circum-pharyngeal central nervous system, a multilayered cuticle, and molting during ontogeny are broader apomorphies of Cycloneuralia and do not specifically support their sister-group relationship [19, 24]. Other Cycloneuralian taxa include Kinorhyncha, Loricifera, and Priapulida (Figure 1). The stem species of Cycloneuralia was likely small, inhabiting marine interstitial spaces of sediment, where its cuticle provided mechanical protection against shifting sand grains [24].
The cylindrical body plan and hexaradial anterior symmetry in the Nematomorpha-Nematoda lineage may represent adaptations to a benthic marine lifestyle [25]. The presence of adhesive glands, which anchor nematodes to substrates, further supports the idea that their ancestors lived in unstable sandy habitats. Since these caudal glands are unique to Nematoda within Cycloneuralia, they likely represent an evolutionary novelty [24]. There is no evidence that adhesive glands were present in the stem species of Nematoda-Nematomorpha and subsequently lost in Nematomorpha due to parasitic adaptation.
In this context, we reject the hypothesis that the three terminal lobes (without glands) in female Paragordius (Nematomorpha) are homologous to the tri-lobed caudal glands of nematodes, despite their similar dorsal and subventral orientation, which is considered plesiomorphic in nematodes. Additionally, because adhesive glands were absent in the Nematoda-Nematomorpha stem species, we also dismiss the idea that hexaradial anterior symmetry evolved in connection with a hemi-sessile lifestyle relying on secretions for substrate attachment [21]. Lastly, historical attempts to homologize nematode caudal glands with the foot glands of Rotifera or Gastrotricha are now considered obsolete [26, 27, 28, 29, 30]. These taxa belong to Lophotrochozoa, the sister group to Ecdysozoa (which includes Nematoda) [31], where adhesive glands primarily were absent.
Typically, three unicellular caudal glands are located in the anterior region of the nematode tail, arranged in tandem with slight overlap, likely due to spatial constraints. We can hypothesise that their arrangement follows a consistent sequence, allowing for the identification of homologous cell bodies (Figure 2). Following the nomenclature proposed by Coomans, et al. [32], we designate these as cg1 (anterior, usually dorsal), cg2 (middle), and cg3 (posterior).
![Figure 2: Caudal glands (cg) in tandem. A) Oncholaimus sp., precaudal gland cells left (l) or right (r) of intestine [115]; B) Synonema cosmopoliticum [44]; C) Araeolaimus filpjevi [231]; D) Thalassogenus tortuosus female, two sets of caudal glands [173]; arrows: extra gland cells.](/fulltextimages/13910/fig_2.png)
Structure and Functioning of Caudal Glands
Glandular Cells for Adhesion and Detachment
The structure of caudal glands has been studied in detail through light and electron microscopy in only a few species, including Chromadorina germanica (Chromadorida) [33], Gonionchus australis (Monhysterida) [34], Theristus caudasaliens (Monhysterida), and Perepsilonema sp. (Desmodorida) [35]. Each gland cell extends into a duct lined by a thin layer of cytoplasm [33] and is flanked by a circomyarian muscle cell. The ducts primarily open separately at the tail tip (Figure 3A-E; 5B), but in most taxa, they merge with those of other gland cells to form a common pore. The secretory cells are relatively large and exhibit features indicative of high metabolic activity. Each gland cell contains a prominent nucleus with multiple strands of chromatin and numerous dense secretory vesicles in the peripheral cytoplasm. Light microscopy of C. germanica revealed large, globular caudal gland cells with conspicuous nuclei. The presence of RNA was confirmed using acridine orange fluorescent staining. Ultrastructural analysis further supported the secretory function of these glands, showing an abundance of rough endoplasmic reticulum, mitochondria, and a well-developed Golgi complex. Additionally, microvilli containing ~20 nm particles were observed projecting into the gland cell lumen (or duct). These particles were proposed as precursors to secretory granules, which range from 0.5 to 2.0 µm in size; however, the chemical composition of the secretion remains unknown.
![Figure 3: Apical view of tail tip. The dorsal side to the top. A) _Dorylaimopsis variabilis_ with 3 openings, 1 dorsal [133]; B) _Leptonemella brevipharynx_ with 3 openings, 2 dorsal, and a small central pore [12]; C) _Pternepsilonema servaesae_ [133]; D) _Triepsilonema tripapillata_ [133]; E) _Epsilonema pustulatum_ [155]; F) _Zalonema ditlevseni_ with a single pore [232]; G-J) Two openings. G) _Laxus sakihariiae_, the dorsal opening larger [51]; H) _Eubostrichus hopperi_ [12]; J) _Bathyepsilonema lopheliae_ male with a medioventral pore and a curved slit-like dorsal opening [133].](/fulltextimages/13910/fig_3.png)
Figure 3: Apical view of tail tip. The dorsal side to the top. A) Dorylaimopsis variabilis with 3 openings, 1 dorsal [133]; B) Leptonemella brevipharynx with 3 openings, 2 dorsal, and a small central pore [12]; C) Pternepsilonema servaesae [133]; D) Triepsilonema tripapillata [133]; E) Epsilonema pustulatum [155]; F) Zalonema ditlevseni with a single pore [232]; G-J) Two openings. G) Laxus sakihariiae, the dorsal opening larger [51]; H) Eubostrichus hopperi [12]; J) Bathyepsilonema lopheliae male with a medioventral pore and a curved slit-like dorsal opening [133].
To enable temporary adhesion, nematode caudal gland cells rapidly release an adhesive secretion under nervous control. This secretion hardens immediately in water, allowing the nematode to adhere to various substrates. The attachment is strong yet reversible, enabling frequent detachment and reattachment of the tail tip within short time intervals. In an experiment with the monhysterid Sphaerolaimus gracilis, Turpeenniemi, et al. [36] demonstrated that the nematode could anchor itself to a sediment particle by secreting a cement-like substance from the caudal glands, forming a thread more than one body length long. The adhesion is so firm that detachment by mechanical means such as using a needle or pipette suction is mostly unsuccessful Schiemer, et al. [37, 38] suggesting a chemical component is required for releasing. The dual function of attachment and release is accomplished by a two-part glandular system, recorded in three species: Perepsilonema conifer, Sphaerolaimus gracilis, and Theristus caudasaliens [35, 36]. This system consists of two or three voluminous adhesive gland cells that secrete a rapidly hardening adhesive, along with approximately two smaller releasing gland cells thought to produce a “de- adhesive” substance that facilitates detachment by chemical means. Similar duo-gland adhesive systems are known from various interstitial meiofaunal taxa [35, 39]. We propose that this cooperative adhesive-releasing system, consisting of 3+2 glandular cells, is homologous across different nematode taxa and was likely present in the last common ancestor of Desmodorida (Perepsilonema) and Monhysterida (Sphaerolaimus, Theristus) (Figure 4, species X). It will be shown that this ancestral species had three separate glandular outlets.
![Figure 4: Phylogenetic diagram of Nematoda compiled from data in [5,9,10,16,17,81]. Paraphyletic taxa after Ahmed, et al. [17] in parentheses. The greyish circles mark points not resolved into dichotomies so far. Following our hypothesis that the stem species had three caudal glands with separate outlets, the lineages are marked where the three outlets must have joined to a single one and where caudal glands were completely reduced.](/fulltextimages/13910/fig_4.png)
Figure 4: Phylogenetic diagram of Nematoda compiled from data in [5, 9, 10, 16, 17, 81]. Paraphyletic taxa after Ahmed, et al. [17] in parentheses. The greyish circles mark points not resolved into dichotomies so far. Following our hypothesis that the stem species had three caudal glands with separate outlets, the lineages are marked where the three outlets must have joined to a single one and where caudal glands were completely reduced.
Hypotheses on the Existence of Releasing Cells and their Origin
Releasing gland cells may have been frequently overlooked in light microscopy studies, particularly when only three caudal gland cells were expected, or they may not have been resolvable in many cases. However, descriptions of several species suggest the presence of additional gland cells. For instance, two extra cells were illustrated in Belgopeltula belgica (Araeolaimida) [40], Monhystera (= Diplohystera) inflata [41], Linhomoeus parmacramphis (Monhysterida) [42], Plectus acuminatus (Plectida) [43], and Synonema cosmopoliticum (Desmodorida) [44] (Figure 2B).
Four to five glandular cells were documented in Caprionchulus diversipapillatus (Triplonchida) [45], while five gland cells were reported in Phanodermopsis nana (Enoplida) [46]. Chitwood described five uninucleate glands arranged in tandem anterior to the anus in Eurystomina americana (Enoplida) [47], likely comprising three caudal glands and two additional supplementary glands. Similarly, Aphanolaimus pseudoattentus (Plectida) exhibited two extra glandular cells extending dorsally from the intestine to nearly one tail length anterior to the cloaca, which may represent releasing gland cells [48].
Occasionally an additional cell was observed in Chromadorina germanica [33], positioned posterior to the main caudal gland cell but lacking a separate duct. Various authors have depicted additional gland-like cells in species across Araeolaimida, Chromadorida, Desmodorida, Desmoscolecida, Enoplida, Monhysterida, Mononchida, Plectida, and Triplonchida, often shading or greying them in illustrations to indicate structural or content differences (Table 1). These cells are typically found anteriorly adjacent or dorsal to the caudal gland cells, though some cases show a posterior or even ventral position (Figure 2C). Despite the lack of commentary by many authors, these observations warrant careful consideration. A targeted reexamination, guided by the hypothesis of additional releasing gland cells, is necessary. However, alternative functions for these adjacent cells should also be explored. In Plectus species, for example, these cells were interpreted as (pseudo)coelomocytes by Andrássy [49, 50], a view also proposed for Hemiplectus [6].
| species name | group | author |
|---|---|---|
| Brevitobrilus graciloides | Tobrilidae, Triplonchida | Abebe, et al. [234] |
| Epitobrilus setosus | Tobrilidae, Triplonchida | Abebe, et al. [234] |
| Triodontolaimus acutus | Triodontolaimidae, Triplonchida | Lorenzen [235] |
| Bathylaimus arthropappus | Tripyloididae, Enoplida | Wieser, et al. [111] |
| Tripyloides imitans | Tripyloididae, Enoplida | Wieser [134] |
| Synonchus dubius | Leptosomatidae, Enoplida | Wieser [42] |
| Phanoderma segmentum | Phanodermatidae, Enoplida | Murphy [236] |
| Iotonchus singaporensis | Iotonchidae, Mononchida | Ahmad, et al. [237] |
| Miconchus sp. | Iotonchidae, Mononchida | Ahmad, et al. [238] |
| Mylonchulus brachyuris | Mononchidae, Mononchida | Zell [239] |
| Chromadora macrolaima | Chromadoridae, Chromadorida | Steiner [116] |
| Chromadora micropapillata | Chromadoridae, Chromadorida | Wieser [240] |
| Chromadora quadrilineoides | Chromadoridae, Chromadorida | Chitwood [47] |
| Chromadorella filiformis (= C. filiformoides) | Chromadoridae, Chromadorida | Chitwood [47] |
| Chromadorina germanica (= C. minor), C. laeta | Chromadoridae, Chromadorida | Wieser [240] |
| Chromadorita mucrodonta | Chromadoridae, Chromadorida | Steiner [116] |
| Dichromadora amphidiscoides | Chromadoridae, Chromadorida | Kito [56] |
| Sabatiera abyssalis | Comesomatidae, Chromadorida | Timm [147] |
| Sabatieria longispinosa, S. pulchra | Comesomatidae, Chromadorida | Tchesunov [241] |
| Paracyatholamus lewisi | Cyatholaimidae, Chromadorida | Coomans, et al. [242] |
| Cobbionema brevispicula | Selachinematidae, Chromadorida | Ahmed, et al. [243] |
| Microlaimus porosus | Microlaimidae, Microlaimida | Miljutin, et al. [244] |
| Synonema braziliense | Aponchiidae, Desmodorida | Wieser [62] |
| Spirinia parasitifera | Desmodoridae, Desmodorida | Vincx, et al. [245] |
| Protricoma squamosa | Desmoscolecidae, Desmoscolecida | Decraemer [246] |
| Tricoma similis | Desmoscolecidae, Desmoscolecida | Decraemer [247] |
| Linhomoeus buculentus | Linhomoeidae, Monhysterida | Wieser [134] |
| Linhomoeus elongatus | Linhomoeidae, Monhysterida | Steiner [116] |
| Linhomoeus hirsutus | Linhomoeidae, Monhysterida | Wieser [62] |
| Linhomoeus ponticus | Linhomoeidae, Monhysterida | Schneider [249] |
| Monhystera rolandi | Monhysteridae, Monhysterida | Khan, et al. [250] |
| Araeolaimus filipjevi | Diplopeltidae, Araeolaimida | De Coninck et al. [231] |
| Diplopeltula sundensis | Diplopeltidae, Araeolaimida | Jensen [251] |
| Intasia monohystera | Diplopeltidae, Araeolaimida | Tchesunov, et al. [252] |
| Aphanolaimus aquaticus | Aphanolaimidae, Plectida | Meyl [253] |
| Dagda bipapillata | Camacolaimidae, Plectida | Holovachov, et al. [221] |
| Stephanolaimus elegans | Camacolaimidae, Plectida | Holovachov, et al. [221] |
| Anaplectus granulosus | Plectidae, Plectida | Holovachov, et al. [254] |
| Arctiplectus alaskanus | Plectidae, Plectida | Andrássy [50] |
| Ceratoplectus armatus | Plectidae, Plectida | Andrássy [43] |
| Chiloplectus andrassyi | Plectidae, Plectida | Andrássy [43] |
| Chiloplectus loricatus | Plectidae, Plectida | Andrássy [43] |
| Hemiplectus muscorum | Plectidae, Plectida | Holovachov, et al. [6] |
| Perioplectus secundus | Plectidae, Plectida | Andrássy [50] |
| Plectus antarcticus, P. belgicae, P. frigophilus, P. insolens, P. meridianus, P. tolerans | Plectidae, Plectida | Andrássy [49] |
| Plectus auracanorum | Plectidae, Plectida | Andrássy [256] |
| Plectus australis, P. intermedius, P. pusteri, P. zelli | Plectidae, Plectida | Zell [255] |
| Plectus cirratus, P. palustris, P. silvaticus, P. tenuis | Plectidae, Plectida | Andrássy [43] |
| P. communis, P. elegans, P. montanus, P. rotundilabiatus, P. similis, P. thornei | Plectidae, Plectida | Zell [255] |
| Plectus infundibulifer, P. parietinus | Plectidae, Plectida | Andrássy [43] |
| Plectus murrayi | Plectidae, Plectida | Zell [255] |
| Plectus parietinus | Plectidae, Plectida | Steiner [257] |
| Plectus sambesii | Plectidae, Plectida | Zell [258] |
| Plectus spicacaudatus | Plectidae, Plectida | Andrássy [50] |
| Tylocephalus cornutus | Plectidae, Plectida | Zell [259] |
Table 1: Existence of additional releasing cell(s) or pseudocoelomocyte(s).
If only three caudal gland cells exist without additional releasing cells, this does not necessarily imply that nematode detachment is mechanically driven by muscle action. Besides the previously discussed system of three adhesive and two releasing cells, an alternative duo-gland system could exist, wherein all three gland cells perform either adhesion or release functions. This possibility is suggested by the presence of two distinct glandular outlets in some species, such as Laxus sakihariiae (Desmodoridae), where one of the pores is dorsally positioned and slightly larger (Figure
3G) [51]. A stronger indication of functional differentiation among the three gland cells comes from observations of secretory vesicle differences. In Gonionchus australis, for example, one of the three gland cells contained secretory vesicles distinct from those in the other two [34], possibly representing a releasing function.
Similarly, in Paramphimonhystrella species (Xyalidae), the anterior-most caudal gland cell (cg1) is particularly voluminous [52], with structural and staining differences also noted in Eumonhystera filiformis [53], Gammarinema gammari and G. ligiae [54, 55], Halomonhystera (= Monhystera) cameroni [2], Monhystera refringens [56], M. disjuncta and Odontobius ceti [55], and possibly Chromadorina germanica [33] and Daptonema conicum [57].
Notably, in Cryonema crassum and C. tenue, it is the middle gland cell (cg2) that differs in secretion content [55]. Except for Chromadorina, all these species belong to the closely related families Monhysteridae or Xyalidae.
Despite its location in the thickest part of the tail, cg1 is often the smallest of the three gland cells, as observed in numerous monhysterids, including Daptonema hirsutum [57], Geomonhystera glaciei [58], Halomonhystera cameroni [2], H. parasitica [59], Monhystera floreanae and M. somereni [60], M. tanae [61], Paramonohystera proteus [62], Theristus caudasaliens [35], T. polychaetophilus [63], T. pratti [64], T. denticulatus and T. roscoffiensis [57].
This size disparity suggests a functional difference between cg1 and the other two cells. This pattern provides insight into species that possess only two caudal gland cells (Table 4), where cg1 appears to be entirely reduced. In such cases, cg1 may not have been responsible for releasing substances, and the presence of additional, undiscovered releasing cells similar to those found in Sphaerolaimus gracilis remains a possibility. This species possesses only two caudal glands in contrast to other species of Sphaerolaimus [65], raising the possibility that both caudal glands perform distinct functions, with one producing the releasing substance. How attachment and release occur in species described with only a single functional caudal gland cell, remains an open question. This has been reported in several Chromadorida species, such as Chromadorita tenuis, C. tentabunda, and Graphonema amokuroides [38], as well as in Cryptonchidae (Mononchida), including Cryptonchus nudus [66] and C. tristis [67]. Reports of species with only one gland, including members of Desmodorida (Draconema, Epsilonema, Notochaetosoma, Prochaetosoma), Desmoscolecida (Desmoscolex), and Triplonchida (Neotobrilus), require re-examination.
From these observations, we infer the existence of two distinct adhesive systems in nematodes. However, a key evolutionary question remains: which system is ancestral, and how and in which lineage did the transition between systems occur? Due to gaps in available data, any evolutionary hypothesis remains speculative, but these uncertainties should inspire further investigation. Since no additional releasing cells have been reported in species of Enoplida or Chromadorida, we propose that the ancestral nematode had three caudal glands, likely differentiated into two adhesive cells and one releasing cell. Each gland cell probably had its own duct and separate outlet at the tail tip. Arguments supporting the ancestral presence of three separate outlets will be discussed later. We hypothesise that the dorsal opening was associated with cg1, functioning as the releasing gland, while the two adhesive glands opened subventrally. The emergence of two additional releasing cells would thus represent a novel trait that may have originated in the stem ancestor of Desmodorida and Monhysterida (Figure 4, species X) or evolved independently in both clades.
The positioning of the two additional releasing glands suggests they may have originated through duplication of the ancestral anterior releasing cell. These new glands could have opened into a central pore, as seen in Leptonemella brevipharynx (Desmodorida) (Figure 3B) [12] and possibly Desmoscolex (Desmolorenzenia) gourbaultae (Desmoscolecida) [68]. Alternatively, they may have joined the dorsal outlet, contrasting with lineages where two subventral outlets presumably fused. This fusion could explain the glandular arrangement observed in Laxus sakihariiae (Figure 3G). The emergence of two additional releasing cells may have altered the function of the primary releasing cell, either shifting it toward adhesive secretion or leading to its reduction. While this hypothesis appears promising, new ultrastructural investigations are needed to critically assess the many assumptions made in this context.
Biological Role of Caudal Glands
Caudal glands play a crucial role in the survival and feeding behavior of aquatic nematodes, enabling them to anchor their tail tips temporarily to the substrate. This adhesion helps them resist water currents and turbulence, allowing specialized species to inhabit dynamic environments such as shifting sands on shores, riverbeds, and even torrents without being swept away. In more stable substrates like muddy sand, some nematodes adopt a hemi-sessile lifestyle. For instance, the long-tailed enoplid Trefusia spp. remains anchored by its tail tip while making exploratory movements in various directions. When disturbed, it can rapidly retreat, coiling its flagelliform tail or even its entire body into compact spirals [37]. Similarly, Chromadorita tentabunda maintains near-permanent attachment to the substrate via its tail while conducting intensive searching movements with its anterior end [38].
In terrestrial environments, adhesive glands may seem redundant. However, they persist in certain soil-dwelling adenophorean lineages, despite no longer being essential for locomotion [69]. The function of caudal glands in terrestrial nematodes such as those in Chromadorida, Monhysterida, Mononchida, Plectida, and Triplonchida remains uncertain. One untested hypothesis suggests that these glands may help prevent nematodes from being washed into deeper soil layers during heavy rainfall. Additionally, species with vestigial caudal glands lacking external outlets may still utilize them for unknown internal physiological functions.
In marine environments, caudal glands are integral to the distinctive movements of Draconematidae and Epsilonematidae. These nematodes exhibit a looper-like crawling behavior on marine plants, alternating between attachment and release of their anterior and posterior ends via caudal gland secretions [70]. A peculiar hopping locomotion is observed in Theristus species (Xyalidae) and Innocuonema tentabunda (= Chromadorita tentabunda) (Chromadoridae), which use their leftward-bent tails to adhere to sand grains. By straightening and curling the posterior part of their bodies, they propel themselves forward in a jumping motion, sometimes alternating attachment between the tail and the anterior end [71]. Interestingly, Innocuonema tentabunda, despite having only a single caudal gland, performs this movement in the same manner as related species with three glands [38]. Similarly, in Daptonema setosum (Xyalidae), caudal glands assist in locomotion [72].
Observations dating back to Cobb describe Chromadorina germanica (= C. minor, Chromadoridae) using an unusual mode of movement [73]. This species anchors itself by suctioning its mouth onto glass slides, then loops its body by bringing its tail close to its head and securing a hold via the caudal spinneret. It then advances by releasing its head, executing a movement similar to the inching motion of geometrid moth larvae.
These diverse adaptations underscore the multifunctionality of caudal glands in nematodes, aiding both adhesion and locomotion in aquatic and terrestrial environments. However, the precise function of these glands in certain soil-dwelling species remains an open question, meriting further investigation.
Caudal glands serve additional functions in certain marine nematode lineages beyond adhesion and locomotion. In some species, these glands secrete mucus to construct microtubes, providing a protective habitat within soft sediments [74]. The epizoic monhysterid Gammarinema utilizes its caudal gland secretions to assemble clumps of detritus and feces, creating a substrate on its crustacean host for egg deposition [75].
An intriguing foraging-related function of caudal glands was proposed in the “mucus-trap hypothesis” [76]. The authors suggested that slimy secretions from caudal glands along with those from the pharyngeal, ventral (renette), and epidermal glands can trap detritus particles, microorganisms, and macromolecules, which are subsequently ingested by the nematode. A remarkable example in this context is that Schneider (p. 55) documented an enoplid producing a several-millimeter-long thread of secretion from its “silk glands” on a glass slide [77].
Another significant role of caudal gland secretions may be in “enzyme sharing” between nematodes and microbes, facilitating the degradation of organic detritus. These secretions appear to agglutinate detritus into mucus- laden threads [78], which may later be decomposed by microbial activity. In Desmodora schulzi, Riemann [79] frequently observed sediment particles aggregating on the ventral side of the female’s body between the vulva and anus forming a ‘micro-garden’ of coccoid, filamentous, and septate microorganisms, likely promoted by caudal gland secretions.
Some members of Anoplostomatidae_,_ Oncholaimidae, and Comesomatidae have been reported to foster microbial growth around tail-secreted threads, potentially contributing to cellulose degradation. Their role in detritus processing is further highlighted by their apparent inability or reluctance to graze on the microorganisms responsible for cellulose breakdown [80]. These diverse functions underscore the multifaceted role of caudal glands in nematodes, extending beyond adhesion and locomotion to include habitat construction, foraging, and microbial interactions that may influence nutrient cycling in sedimentary environments.
The Stem Species of Nematodes Had Three Caudal Gland Cells with Separate Openings
The Discovery of Separate Gland Outlets
It can be assumed that three unicellular caudal glands were already present in the stem species of Nematoda [22]. These glands may have evolved as specialised derivatives of distal epidermal gland cells, arranged terminally in a triplet as a functional unit. As reasoned below, this set of large glands likely consisted of one dorsal cell and two subventral cells.
Their pyriform or ellipsoidal bodies are confined to the upper part of the tail, each containing a large nucleus with a prominent nucleolus. Their elongated ducts open terminally, reinforcing a triradiate organization at the posterior end of the nematode’s body similar to the anterior end, where bilateral symmetry is also overridden by triradiate structures. Contrary to the general view (eg. [22]), we agree with Lorenzen [82], that each gland cell originally had its own separate outlet rather than a common pore (Figure 5B).
![Figure 5: A) Three caudal glands with one terminal opening and spinneret; B) three separate outlets (exemplified by Diplopeltis incisus); C) three-pointed tail in a Secernentea lacking caudal glands (exemplified by Molinostrongylus). Sketches by Osche [107], newly arranged.](/fulltextimages/13910/fig_5.png)
The first documented case of separate glandular outlets was reported in Pseudodilaimus pilosus [83], which was described as having a truncated female tail where one gland opens dorsally and two open ventrally or subventrally (Figure 10D). Though the description of the species is quite well, it has never been found again, was classified as incertae sedis within the “Adenophorea”, and its existence of separate gland outlets was never cited. It happened similarly to Schulz [84], who observed a thin glandular duct opening laterally near the tip of the swollen tail in Sphaerolaimus makrolasius and S. sabulosus (Monhysterida), instead of a terminal opening, and hypothesised that there were three such ducts corresponding to three gland cells. This hypothesis was later supported by findings in S. gracilis and S. occidentalis, where three distinct glandular outlets were confirmed [85].
The hypothesis stating that separate caudal gland openings one dorsal and two subventral represent a plesiomorphic condition in Nematoda, was first proposed by Osche [86]. He observed remnants of a “triply-pointed” tail in a high percentage of females in a population of Pellioditis (= Rhabditis) papillosa, which he interpreted as an atavistic trait. Additionally, he noted similar tail structures in various nematode lineages, particularly among Secernentean parasites, where the tail exhibited three distinct cone-like protuberances arranged in the same way (Figure 5C). Osche suggested that this triply-pointed tail persisted as a cryptic feature of nematode body architecture for a prolonged evolutionary period, resurfacing repeatedly, particularly within Secernentea [87].
Osche further pointed out the occurrence of “archaic” three-pointed tails in females of the Strongylacanthinae (Trichostrongylidae) [88], implying that this trait may have been present in the stem species of Secernentea but was subsequently lost multiple times at the phenotypic level. However, we reject the idea of phenotypic continuity due to the improbability of such frequent reductions across nearly all lineages of Strongylacanthinae_._ Instead, we infer the persistence of the underlying genetic information for a three-pronged tail structure, followed by parallel phenotypic reversals driven by the same gene complexes. Although gland vestiges were never observed within the three tail cusps, their positional correspondence with the glands of “Adenophorea” is striking. In this context, Osche [86] referenced an earlier observation by Gerlach [89], who reported that two species of Diplopeltidae possessed three caudal glands with separate openings one dorsal and two subventral (Figure 5B).
Separate Outlets is a Plesiomorphic Character
Subsequent studies, influenced by Osche’s hypothesis, carefully examined gland termination and confirmed separate glandular outlets in approximately 77 species (4 questionable) across 39 genera (3 questionable) spanning Microlaimida, Desmodorida, Ceramonematoidea, Desmoscolecida, Monhysterida, and Araeolaimida, with possible occurrences in Enoplida (Table 2).
This trait is considered homologous across these taxa. In a few lineages where the dorsal gland was completely reduced (Daptonema trabeculosum, Echinotheristus teutonicus, Paramonohystera concinna, Pseudechinotheristus nudus, and likely Eubostrichus hopperi, Laxus spp., and Rhynchonema spp.), the two remaining gland outlets shifted laterally and slightly anterior to the tail tip [82]. This modification appears to have occurred independently multiple times.
| species name | group | author(s) |
|---|---|---|
| Pseudodilaimus pilosus (?) | Enoplida incertae sedis | Kreis [83] |
| Ixonema deleyi | Microlaimidae, Microlaimida | Muthumbi, et al. [153] |
| Ixonema sordidum | Microlaimidae, Microlaimida | Lorenzen [98] |
| Ixonema sp. | Microlaimidae, Microlaimida | Raes, et al. [133] |
| Desmodora bilacinia (possibly 2, dorsal/ ventral) | Desmodoridae, Desmodorida | Leduc, et al. [129] |
| Akanthepsilonema sinecornibus | Epsilonematidae, Desmodorida | Raes, et al. [133] |
| Bathyepsilonema lopheliae male | Epsilonematidae, Desmodorida | Raes, et al. [133] |
| Epsilonema multispiralum | Epsilonematidae, Desmodorida | Raes, et al. [132] |
| Epsilonema pustulatum | Epsilonematidae, Desmodorida | Karssen, et al. [155] |
| Triepsilonema tripapillata | Epsilonematidae, Desmodorida | Decraemer [100] |
| Eubostrichus hopperi (2, dorsal/ventral) | Stilbonematinae, Desmodorida | Armenteros, et al. [12] |
| Eubostrichus topiarius | Stilbonematinae, Desmodorida | Berger, et al. [260] |
| Laxus parvum (2, dorsal/ventral) | Stilbonematinae, Desmodorida | Armenteros, et al. [12] |
| Laxus sakihariiae (2, dorsal/ventral) | Stilbonematinae, Desmodorida | Leduc, et al. [51] |
| Leptonemella brevipharynx | Stilbonematinae, Desmodorida | Armenteros, et al. [12] |
| Leptonemella vestari | Stilbonematinae, Desmodorida | Hoschitz, et al. [261] |
| Robbea hypermnestra | Stilbonematinae, Desmodorida | Ott, et al. [152] |
| Diplopeltoides asetosus, D. bulbosus, D. grandis, D. linkei, D. longicaudatus, D. nudus, D. pumilus, D. suecicus | Ceramonematoidea | Holovachov, et al. [262] |
| Diplopeltoides axayacatli, D. santaclarae, D. paramastigia | Ceramonematoidea | Holovachov, et al. [263] |
| Diplopeltoides mastigia | Ceramonematoidea | Tchesunov [264] |
| Desmoscolex gourbaultae (not quite sure) | Desmoscolecidae, Desmoscolecida | Decraemer [68] |
| Tricoma tripapillata | Desmoscolecidae, Desmoscolecida | Soetaert [265] |
| Desmolaimus zeelandicus (?) | Linhomoeidae, Monhysterida | Meldal [81] |
| Metasphaerolaimus crassicauda, M. hamatus | Sphaerolaimidae, Monhysterida | Jensen [266] |
| Sphaerolaimus gracilis (2) | Sphaerolaimidae, Monhysterida | Turpeenniemi, et al. [36] |
| Sphaerolaimus occidentalis (2) | Sphaerolaimidae, Monhysterida | Turpeenniemi [85] |
| “several species” | Sphaerolaimidae, Monhysterida | Lorenzen [82] |
| Amphimonhystera anechma | Xyalidae, Monhysterida | Lorenzen [57] |
| Amphimonhystrella bullacauda | Xyalidae, Monhysterida | Tchesunov, et al. [267] |
| Cobbia sp. (presumably) | Xyalidae, Monhysterida | Lorenzen [57] |
| Daptonema conicum, D. fistulatum, D. hirsutum, D. trabeculosum (2) | Xyalidae, Monhysterida | Lorenzen [57] |
| Daptonema normandicum, D. oxycerca | Xyalidae, Monhysterida | Blome [65] |
| Echinotheristus cimbricus (2), E. teutonicus (2) | Xyalidae, Monhysterida | von Thun, et al. [101] |
| Elzalia spp. | Xyalidae, Monhysterida | Hope, et al. [268] |
| Gonionchus inaequalis, G. sensibilis | Xyalidae, Monhysterida | Lorenzen [57] |
| Metadesmolaimus gelana | Xyalidae, Monhysterida | Lorenzen [57] |
| Metadesmolaimus varians | Xyalidae, Monhysterida | Lorenzen [93] |
| Paramonohystera (P.) concinna (2), P. (P.) levicula | Xyalidae, Monhysterida | Lorenzen [57] |
| Pseudechinotheristus nudus (2) | Xyalidae, Monhysterida | Blome [269] |
| Rhynchonema lyngei (2), R. separatum (2) | Xyalidae, Monhysterida | Lorenzen [102] |
| Steineria pilosa | Xyalidae, Monhysterida | Lorenzen [270] |
| Stylotheristus mutilus | Xyalidae, Monhysterida | Lorenzen [57] |
| Theristus (T.) balticus, T. (T.) bastiani, T. (T.) denticulatus, T. (T.) roscoffiensis | Xyalidae, Monhysterida | Lorenzen [57] |
| Theristus biarcospiculoides | Xyalidae, Monhysterida | Blome [65] |
| Theristus caudasaliens (2) | Xyalidae, Monhysterida | Adams, et al. [35] |
| Valvaelaimus maior | Xyalidae, Monhysterida | Lorenzen [57] |
| Xyala riemanni | Xyalidae, Monhysterida | Lorenzen [57] |
| Dorylaimopsis variabilis | Comesomatidae, Araeolaimida | Muthumbi, et al. [154] |
| Diplopeltula breviceps | Diplopeltidae, Araeolaimida | Gerlach [89] |
| Mudwigglus nellyae, M. patumuka | Diplopeltidae, Araeolaimida | Leduc [303] |
| Neodiplopeltula barentsi (= Diplopeltula cuspidiboja) | Diplopeltidae, Araeolaimida | Leduc, et al. [148] |
| Neodiplopeltula bathmanni, N. onusta | Diplopeltidae, Araeolaimida | Holovachov, et al. [271] |
| Neodiplopeltula (Diplopeltis) incisa | Diplopeltidae, Araeolaimida | Gerlach [89] |
| Pararaeolaimus nudus | Diplopeltidae, Araeolaimida | Lorenzen, et al. [99] |
| Aegialoalaimus elegans | incertae sedis, Araeolaimida | Tchesunov [264] |
| Aegialoalaimus setosus | incertae sedis, Araeolaimida | Holovachov pers. com. |
Table 2: Three (or two) separate outlets of caudal glands.
The most parsimonious interpretation of these findings is that the presence of three separate gland openings represents a plesiomorphic trait within nematodes. Thus, the last common ancestor of Microlaimida, Desmodorida, and Secernentea likely possessed distinct caudal gland outlets, one dorsal and two subventral (Figure 4, species X). This same ancestor is reconstructed to have had a dual gland system consisting of three adhesive and two release gland cells, though it remains uncertain how each gland type corresponded to the three openings. As explored in the next section, the transition to a single common opening likely occurred multiple times through gradual tail-tip transformations and remodelling of the original three outlets.
What type of glandular openings might have been present in the outgroup of the clade containing stem species X? So far, no separate glandular outlets have been observed in the early-diverging lineages of the nematode tree. The only documented case was Pseudodilaimus pilosus [83], mentioned earlier (Figure 10D), which might be an artifact, but no similar structures have been reported since.1 Nevertheless, from a functional perspective, it can If it turns out to be right, as suggested, that the Muspiceida belongs to the Trichinellida clade [96], they provide an example of separate openings of the two caudal glands in one of the early nematode lineages ("Adenophorea"). However, presumably, this taxon belongs to Secernentea and more precise ly to the Spiroascarida [97], while the paired glands represent phasmids, which will be discussed in the chapter XIV on phasmids.
be argued that even the stem species of Nematoda likely possessed separate gland outlets. This reasoning stems from the improbability of a functional transition from a common opening potentially involving a complex spinneret apparatus (see below) and possibly a shared ampulla to three entirely separate outlets. Such a drastic shift in glandular architecture seems less parsimonious than the alternative scenario: the stepwise evolution of a common opening via the progressive invagination of the tail end, gradually consolidating the three separate outlets. Given the multiple independent occurrences of a common opening across nematode lineages (as explored in the section after the next), this evolutionary trajectory appears far more plausible.
Historical Remnants of Former Three Outlets
Caution is advised when interpreting caudal gland data reported in taxonomic descriptions, certain details have been elucidated. Cobb [90] reported that in various nematodes including Synodontium fecundum (Araeolaimida), Mesonchium poriferum (Chromadorida), Chromaspirina (= Bolbolaimus) pellucida and Synonema braziliense (Desmodorida), Tripylium carcinicola (Monhysterida), and Acontiolaimus (= Digitonchus) uniformis (Plectida) the caudal glands discharge through separate ducts. In close relatives of these species, three glandular outlets are frequently observed. Notably, in Synodontium, “the ducts of the caudal glands are separate, practically to the spinneret pore” (p. 280), indicating that while three ducts remain distinct for most of their length, they ultimately converge at a single common spinneret rather than terminating in three separate pores at the tail tip (Figure 10F). Similarly, in Oxyonchus spp. (Enoplida), “the caudal glands open by three ducts through a spinneret” [91]. Deontolaimus papillatus (Plectida) (Figure 10C) also possesses a common spinneret [92], contradicting Lorenzen’s [93] assumption based on Meyl’s [94] depiction of seemingly three terminal tubes that it had separate outlets.
However, it is of much interest that in some cases, the three gland ducts expand into three distinct ampullae before ultimately merging into a single terminal outlet. This structural organisation may be a remnant of an ancestral condition in which three separate openings were present in the nematode body plan. Such configurations have been observed in Cynura uniformis (Araeolaimida), Gammanema ferox (Chromadorida), Synonema braziliense (Desmodorida), Chromaspirina (= Bolbolaimus) pellucida (Desmodorida), and Trissonchulus oceanus (Enoplida) (all reported in [90]). Additionally, similar glandular arrangements have been noted in monhysterids such as Gammarinema gammari [55], Halomonhystera cameroni [2], Monhystrium inquilinus [55], M. wilsoni [90], and Theristus meyli [95].
A particularly intriguing case is Phanoderma (= Cophonchus) ocellatum (Enoplida), where it appears that one of the glands “the one on the ventral side” leads to a separate ampulla, while the other two ducts run parallel and seem to join within a single, much larger ampulla [90]. Given the structural diversity of glandular arrangements across taxa, a more detailed investigation would likely reveal additional examples of this intermediate condition. However, most Adenophorean taxa appear to have a single common ampulla, reinforcing the idea that the fusion of glandular ducts into a single opening occurred repeatedly and independently in nematode evolution.
A Cryptic Base for the Three-Pointed Tail Tip
In Ixonema sordidum and Pararaeolaimus nudus, the tail end features three protuberances, each serving as a separate outlet for the three caudal glands [98, 99]. Similarly, in Triepsilonema tripapillata, the three glandular outlets are positioned on prominent caudal papillae (Figures 3D & 10H) [100]. In contrast, Echinotheristus cimbricus and E. teutonicus possess only two caudal glands, which open separately on distinct protuberances or tubercles [101]. A similar arrangement has been observed in species of Rhynchonema [102].
Interestingly, in unrelated lineages of Secernentea where caudal glands are entirely absent prominent projections or cusps often appear dorsally and/or subventrally at the tail end, either as species-specific traits or as features of a particular developmental stage or sex (Figure 5C). For example, in the diagnosis of Histiostrongylus (Strongyloidea) (Figure 10E), Yorke, et al. [103] (p. 132) noted: “the posterior extremity of the female is provided with three strong spines, one dorsal and two subventral, between which the atrophied tail is found”.
This suggests that once caudal glands are lost, the tail shape is no longer constrained by the functional necessity of terminal glandular openings, potentially allowing the reactivation of an underlying developmental program for such protuberances. Slight dorsal and subventral protrusions could then be selectively enhanced, especially if they contribute to anchoring the nematode to its host. This may partly explain the recurrent evolution of cusps or mucros at the tail end in many unrelated, predominantly zooparasitic nematodes of Secernentea, as well as in multiple independent lineages of Strongylina parasitising bats (Chiroptera) [88]. However, the potential selective advantage of these structures remains unknown.
If a three-pointed tail is a cryptic feature of the nematode bauplan rather than being exclusive to Secernentea its occasional reappearance should also be expected in certain taxa of “Adenophorea” where caudal glands have been lost. However, such occurrences appear to be rare. In species of Lenonchium (Dorylaimida), both sexes exhibit three or four finger-like projections at the tip of their elongated tails (Figure 10A) [104, 105].
An exceptional case was reported in a female Benthimermis sp., which displayed three terminal lobes in dorsal and subventral positions [106]. Osche also cited Chronogaster (Plectida) as an example [107], but subsequent observations indicate that species in this genus primarily possess hook-like spines or other distinct mucro shapes at the tail terminus [108]. In one peculiar case, a male Trefusialaimus monorchis (Enoplida) (Figure 10B) was found with three cusps at the terminal end of its cylindrical tail. However, rather than representing an ancestral trait, these structures were suspected to result from wound healing [109].
Towards A Common Outlet and A Spinneret Apparatus
We hypothesise that three caudal glands with three separate terminal openings represent the ancestral state in nematodes, and a common exit realized by fusion is apomorphic. This hypothesis is supported by the presence of separate openings in multiple species within the clade that includes species X as an ancestor (Figure 4). Additionally, some taxa with a common opening retained three separate ampullae. In contrast, others rarely exhibited remnants of a three-pointed tail tip.
A unique relic of this ancestral condition is found in Pternepsilonema servaesae, where a triradial tube with a triangular aperture outlines three duct orifices (Figure 3C) [110]. In contrast, nematodes with a single outlet typically possess a cuticularized tube with a circular cross-section.
Similarly, a three-lobed spinneret is observed in some Araeolaimida, such as Synodontium fecundum (Figure 10F) [90] and Axonolaimus hexapilus [111], while a tripartite supporting structure is present in the mononchid Oionchus obtusicaudatus [112].
A functional argument against the presence of a common exit in the nematode stem species is the improbability of losing a complex and highly efficient (effective) spinneret apparatus while gland cells remained ecologically functional. A possible intermediate stage in this evolutionary transition would involve the retention of three separate ampullae, with early signs of fusion evident in the structure of the opening. This suggests a gradual transformation from three distinct gland ducts, ampullae, and openings to a configuration with at least one common ampulla, a valve, and a joint opening (Figure 6).
If this reasoning is correct, then the common opening seen in the vast majority of adenophorean species is not an ancestral trait but rather a unique feature that evolved multiple times independently. This pattern of convergent evolution is reflected in the phylogenetic tree (Figure 4) and will be further discussed in the next section.
Figure 6: Different characteristics of the caudal glands complex and terminology. A) Beginning with three separate openings; B) in a second step with a common pore and a valve represented by a cuticularised plug in a funnel, each gland with an ampulla; C) the separate ampullae are joined to a common ampulla and the simple pore is developed to a cuticular spinneret tube; gland cells in tandem, numbered from anterior to posterior; A-B) gland cells grouped.
The common opening is typically part of a functional complex that Cobb first referred to as a “spinneret” [113], drawing an analogy to the silk-spinning organs of spiders. In Mononchulus ventralis, M. nodicaudatus and Mononchus lacustris [114, 115]), Cobb described a sophisticated apparatus associated with this structure [113]. A similar mechanism was noted in Klugea (= Enchelidium) polaris (Enoplida) [116], although without illustration. This apparatus consists of a conical plug housed within a cuticular funnel (Figure 6), functionally resembling a needle valve that finely regulates secretion flow. Small retractor muscles open a ring-shaped passage around the plug, which is always positioned obliquely to the dorsal body wall, allowing secretions to be released. The valve then closes due to the turgor pressure of the body fluid [33], enabling secretion to be controlled under neural regulation. The term “spinneret” is often applied to a simple terminal opening (pore), but more commonly, it refers to a mucro-like offset terminal tube (historically termed an “adhesive tube”), whose length and shape are often diagnostic in species identification (e.g., in Monhystera [117]). However, inconsistencies exist: some authors refer to the valvular apparatus as the “spinneret” [115, 116, 117, 118, 119], others use the term to describe the apparatus plus the terminal tube [55], and some remain ambiguous in their usage [120, 121, 122].
We emphasize the importance of distinguishing between a “spinneret valve” and a “spinneret” (Figure 6). The term “spinneret” should be used as a general descriptor for the common outflow of the caudal glands, given its widespread usage in species descriptions. Based on structural differences, we propose the following terminology: (1) a simple opening, as depicted in Cobb’s illustrations, should be referred to as a “pore” or “spinneret pore”; (2) a cuticular tube should be designated as a “spinneret tube” or “tubular spinneret”; and (3) specialized structures (valves) should be described more precisely. As these structures have evolved multiple times independently, these terms serve as functional descriptors within our evolutionary framework.
In many lineages, the separate ducts of the glands, which form part of the gland complex, converge into a common ampulla for secretion storage, enabling immediate release when needed. During evolution, a distinct ampulla or expanded collecting duct likely emerged from the fusion of three ducts or from separate ampullae, as observed in Monhystera species [61]. A prominent ampulla is documented across diverse lineages, including Mononchida, Triplonchida, Enoplida, Chromadorida, Desmodorida, Monhysterida, and Araeolaimida, based on various studies. However, estimating the frequency of convergent evolution for this trait is challenging, as it is rarely considered in species descriptions. Notably, an ampulla is absent in Chromadorina and Perepsilonema [33, 35].
According to our hypothesis, the fusion of three ducts into a single ampulla and outlet, along with the formation of a valve or “caudal cone,” likely occurred simultaneously. Malakhov (p. 109) described the cuticular tail cone as forming via the submersion of a cuticle segment located between gland pores [123]. Although comparative studies on this topic are lacking, structural variations of the apparatus among unrelated taxa are expected. Pternepsilonema servaesae, for instance, possesses a triangular aperture, which may represent a transitional stage within the Epsilonematidae family. Gerlach described three cuticular components (one dorsal, two subventral) forming the outlet in Cynura uniformis (Leptolaimidae) [124]. Similarly, Kreis noted three sclerotized structures laterally positioned around the outlet in Symplocostoma papillatum (Enchelidiidae), however one ventral and two subdorsal [125].
Information on the internal sclerotized valve structure across taxa remains limited, and structural differences among lineages such as Mononchida, Chromadorida, and Monhysterida are likely (for monhysterids, see [55]). The homology of the spinneret apparatus in these taxa was questioned [33]. However, at present, we assume that these species exhibit structural similarity with only minor variations, such as a needle-shaped versus conoid spinneret plug, and differences in the number of retractor muscles ranging from one [33] to two [126] or several [127]. Wilsonematinae species lack needle valves [128]. The closure mechanism in species with separate outlets and no valves remains unknown [35, 36].
Multiple Transformations to A Single Opening
It is highly probable that the stem species of Nematoda possessed three distinct apertures for the caudal glands, a trait that has been retained in many descendants of species X (Figure 4). However, most Adenophorean taxa share a common outlet, suggesting that multiple lineages independently converged on this trait. This study aims to determine where these transformations occurred in phylogeny (see tree for comparison; Figure 4).
The absence of distinct gland openings in all early- branching species of “Adenophorea” supports the hypothesis that a common outlet evolved independently in Chromadorida, Enoplida/Triplonchida, and the clade encompassing Mononchus and Trichinella (Figure 4). In closely related species of the clade defined by the stem species X both separate and common gland openings are present. Further investigation is required to clarify evolutionary transitions within these taxa (Figure 7).
Microlaimida Among Microlaimidae, only Ixonema exhibits separate gland outlets. Therefore, we infer that the stem species of Microlaimidae possessed this trait, with at least one subsequent transformation to a common outlet within this family taxon.
Aponchiidae (Aponchium, Synonema), Monoposthiidae (Monoposthia, Monoposthioides, Nudora, Rhinema), and Richtersiidae (Desmotersia, Richtersia) all possess a spinneret.
This suggests that the stem species of Microlaimida originally had separate gland openings, which were independently modified into a common pore in at least two lineages.
Desmodorida Desmodoridae: The condition in Desmodora bilacinia remains uncertain, as there appear to be two superimposed openings (Figure 8E). Aside from this ambiguous case and the Stilbonematinae subfamily, all Desmodoridae exhibit a common outlet with a spinneret [129].
Stilbonematinae: Since multiple species in this subfamily retain separate apertures, it is likely that both the stem species of Stilbonematinae and the broader Desmodoridae initially had separate gland openings.
While Adelphos may possess a single terminal pore [130], historical claims of a “spinneret present” in Catanema and Stilbonema [120] require verification.
Eubostrichus includes species with both separate gland openings (Figure 3H) and a single spinneret [131], indicating at least two independent transitions to a common outlet within Stilbonematinae.
At least a third independent transformation must be postulated for the remaining Desmodoridae. • All species of Draconematidae have a single outlet for the caudal glands and usually a spinneret, which is interpreted as an apomorphic trait.
• In Epsilonematidae, some genera exhibit three separate outlets (Akantepsilonema, Epsilonema, Triepsilonema: Figures 3D,3E), while others have a single outlet (Glochinema, Metepsilonema, Perepsilonema). In Bathyepsilonema, there is one pore and a slit in males (Figure 3J) and a single pore in females [132, 133], which may indicate the fusion of two pores. The presence and structure of pores in Polkepsilonema and Pternepsilonema remain ambiguous [110]. It is likely that the stem species of Epsilonematidae had three separate gland openings, with at least one independent transformation to a joint opening within this group. Similarly, at least five independent transformations to a common duct for caudal glands must be assumed within Desmodorida.
Ceramonematoidea Unlike other taxa within Ceramonematoidea, species of Diplopeltoides possess separate gland openings. This suggests that the transition from separate openings in the stem species to a single joint pore occurred at least once within this group.
Desmoscolecida The only species of Desmoscolecida known to have separate outlets is Tricoma tripapillata, though they may also be present in Desmoscolex gourbaultae [68]. Since a terminal spinneret is present in other species of Tricoma, it suggests that the separate outlets reconstructed for the stem species of Desmoscolecidae were modified into a terminal spinneret at least twice independently within this group.
Monhysterida • Linhomoeidae: All species possess a spinneret. Meldal reported separate openings in Desmolaimus zeelandicus [81], but other authors have described only a single terminal pore in Desmolaimus species [84, 134]. Therefore, we question the validity of Meldal’s observation. At least one transition to a single duct must be assumed here.
• Sphaerolaimidae: All species should possess three separate outlets. However, there are reports of a spinneret in Doliolaimus agilis [135] and various Sphaerolaimus species [62, 134, 136].
• Monhysteridae: All taxa have a common opening for the glands, which is considered an apomorphy supporting the monophyly of the group [82].
• Xyalidae: The stem species of Xyalidae undoubtedly had separate outlets for the three caudal glands, a feature retained in most genera (Amphimonhystera, Daptonema, Echinotheristus, Elzalia, Gonionchus, Metadesmolaimus, Pseudechinotheristus, Stylotheristus, Valvaelaimus). However, some genera contain both species with three outlets (references in Table 2) and a single pore or spinneret, such as Paramonhystera [56], Rhynchonema [137], Steineria [138, 139], Theristus [140, 141, 142], and Xyala [143, 144]. Other genera consistently exhibit a common outlet (Capsula, Dactylaimoides, Hofmaenneria, Lamyronema, Promonhystera, Sphaerotheristus). The character state in Cobbia remains uncertain [57, 144]. If these character states are correctly assessed, approximately eleven independent transitions to a joint outlet happened within Xyalidae.
Thus, within Monhysterida, there have been approximately thirteen transitions to a common pore, one each in the lineages leading to Linhomoeidae and Monhysteridae, and eleven transformations within Xyalidae.
Araeolaimida • Comesomatidae: Three gland pores (one dorsal, two subventral) have been reported only for Dorylaimopsis variabilis (Figure 3A), distinguishing it from other, spinneret-bearing Dorylaimopsis species [145, 146] and all other genera within the family. This suggests that a transformation to a common duct occurred at least twice within Comesomatidae.
• Diplopeltidae: Caudal glands open via three separate pores in Diplopeltula, Mudwigglus, Neodiplopeltula, and Pararaeolaimus nudus, whereas other species of Pararaeolaimus [147, 148] and all other genera possess a spinneret. Notably, caudal glands are absent in Edalonema. These observations indicate that Diplopeltidae inherited separate openings from its stem species, with at least two independent transitions to a common spinneret within this group.
Aegialoalaimus: At least two species of Aegialoalaimus possess three separate openings (Table 2), while others exhibit a common pore [16]. It is hypothesised that fusion occurred only once within Aegialoalaimus. An intriguing observation by Tchesunov suggests that in A. elegans, adults exhibit three separate outlets, whereas juveniles appear to have a common pore [149]. However, before proposing that a common pore represents an ancestral state in juveniles, potentially challenging our hypothesis of separate pores as plesiomorphic, further confirmation of character states in both developmental stages is needed. Consequently, caudal glands have independently evolved to discharge via a common opening at least five times in the lineage of Araeolaimida.
Plectida The timing of the evolution of a single duct in Plectida remains uncertain. In the Secernentea/Teratocephalida lineage, caudal glands were completely reduced. It is assumed that the three-pointed tails found within Secernentea represent a plesiomorphic condition corresponding to the existence of separate outlets. This implies that a joint outlet was a derived trait, emerging either in the lineage to Plectida or in the sister lineage to Metateratocephalidae (Figure 4), suggesting at least one such transformation. However, the exact point of this transition remains undetermined (Figure 4).
![Figure 9: Phylogenetic diagram of Enoplida appraising the data in [11,13,17]. Paraphyletic taxa after Ahmed, et al. [17] in parentheses. The greyish circles mark points not resolved into dichotomies so far.](/fulltextimages/13910/fig_9.png)
In conclusion, in the clade derived from stem species X, caudal glands evolved to open via a single pore at least 29 times independently (Figure 4). Additionally, as noted earlier, three similar transformations could be inferred within the early lineage of “Adenophorea” (Figure 4). Thus, it can be hypothesised that the single caudal gland outlet evolved independently about 32 times in Nematoda. This exceptionally frequent parallel evolution suggests a selective advantage for a single outlet, although its functional benefits remain unclear. A newly developed valve may regulate secretion flow precisely, but the challenge of coordinating the discharge of secretions from both adhesive gland cells and releasing gland cells through the same pore requires further investigation. The convergent evolution of joint pores suggests that spinneret structures might differ across lineages, of which three-lobed spinnerets give examples. A comparative SEM study could provide valuable insights into their structural diversity and functional implications.
Dorsoventral Positioning of Caudal Glands
Two distinct orientations of caudal glands can be observed within Nematoda. The most common configuration consists of an anteriorly positioned dorsal gland and two ventrally located caudal glands or their ducts. However, the exact positions of these glands have seldom been described in the literature, with exceptions such as Mononchus italicus [150] or illustrated cases like the mononchid Parahadronchus shakili [151] and the monhysterid Theristus caudasaliens [35].
![Figure 10: Structure of tail terminus in different nematode taxa. A) Lenonchium fimbricaudatum [104]; B) Trefusialaimus monorchis [109]; C) Deontolaimus papillatus; D) Pseudodilaimus pilosus [83]; E) Histiostrongylus spineus [233]; F) Synodontium fecundum [90]; G) Caribplectus magdalenae [201]; H) Triepsilonema tripapillata [133].](/fulltextimages/13910/fig_10.png)
Instances where glands open through three separate apertures at the tail tip show a consistent pattern: one dorsal and two subventral openings [82, 83, 89]. Examples include Robbea hypermnestra [152] in Desmodoridae, Ixonema deleyi [153] in Microlaimidae, and Dorylaimopsis variabilis [154] in Araeolaimida. A similar arrangement is found in glandless Secernentea, where three-pointed tails also feature one dorsal and two subventral protrusions.
An unusual deviation from this pattern was reported by Lippens (p. 182) in Chromadorina germanica and Chromadora nudicapitata (Chromadoridae), where “the duct of the anterior gland cell is located ventrally... [and] the ducts of the two posterior cells assume a dorso-sublateral position” [33]. A similar reversed orientation was noted in Tuerkiana (=Thoracostoma) strasseni (Leptosomatidae, Enoplida), where Türk (p. 308) observed a ventromedial gland duct alongside two subdorsal ducts [1].
This orientation is also seen in Epsilonematidae (Desmodorida), as reported for Akanthepsilonema sinecornibus, Epsilonema pustulatum, and Triepsilonema tripapillata, which possess one medioventral and two subdorsal outlets [155]. Furthermore, SEM imaging of Leptonemella brevipharynx (Desmodoridae) reveals two laterodorsal outlets and one ventral opening (Figure 3B).
Despite these observations, none of the cited authors have addressed the significance of these variations, and to our knowledge, this discrepancy has not been discussed in the literature. One possible explanation could be situs inversus, as observed in the triradiate stoma of five species within “Rhabditidae” by Osche [156], suggesting that similar developmental modifications may have influenced the caudal gland arrangement in these nematode groups.
The inverse orientation of the three duct orifices in Pternepsilonema servaesae [110] may also be indicative of a corresponding inversion in caudal gland positioning. In this context, the sclerotizations observed at the outlet of Symplocostoma papillatum (Enoplida) by Kreis [125] may be relevant. Similarly, in Phanoderma ocellatum (Enoplida), the orientation of the caudal glands might resemble that of Chromadoridae [90]. However, these interpretations require confirmation, as the available data remain too imprecise for meaningful phylogenetic discussion.
Based on the available evidence, an apomorphic inverse orientation of caudal glands appears to have evolved independently in at least four distantly related lineages: Leptosomatidae, Chromadoridae, Epsilonematidae, and likely within Desmodoridae. Some of the inferred transformations of gland positions and their respective openings are outlined in Figure 7.
Location and Arrangement of Caudal Gland Cells, Especially in Enoplids
Caudal glands in nematodes typically follow one of two main arrangements: a tandem sequence (cg1-cg3) (Figure 6D) or a grouped configuration, where cg1 is positioned anteriorly and cg2 and cg3 lie side by side at the same level (Figure 6B, C).
In some cases, these differences serve as taxonomic markers for distinguishing closely related species. However, no clear pattern has emerged in the distribution of these arrangements, suggesting that both configurations have evolved multiple times in different directions. It is plausible to assume that the grouped arrangement was ancestral in nematodes [21, 115].
Based on available data, caudal glands in nematodes can be categorized into two primary spatial configurations: incaudal and precaudal.
Incaudal: The glands are predominantly confined to the tail, with cg1 usually reaching the level of the anus/cloaca or extending slightly beyond (Figure 2B, C). In some cases, cg1 and even cg2 may extend anterodorsally beyond the rectum, occasionally reaching the end of the intestine. This condition is termed “suprarectal.” Meanwhile, cg3 remains entirely within the tail.
Precaudal: All three gland cells, including their nuclei, are located anterior to the anus/cloaca, typically arranged sequentially without overlap. They are aligned parallel to the intestine and often the gonads (Figure 2A). This precaudal positioning allows the gland cells to enlarge and potentially produce greater secretions.
Even within the incaudal configuration, the extension of cg1 (and sometimes cg2) anterior to the anus/cloaca likely serves to increase secretion output based on species-specific ecological demands. In some cases, this extension is sexually dimorphic being more pronounced in females than in males. A possible explanation is that, in males, the anterior expansion of caudal glands may be constrained by the presence of copulatory structures such as spicules, the gubernaculum, and associated muscles. However, exceptions exist: in Enoplus [23] and Synonchus fasciculatus [73], caudal glands extend further anteriorly in males, while remaining within the tail in females. Similarly, in Graphonema antarcticum (Chromadoridae), all caudal gland cells are located anterior to the cloaca in males, whereas only one gland is preanal in females [157].
In contrast, taxa with gland bodies located far precaudally are particularly notable and occur frequently in Enoplida. The mixed distribution of incaudal and precaudal gland positioning within this group (or clade) allows for multiple evolutionary interpretations: (1) caudal glands were originally confined to the tail, with precaudal displacement occurring repeatedly in parallel; (2) a single primary shift to a precaudal position occurred, with occasional reversions; or (3) continuous back-and-forth displacements happened throughout evolutionary history. Lorenzen proposed an explanation based on the concept of “underlying apomorphy” [23, 158] suggesting that a genetic predisposition for precaudal displacement was inherited from a common ancestor with incaudal glands. According to this hypothesis, minor transformations could repeatedly lead to an anterior shift. We agree with Lorenzen that the precaudal gland position in Enoplida is homologous in origin. However, distinguishing between evolutionary scenarios (1) and (2) remains challenging.
To explore this question, we consider the possibility of independent precaudal shifts. If unrelated taxa exhibit differences in gland positioning relative to the intestine and rectum, this could support the hypothesis of multiple independent shifts. Unfortunately, data to test this hypothesis are scarce.
Based on our observations, we propose a positional formula for precaudal gland cell arrangement - left (l) or right (r) of the intestine - in the sequence cg1-cg3: Phanoderma tuberculatum [21]: <l-l-l> Oncholaimus sp. [21], see Fig. 2A: <l-r-l> Viscosia viscosia [159]: <r-l-l> Eurystomina terricola [160]: <r-l-r> Oncholaimus jessicae [32]: <r-l-r>, but some specimens exhibit <l-r-l>.
This pattern highlights variation in gland positioning, which could provide further insights into the evolution of caudal gland arrangements in nematodes.
In Triplonchida, the supposed sister group of Enoplida, the three caudal glands are invariably incaudal, which is likely plesiomorphic, suggesting that the ancestral condition in the Enoplida lineage was also incaudal. Consequently, within Enoplida, the incaudal condition is presumed always to be the ancestral state, ruling out any reversals from a precaudal site back to the original configuration. While the exact evolutionary relationships within Enoplida remain uncertain (Figure 9), the potential positioning of caudal glands in well-supported monophyletic subgroups of Enoplida can be outlined. Additionally, it is assumed that most of the genera listed below are monophyletic in this preliminary analysis. The data sources include various species descriptions, with references provided only in exceptional cases. Estimating the number of parallel evolutionary events is challenging, particularly given the lack of robust phylogenetic trees. However, it is hoped that studies such as the present one will encourage further research on this subject.
Campydoroidea: All taxa (Campydoroides, Udonchus, Rhabdolaimus, Rogerus, and most species of Syringolaimus) possessing caudal glands exhibit an incaudal arrangement.
Alaimidae: In the stem species of Alaimus and Amphidelus, caudal glands were lost. However, their ancestral lineage likely possessed incaudal glands.
Ironidae: In most genera (Dolicholaimus, Ironus, Syringolaimus, Trissonchulus), the caudal glands remain within the tail or extend suprarectally. However, the precaudal positioning of glands in Conilia, T. jungi within Thalassironus, and possibly in Ironella suggests at least three independent gland dislocations within Ironidae.
Trefusiidae: In genera such as Africanema, Rhabdocoma, Trefusia, Tripylina, and Trischistoma, caudal glands are always in the tail, rarely extending suprarectally.
Tripyloididae: In Bathylaimus, Paratripyloides, and Tripyloides, caudal glands are consistently confined to the tail.
Oxystominidae: This group may be paraphyletic. The plesiomorphic incaudal condition is observed in Halalaimus, certain species of Litinium (e.g., L. bananum), Nemanema (N. rectoresectum) and Thalassoalaimus (T. longicaudatus, T. montemari). However, Oxystomina (O. astridae, O. clavicauda, O. exilis) and most species of Litinium, Nemanema, Paroxystomina, and Thalassoalaimus exhibit a suprarectal gland position. Thus, at least five independent precaudal gland shifts are expected within Oxystominidae.
Rhaptothyreidae: In Rhaptothyreus, the caudal glands remain incaudal.
Enchelidiidae: Plesiomorphic caudal glands are incaudal or suprarectal in Belbolla heptabulba, species of Ledovitia, Eurystomina (E. bilineata, E. chilensis, E. vincxae), and Polygastrophora (P. hexabulba, P. septembulba). However, they are often arranged in tandem anterior to the anus/cloaca or at a much greater distance in Calyptronema (= Catalaimus), Ditlevsenella, Illium, Symplocostoma, Symplocostomella, Thoonchus, and Belbolla sundaensis, as well as in most species of Eurystomina and Polygastrophora. In Bernardius lineatus, a transitional state is observed, with two caudal glands extending up to two spicule lengths anterior to the cloaca, while cg3 terminates at the cloacal level [161]. Within Enchelidiidae, at least eight to nine independent gland displacements have occurred. However, the loss of caudal glands in Abelbolla, Bathyeurystomina, Megeurystomina, and Pareurystomina cannot be precisely traced to an incaudal or precaudal origin.
Oncholaimidae: This group appears to be paraphyletic. The presumed plesiomorphic incaudal and possibly suprarectal gland positions are seen in Pelagonemella, Thalassogenus, and certain species of Adoncholaimus, Oncholaimus, and Viscosia. However, most species in these genera exhibit the apomorphic precaudal condition. Transitional states are observed in Dioncholaimus brevicavatus and Metoncholaimus isopapillatus, where two caudal gland cells are precaudal while one remains incaudal [162, 163]. In other species of Metoncholaimus and other genera (Anoncholaimus, Krampia, Metoncholaimoides, Meyersia, Mononcholaimus, Oncholaimellus, Pelagonema, Pontonema, Prooncholaimus, Pseudoncholaimus, Pseudopelagonema, and Vasculonema), the precaudal condition is well established. A cautious estimation suggests that the precaudal gland position evolved independently at least 16 times in Oncholaimidae.
Anoplostomatidae: In Anoplostoma and Chaetonema, the caudal glands are precaudal, suggesting an apomorphic state for the family.
Anticomidae: The caudal glands are incaudal to suprarectal in Anticoma, Anticomopsis, and Paranticoma.
Leptosomatidae: This possibly paraphyletic group shows incaudal to suprarectal glands in Leptosomatum caecum, L. groenlandicum, Orthophallonema, Paratuerkiana, Pseudocella, Synonchus, and Tuerkiana. However, precaudal glands are found in Bongersia, Corythostoma, Cylicolaimus, Deontostoma, Leptosomella, Leptosomatides, Paraleptosomatides, Thoracostoma, and certain Leptosomatum species. Due to limited data and uncertain phylogenies, nine precaudal shifts are estimated within Leptosomatidae.
Thoracostomopsidae: Many taxa exhibit incaudal or suprarectal glands, including Fenestrolaimus, Filipjevia, Metenoploides, Parenoplus, Thoracostomopsis galeata, and several species of Enoploides, Enoplolaimus, Epacanthion, Mesacanthion, Oxyonchus, Paramesacanthion, Saveljevia, and Trileptium. However, approximately 13 independent precaudal transitions have occurred (in Africanthion, Fleuronema, Mesacanthoides, Okranema, Enoplolaimus pectinatus, Epacanthion, Oxyonchus dentatus, Paramesacanthion tricuspis, Saveljevia kolaensis, Thoracostomopsis barbata, Trileptium sp. and within Enoploides and Mesacanthion).
Phanodermatidae: Caudal glands are generally incaudal or suprarectal. However, in Phanoderma campbelli and Paraphanoderma robynae, they are precaudal, indicating two independent shifts.
Enoplidae: In Enoplus, caudal glands extend precaudally in males, whereas they remain incaudal to suprarectal in females.
Overall, caudal glands remain largely restricted to the tail in six of the 15 groups analyzed, supporting their plesiomorphic nature. However, gland migration anterior to the anus/cloaca has occurred approximately 58 times independently. This high frequency suggests that gland dislocation is genetically predisposed and phenotypically adaptable, reinforcing the concept of underlying apomorphy within Enoplida.
In the outgroup of Enoplida the caudal glands are located in the anterior part of the tail, but frequently cg1 and sometimes also cg2 extend suprarectally near the end of the intestine while cg3 still remains entirely incaudal. Possibly there are few exceptions of species of Araeolaimida, Chromadorida, Desmodorida and Monhysterida (Table 3) where all gland bodies are situated suprarectally anterior to anus/cloaca.
In Onyx adenophorus (Desmodoridae) they are located considerably anterior similar to some enoplids [164]. In comparison with their closely related taxa, the species in Table 3 demonstrate continuity between both conditions (incaudal to suprarectal and precaudal), which are defined with respect to the location of the posterior-most gland cell (more precisely: the cell portion of cg3 containing the nucleus) posterior versus anterior to anus/cloaca. In Diplopeltula striolata (Araeolaimida) two gland cells extend far anterior but cg3 just reach the cloaca [165].
| species name | group | author(s) |
|---|---|---|
| Steineridora (= Spilophora) loricata | Chromadoridae, Chromadorida | Steiner [116] |
| Onyx adenophorus | Desmodoridae, Desmodorida | Blome, et al. [164] |
| Xenonema obesum | Desmodoridae, Desmodorida | Cobb [90] |
| Dracograllus filipjevi | Draconematidae, Desmodorida | Rho, et al. [272] |
| Draconema cephalatum | Draconematidae, Desmodorida | Steiner [21] |
| Draconema micoletzkyi | Draconematidae, Desmodorida | Kreis [83] |
| Perepsilonema conifera | Epsilonematidae, Desmodorida | Adams, et al. [35] |
| Polkepsilonema mombasae | Epsilonematidae, Desmodorida | Verschelde, et al. [110] |
| Desmolaimus zeelandicus | Linhomoeidae, Monhysterida | Meldal [81] |
| Thalassomonhystera diplops | Monhysteridae, Monhysterida | Cobb [73] |
| Diplopeltula striolata | Diplopeltidae, Araeolaimida | Blome, et al. [164] |
Table 3: All caudal glands anterior to anus/cloaca in the outgroup of Enoplida.
Change in the Number of Caudal Glands
Additions As discussed earlier, a special cell associated with the three gland cells could either be a releasing cell or a pseudocoelomocyte. However, in many cases, additional cells cannot be distinguished from caudal glands. In such instances, the multiplication of gland cells may have occurred, resulting in a total of four to six cells.
Four gland-like cells have been reported in the Enoplida species Bathylaimus arthropappus and Tripyloides imitans, as well as in Linhomoeus spp. (Monhysterida), Aphanolaimus aquaticus, and various Plectus species (Plectida) (Table 1). In Steineridora (= Spilophora) loricata (Chromadoridae), Steiner depicted four gland cells located precaudally on the left side, except for the third gland, which was positioned to the right of the intestine [116].
In the section on releasing glands, two supernumerary gland cells were noted in Caprionchulus, Eurystomina, and Phanodermopsis. Additionally, a series of unidentified cells potentially gland cells or pseudocoelomocytes have been documented in the Mononchida species Miconchus studeri [166] and Mylonchulus obtusicaudatus [167], as well as in Triplonchida species such as Eutobrilus [168, 169] and Tobriloides [170, 171]. Similar features were observed in Anticyclus exilis (Monhysterida) [90].
Notably, species of Thalassogenus (Oncholaimidae) exhibit six caudal glands arranged in two sets of three in tandem (Figure 2D), each with a separate duct leading to a large common ampulla [172, 173]. This configuration suggests a duplication of the entire gland complex in the stem species of Thalassogenus.
Reduction in the number of gland cells While increases in gland cell numbers are rare, reductions have occurred many times independently. In several lineages, one gland has been lost, leaving only two gland cells. This phenomenon has been documented in approximately 40 species across Chromadorida, Desmodorida, Enoplida, Monhysterida, Mononchida, and Triplonchida (Table 4), where closely related species typically possess three glands.
If these reports are accurate, the complete reduction of one gland cell would have occurred at least 24-27 times independently, though it is likely to have happened even more frequently.
| species name | group | author(s) |
|---|---|---|
| Pandolaimus spp. | Pandolaimidae, Triplonchida | Lorenzen [23] |
| Rhabdodemania major | Rhabdodemaniidae, Triplonchida | Naveen, et al. [273] |
| Rhabdodemania minor | Rhabdodemaniidae, Triplonchida | Murphy, et al. [274] |
| Anticoma arctica | Anticomidae, Enoplida | Steiner [116] |
| Trileptium sp. | Thoracostomopsidae, Enoplida | Smol, et al. [195] |
| Miconchus japonicus | Anatonchidae, Mononchida | Ahmad, et al. [238] |
| Cryptonchus sp. | Cryptonchidae, Mononchida | Lorenzen [23] |
| Prionchulus japonicus, P. punctatus | Mononchidae, Mononchida | Ahmad, et al. [238] |
| Chromadorita heterophya | Chromadoridae, Chromadorida | Allgén [275] |
| Dichromadora geophila | Chromadoridae, Chromadorida | Schiemer, et al. [38] |
| Latronema orcinum | Selachinematidae, Chromadorida | Blome [276] |
| Eubostrichus hopperi (?) | Stilbonematinae, Desmodorida | Armenteros, et al. [12] |
| Laxus parvum (?) | Stilbonematinae, Desmodorida | Armenteros, et al. [12] |
| Monhystrella hastata (?) | Monhysteridae, Monhysterida | Abebe, et al. [60] |
| Monhystrium transitans | Monhysteridae, Monhysterida | Cobb [90] |
| Monhystrium aff. transitans | Monhysteridae, Monhysterida | Riemann [30] |
| Thalassomonhystera bathislandica | Monhysteridae, Monhysterida | Riemann [277] |
| Thalassomonhystera oxycephalata | Monhysteridae, Monhysterida | Tchesunov, et al. [267] |
| Tripylium carcinicola | Monhysteridae, Monhysterida | Riemann [30] |
| Astomonema otti | Siphonolaimidae, Monhysterida | Vidakovic, et al. [278] |
| Sphaerolaimus gracilis | Sphaerolaimidae, Monhysterida | Turpeenniemi, et al. [36] |
| Sphaerolaimus macrocirculus | Sphaerolaimidae, Monhysterida | Rosli, et al. [279] |
| Sphaerolaimus occidentalis | Sphaerolaimidae, Monhysterida | Turpeenniemi [85] |
| Subsphaerolaimus brevicauda | Sphaerolaimidae, Monhysterida | Jensen [266] |
| Daptonema trabeculosum | Xyalidae, Monhysterida | Lorenzen [57] |
| Echinotheristus cimbricus, E. teutonicus | Xyalidae, Monhysterida | von Thun, et al. [101] |
| Gnomoxyala spina | Xyalidae, Monhysterida | Lorenzen [57] |
| Pseudechinotheristus nudus | Xyalidae, Monhysterida | Blome [269] |
| Amphimonhystera marisalbi | Xyalidae, Monhysterida | Tchesunov, et al. [280] |
| Paramonohystera concinna | Xyalidae, Monhysterida | Lorenzen [57] |
| Rhynchonema amakusanum | Xyalidae, Monhysterida | Bezerra, et al. [137] |
| Rhynchonema fossum, R. lyngei, R. megamphidum, R. ornatum, R. semiserratum, R. separatum, R. tremendum | Xyalidae, Monhysterida | Lorenzen [102] |
| Theristus acer | Xyalidae, Monhysterida | Lorenzen [57] |
| Theristus flevensis, T. pertenuis | Xyalidae, Monhysterida | Lorenzen [57] |
Table 4: Only two caudal glands stated.
The literature contains depictions of only two caudal glands in approximately 100 additional species across 22 family-level taxa, though without explicit mention of the observed number. This raises uncertainty as to whether these species truly possess only two glands or whether some illustrations were merely schematic, omitting a third gland. Cases were excluded where only two glands were depicted in illustrations but three were stated in the text. Accurately determining the number of glands is challenging, as authors sometimes correctly note that only two caudal glands were “visible.” In some species, two posterior gland cells may be positioned parallel, appearing as a single structure in lateral view. Additionally, one gland may be obscured and thus overlooked, as mentioned for various species of Monhysterida [169].
In Monhysterida, it is evident that the most anterior and presumably always dorsal gland cell (cg1) has been reduced in size. In other groups, we have not attempted a comparative analysis with closely related species possessing three gland cells to determine whether a different gland was lost in certain species. This would be particularly relevant in Chromadorida species, where the glands have an inverse orientation.
The reduction to a single gland cell, as discussed in the section on caudal gland function, has occurred more than five times independently. There is no indication that this reduction happened in a stepwise manner, with an intermediate stage of two retained cells. Moreover, the exact reasons for the complete loss of one or two gland cells remain unclear.
Processes of Reduction of Caudal Glands
Degenerative evolution leading to the loss of caudal glands has been observed across multiple taxa. In various populations of Plectus species, researchers have identified aberrant specimens lacking caudal glands and a spinneret [108]. Similar observations have been made in Chiloplectus andrassyi [174].
Within Mononchida, the complete reduction of caudal glands has occurred across numerous lineages, with several indications of ongoing degeneration. For instance, species of Clarkus, Coomansus, and Prionchulus exhibit vestigial tail glands that lack an orifice [175]. Similar conditions have been reported in species of Crassibucca, Iotonchus, Jensenonchus, and Mulveyellus [176]. In species descriptions, terms such as “caudal glands weakly developed” or “reduced” are frequently used, but these should be qualified with descriptors like “partially” or “entirely.” In some cases, these terms indicate a process of gradual reduction. Table 5 presents 38 such instances. Transitional stages are evident in species where caudal glands are mostly absent or vestigial in only a few specimens, such as Clarkus papillatus [177] and Margaronchulus adenticulatus [178].
| species | caudal glands | terminal opening | author(s) |
|---|---|---|---|
| Clarkus enigmatus | indistinct | lacking | Ahmad, et al. [238] |
| Clarkus papillatus | absent, rudimentary in one specimen | absent | Tahseen, et al. [177] |
| Clarkus parvus | rudimentary | absent | Rizvi [282] |
| Clarkus pulcherrimus | rudimentary | absent | Andrássy [178] |
| Clarkus sheri | weakly developed | lacking | Khan, et al. [283] |
| Comiconchus trionchus | rudimentary | absent | Loof [284] |
| Coomansus garhwalicus, C. minor | rudimentary | absent | Rizvi [282] |
| Coomansus intestinus, C. silvius, C. splendidus | reduced | absent | Andrássy [285] |
| Coomansus magellanicus, C. meridionalis | absent in female, reduced in male | absent in female, present in male | Guirado, et al. [179] |
| Coomansus (= Mononchus) mesadenus | present, reduced | present in male, not in female | Clark [182] |
| Granonchulus decurrens | reduced | absent | Andrássy [286] |
| Iotonchus arcuatus | very weakly developed | absent | Ahmad, et al. [238] |
| Iotonchus cuticaudatus | present | present in female, absent in male | Jana, et al. [181] |
| Iotonchus darreni, I. singaporensis | weakly developed | weakly developed | Ahmad, et al. [237] |
| Iotonchus indicus | poorly developed | zpresent | Rizvi [282] |
| Iotonchus maragnus, l. parazschokkei | present | absent | Clark [287] |
| Iotonchus sphagni | rudimentary or absent | absent | Loof [284] |
| lotonchus stockdilli | non-functional | not distinct | Yeates [288] |
Table 5: Remnants of caudal glands or associated structures in Mononchida.
- Jensenonchus sp. rudimentary absent
- Andrássy [176]
- Margaronchulus adenticulatus rudimentary mostly absent, weak if
- Miconchus japonicus poorly developed absent
- Ahmad, et al. [238]
- Mononchus papillatus present absent
- Clark [290]
- Mylonchulus caespitosus inconspicuous absent
- Andrássy [286]
- Mylonchulus psammophilus apparently vestigial absent
- Yeates [291]
- Mylonchulus striatus obscure rudimentary de Bruin [292]
- Parahadronchus egregius reduced lacking
- Andrássy [176]
- Prionchulus iranicus present absent
- Farahmand, et al.
- Prionchulus japonicus weakly developed lacking
- Ahmad, et al. [238]
- Prionchulus koriensis present absent
- Zakaullah, et al. [294]
- Prionchulus mordax reduced absent
- Andrássy [285]
- Prionchulus muscorum rudimentary or absent absent
- Tahseen, et al. [177]
- Sporonchulus ibitensis weakly developed narrow, inconspicuous
- Tahseen, et al. [177]
Table 6: Remnants of caudal glands or associated structures in Mononchida.
Of particular interest is the occurrence of sexual dimorphism, where caudal glands and their outlets are absent in females but present (albeit reduced) in males, as seen in Coomansus magellanicus and C. meridionalis [179]. The reverse situation is apparently observed in Prionchulus longus [180]. In other species, glands exist in both sexes, but the outlet is absent in one, such as in Iotonchus cuticaudatus (pore absent in males [181]) and Coomansus (= Mononchus) mesadenus (pore absent in females [182]). Ontogenetic differences have also been noted, with caudal glands being distinct in juveniles but obscure in adults, as reported in Anatonchus tridentatus and Miconchus digiturus [167, 183]. These observations suggest that, once adhesive secretion became unnecessary in evolutionary history, the initial step in reduction involved the closure of the terminal opening. Some instances of caudal gland cells persisting without an outlet suggest they may have acquired an accessory internal function. This is also observed in Dorylaimida, where remnants of caudal glands were reported in Lenonchium fimbricaudatum (Fig. 10A) [104].
The degeneration of caudal glands has also been documented in marine Enoplida. In Oncholaimidae, Kreis reported rudimentary caudal glands in Pseudoncholaimus elegans, while in Pontonema (= Paroncholaimus) ocellatum, females lacked caudal glands altogether, whereas males retained a single precloacal gland cell [162]. In Nemacoma (= Acoma) borealis (Oxystominidae), Steiner described a small residual bubble-shaped structure without a terminal opening [116]. Among Ironidae, Trissonchulus baldwini possesses weakly developed or rudimentary caudal glands and lacks a spinneret [184], whereas Parironus keiensis exhibits “vestigial or absent” caudal glands [185].
In Triplonchida (Onchulinae), several species exhibit caudal gland cells without a spinneret, including Onchulus flagellatus [171] and Caprionchulus diversipapillatus, where four to five tandem glandular cells persist without a spinneret, though remnants of a tubule are sometimes observed [45]. Similarly, in Monhystera cartaromanae (Monhysterida), caudal glands are in the process of degeneration, with no indication of a spinneret [186]. A comparable condition is found in Mediolaimus obtusicaudatus (Araeolaimida), where the spinneret is absent and caudal glands appear obscure [187], as well as in Leptolaimoides filicaudatus (Plectida) [188].
Unlike in Teratocephalida and Secernentea, vestigial caudal glands appear to be recapitulated in early stages of zooparasitic Benthimermithida and Marimermithida, which usually lack such glands. In Benthimermitidae, poorly developed caudal glands and a terminal pore are present in early juveniles of Trophomera conchicola [8], while in parasitic juveniles of Trophomera granovitchi, a “very thin terminal duct, probably a rudiment of a spinneret” has been observed [189].
Similarly, in Marimermithida, juveniles possess three caudal glands posterior to the anus and a terminal pore, whereas adults are entirely devoid of these structures [190]. The function of these vestigial glands in juveniles remains unknown.
Entire Loss of Caudal Glands
In terrestrial and zooparasitic nematodes, “there is no need for such glutinous glands” [191], leading to their loss in many lineages. Among parasitic groups within “Adenophorea,” this occurred independently in Dioctophymatida/Trichinellida and Mermithida. However, an exception exists in Domorganus (Ohridiidae, Plectida), a parasite in annelid intestines, which retains three caudal glands and a spinneret, likely for attachment to the host’s intestinal wall [192]. The parasitic Benthimermithida and Marimermithida exhibit vestigial caudal glands in juveniles. Their absence in zooparasitic Secernentea is attributed to loss in the stem species of Teratocephalida/Secernentea, which likely inhabited terrestrial environments.
Throughout evolution, many primarily marine nematode lineages transitioned to terrestrial habitats via moist soil. During this ecological adaptation, caudal glands frequently disappeared as they were no longer selectively advantageous. The presence and reduction of caudal glands across different groups, as inferred from phylogenetic relationships (Fig. 4), are discussed below.
Triplonchida In a clade including Bastianiidae, Diphtherophoridae, Odontolaimus, Prismatolaimus, and Trichodoridae, caudal glands are absent [193], despite contrary claims in older literature regarding Bastiania and Prismatolaimus. Nearly all species in this clade inhabit soil, indicating that caudal glands were lost in its stem species in adaptation to terrestrial life. Conversely, the predominantly aquatic sister group retained these glands.
The Onchulidae lineage also supports this trend. While its stem species likely lived in moist soil, caudal glands were retained in Tobriloides/Tobrilonchus and were undergoing degeneration in Caprionchulus and Onchulus. In the Onchulinae lineage (e.g., Kinonchulus, Limonchulus, Stenonchulus, Pseudonchulus inermis), complete gland loss occurred [194]. The presence of vestigial glands in Caprionchulus diversipapillatus, which presumably secondarily invaded freshwater, suggests no reversal to functional glands.
Enoplida The transition to terrestrial habitats within Enoplida led to caudal gland loss in the Alaimina lineage (Alaimus, Amphidelus, etc.). Similar reductions occurred in terrestrial species like Arenasoma terricola (Tripyloididae) and Campydora demonstrans [195]. Even within marine environments, some species lost caudal glands (Table 6).
| species | group | habitat | author(s) |
|---|---|---|---|
| Trissonchulus quinquepapillatus | Ironidae, Enoplida | dune sand | Yeates [291] |
| Platycoma cephalata (?) | Leptosomatidae, Enoplida | marine | Cobb [73] |
| Proplatycoma (= Platycoma) sudafricana | Leptosomatidae, Enoplida | marine | Inglis [295] |
| Proplatycoma tsukubae | Leptosomatidae. Enoplida | marine | Shimada, et al. [296] |
| Filoncholaimus prolatus | Oncholaimidae, Enoplida | marine | Neres [297] |
| Oncholaimus rapax | Oncholaimidae, Enoplida | marine | Kreis [162] |
| Phylloncholaimus immanis | Oncholaimidae, Enoplida | marine | Smol, et al. [195] |
| Halalaimus (= Halalaimoides) acuminatus | Oxystominidae, Enoplida | marine | Wieser [298] |
| Klugea morchella | Phanodermatidae, Enoplida | marine, soft bottom | Wieser [298] |
| Rhabdocoma (= Cytolaimium) obtusicaudata | Trefusiidae, Enoplida | marine | Chitwood [299] |
| Cryptonchus (= Gymnolaimus) exilis | Cryptonchidae, Mononchida | soil | Cobb [66] |
| Anticyathus tenuicaudatus | Linhomoeidae, Monhysterida | marine, sand | Cobb [90] |
| Didelta maculatum | Linhomoeidae, Monhysterida | marine, sea-grass | Cobb [90] |
| Zanema acanthurum | Linhomoeidae, Monhysterida | marine, mud, tide pool | Cobb [90] |
| Monhystera dintheriana | Monhysteridae, Monhysterida | soil | Schneider [249] |
| Siphonolaimus (= Chromagaster) nigricans, S. (= C.) purpureus | Siphonolaimidae, Monhysterida | marine, sand | Cobb [73] |
- Solenolaimus obtusus
- Siphonolaimidae, Monhysterida marine, sand
- Cobb [73]
- Hofmaenneria hazenensis
- Xyalidae, Monhysterida soil
- Mulvey [300]
- Rhynchonema deconincki
- Xyalidae, Monhysterida marine, sand
- Bezerra, et al. [137]
- Theristus aculeatus
- Xyalidae, Monhysterida marine, sand
- Schulz [301]
- Edalonema camerunense
- Diplopeltidae, Araeolaimida soil
- Holovachov, et al.
- Leptolaimoides thermastris
- Leptolaimidae, Plectida silt of salt-marshes
- Lorenzen [23]
- Leptolaimus acicula
- Leptolaimidae, Plectida silt of salt-marshes
- Lorenzen [23]
- Pakira orae
- Plectidae, Plectida dune sand
- Yeates [291]
- Plectus amorphotelus, P. spicacaudatus
- Plectidae, Plectida soil
- Ebsary [302]
- Iotalaimus striatus incertae sedis soil
- Cobb [90]
- Litonema nudum incertae sedis sphagnum swamp
- Cobb [90]
Table 8: Loss of caudal glands in individual species.
This trend extends to limnic Andrassyidae (Andrassya, Malakhovia) [196] and independently within several marine and limnic Ironidae genera (Dolicholaimus, Ironus, Parironus, Pheronous) (various authors). Marine Enchelidiidae also show independent gland loss in Abelbolla [161], Bathyeurystomina, Pareurystomina, and Megeurystomina combesi [195]. These multiple losses in closely related taxa highlight the need for further ecological studies to determine the selective pressures behind caudal gland disappearance in aquatic environments.
Dorylaimida The lineage leading to Dorylaimida experienced caudal gland reduction or loss, while closely related taxa (Bathyodontus, Cryptonchus, Mononchulus, Oionchus) retained them, despite including soil-dwelling species. Lenonchium fimbricaudatum exhibits three gland-like cells (Figure 10A) without duct openings [197], suggesting a stepwise reduction in the ancestral lineage of Dorylaimida til vestiges in the stem species. Dorylaims lacking caudal glands later reinvaded aquatic environments multiple times.
Mononchida This group demonstrates the most extensive loss of caudal glands, likely reflecting multiple independent reductions in terrestrial habitats. Caudal glands are absent in ten genera (e.g., Caputonchus, Doronchus, Prionchulellus, Tricaenonchus) [175, 198]. Other genera (e.g., Clarkus, Coomansus, Granonchulus, Mononchus, Prionchulus and 15 more genera) include both species with and without caudal glands, highlighting an ongoing reduction process (various authors). Given their presumed monophyly, caudal glands appear to have been lost independently in about thirty lineages. It is hypothesised that freshwater mononchids lacking caudal glands secondarily invaded aquatic habitats after terrestrial adaptation (compare [199]).
Monhysterida In this order-taxon, nine species (seven marine, two terrestrial) lack caudal glands (Table 6), indicating multiple independent loss events.
Araeolaimida Terrestrial species such as Edalonema camerunense [200] and closely related Aulolaimidae and Isolaimium lack caudal glands [7]. As these taxa are predominantly terrestrial, their ancestor likely lost caudal glands before a few species secondarily invaded aquatic habitats.
Plectida Many terrestrial taxa within this order retain caudal glands, but they are absent in Pakira orae and several Plectus species [201]. Chronogaster species or their common ancestor lost caudal glands except for C. boettgeri, the only species with glands and a spinneret [108]. As terrestrial Metateratocephalidae lack caudal glands, we hypothesise the loss in its stem species in a terrestrial environment. Consequently, living in freshwater habitats in some Euteratocephalus species is secondary. Additionally, some marine and brackish species of Leptolaimus (e.g., L. acicula) and Leptolaimoides lack caudal glands [201].
Conclusively, a total of nearly 80 documented cases of caudal gland loss have been identified, with approximately 40% occurring in aquatic environments. This challenges the assumption that gland loss primarily occurs in terrestrial lineages. Further research is needed to understand why caudal glands became redundant in marine Enoplida and Monhysterida. Conversely, the persistence of caudal glands in some terrestrial lineages requires investigation. The
presence of limnic and terrestrial species within caudal- gland-lacking groups supports the hypothesis that aquatic taxa in these groups passed through a terrestrial phase (e.g., Alaimidae, Chronogaster, Euteratocephalus, Mononchida, Odontolaimus, Prismatolaimus). Notably, various taxa that lost caudal glands evolved alternative tail structures for attachment, such as mucros, spikes, and hooks. Furthermore, certain nematodes adhere to substrates using tail haptics without glands or anchoring structures, as seen in Rhabditida (own observations) and infective juveniles of parasitic Ascaridoidea (Pseudoterranova decipiens) or Spiruroidea (Spiroxys contortus) [202].
On The Unsettled Problem of the Origin of Phasmids
Steiner [2] emphasized the need for a comparative study of the distribution, morphology, and function of foot glands and phasmids, as well as their reduction and disappearance across various nematode groups. It has been observed that caudal glands are absent whenever phasmids are present and vice versa [28]. These glandular structures in the tail serve as key distinguishing features between the two main groups of Nematoda: the monophyletic Secernentea, which predominantly possess laterally opening phasmids and lack caudal glands, and the paraphyletic “Adenophorea,” where caudal glands with terminal outlets are generally present and phasmids are missing. Three caudal glands are considered plesiomorphic in nematodes, inherited from ancestral species. Phasmids, on the other hand, are paired structures connected to the nervous system and primarily serve a chemosensory function, with glandular activity being rare in extant species [203]. Given the mutually exclusive presence of caudal glands and phasmids, it has been suggested that these structures might be homologous. Steiner [2] hypothesised that phasmidial glands are modified, functionally and numerically reduced caudal glands with laterally shifted outlets. Osche [204] also supported their homology, while Chitwood [47] considered this as possible. However, the Chitwoods (p. 197) were more cautious, recognizing the origin of phasmids as “one of the most perplexing problems in nematology” and considering multiple possibilities for their evolutionary derivation [27].
The transformation of caudal glands with separate outlets into phasmids would theoretically require at least five evolutionary steps: (1) the loss of one of the three primary caudal glands (possibly the dorsal one), (2) a positional shift of the remaining two glands from a subventral to a lateral arrangement, (3) relocation of glandular pores from the tail terminus to the anterior third of the tail (the common position of phasmids), (4) integration of these glands with ciliary structures connected to nerves, resulting in chemosensory function, and (5) a functional shift from adhesive secretion to sensory reception.
A potential example of this transformation can be found in the parasitic Muspiceida, formerly classified within “Adenophorea” [96]. The second-stage larvae of these nematodes are typically ensheathed [205], a trait characteristic of Secernentea. Muspiceida possesses two ampulla-shaped glands that open laterally at approximately 25% of the tail length. These structures were initially identified as phasmids [206], but due to the supposed systematic placement of Muspiceida and the absence of associated neural tissue, they were later reclassified as caudal glands [207]. However, molecular evidence placed Haycocknema perplexum (Muspiceida) within the parasitic Spiroascarida of Secernentea [208], supporting the identification of these glands as phasmids rather than caudal glands. A similar misidentification may have occurred in Breinlia ventricola (Filarioidea), where terminally opening “caudal glands” [209] are more accurately recognized as phasmids.
Examining the “perplexing” situation described by Chitwood, et al. [27], several key points must be considered:
In Secernentea with three-pointed tails the protuberances representing ancestral caudal glands occasionally occur alongside phasmids, indicating that these structures are distinct.
Kaiser described “phasmid-like organs” associated with the nervous system in the mermithid Hexamermis albicans (“Adenophorea”, Fig. 4) located subventrally in the posterior third of the tail in males and at 35% of tail length in preparasitic juveniles [210]. Similar structures were observed in Agamermis decaudata [211], Amphimermis elegans, Hexamermis brevis [210], and several Psammomermis species [212]. However, the homology of these structures with phasmids remains unconfirmed.
Schrage & Gerlach noted lateral pores associated with glands on the ending of the tail in Desmoscolecida (Greeffiellinae), which coexist with caudal glands opening through a spinneret [213]. Chitwood referred to these structures as “phasmata” [47], which also exist in Syringolaimus (Enoplida). The homology of phasmata and phasmids remains uncertain pending evidence of a nervous system connection [214].
Some Mononchida and Chromadorida possess lateral tail pores associated with unicellular epidermal glands, which might be mistaken for phasmids.
In Ascaridoidea (Paranisakis parva) (Secernentea), six tubular gland-like cells extend from rectum level to near the tail tip [115, 123], possibly representing vestigial caudal glands without an external outlet.
The possible homology of two releasing cells with phasmids never seems to be considered.
Hyman’s [28] assertion remains valid: there is no confirmed case of caudal glands and phasmids coexisting in the same organism. The observations discussed suggest that these structures are not homologous but rather coexisted in ancestral nematodes, as proposed by Lorenzen [23]. Under this scenario, caudal glands were lost in lineages where phasmids were present, such as Secernentea and possibly Mermithida, while phasmids were lost in more than thirteen “Adenophorean” lineages that retained caudal glands (Figure 4). However, given the essential role of phasmids in nematode orientation [203], the idea that they were lost multiple times remains questionable. Future studies using transmission electron microscopy are needed to investigate phasmata and phasmid-like structures in mermithids and to reassess their potential homology with phasmids.
Tail Shape and Structures at the Terminal End
Nematodes tail shape varies highly according to the way of living and mode of locomotion of the species. However, caudal glands filling the tail should also affect tail morphology, which represents a compromise between ecological and organismal demands shaping the tail. Although it is beyond the scope of this paper to investigate the relationship between caudal glands and tail shape, it appears worthwhile to compare closely related species of a guild that have caudal glands present or absent, as in Mononchida, or that are incaudal or shifted to a precaudal position, as in Enoplida. We limit ourselves to a few remarks without presenting all the evidence.
The nematodes with three outlets mostly possess a cylindrical tail with a rounded tip. It conforms to tail type 1 of Thistle, et al. [215]. Thus, we can speculate that the stem species of nematodes exhibiting separate gland orifices do not possess an elongated or filiform tail that has been considered plesiomorphic by Decraemer, et al. [216]. Possibly the stem species had three protuberances for the outlets at the blunt end like they exist in Pararaeolaimus nudus [99] or Triepsilonema tripapillata (Fig. 3D) [100].
Very often there exists a condition of caudal glands and a club-shaped tail with a swollen tip bearing an ampulla and a spinneret valve, corresponding with tail type 2 of Thistle, et al. [215]. In a few cases, a swollen terminal end exists (secondarily in our view) also in species with separate orifices (Amphimonhystrella bullacauda, Sphaerolaimus gracilis). In the glandless Secernentea, such tails are rare.
It seems that when the tail has a swollen terminal end there is usually a simple pore and not a tubular spinneret. A swollen tail tip characterises many species of Araeolaimida, Enoplida, Triplonchida, and single species from other groups, while a tapering tail with an offset spinneret tube on a finely rounded terminus is found in many taxa of Chromadorida, Desmodorida, Plectida and others. Exceptionally species of Tobrilus (Triplonchida) [217] and Chromadorida like Marylynnia puncticaudata [218] or Synonchiella hopperi [219] as well as Plectida like Halaphanolaimus rivalis [220], Paraplectonema loofi [221] and some species of Paraphanolaimus [222] have both a terminal swelling and a short protruding spinneret.
Usually, the pore of the glands or the adhesive tube, which became set-off in multiple lines, is terminal to the conical, filiform or clavate tail tip. However, there are many exceptions where the orifice is subterminal. Within Mononchida with a rounded or digitate tail end, but also those with a filiform tail, often the pore has been shifted slightly subdorsally or subventrally. Also, in a few other taxa the pore occupies a somewhat subterminal position. After species descriptions of various authors, the opening is shifted (1) ventrally within genera of Mononchida (Anatonchus, Clarkus, Coomansus, Granonchulus, Iotonchus, Mylonchulus, Parahadronchus, Sporonchulus), Triplonchida (Rhabdodemania), Enoplida (Adoncholaimus, Dolicholaimus, Leptosomatum, Litinium, Metoncholaimus, Mononchulus, Paraleptosomatides, Symplocostoma, Tripyla, Trissonchulus), Chromadorida (Chromadorissa) and Monhysterida (Steineria), respectively (2) dorsally within genus-taxa of Mononchida (Bathyodontus, Cobbonchus, Iotonchus, Mononchus, Mylonchulus, Oionchus, Parahadronchus), Enoplida (Dolicholaimus, Trissonchulus, Viscosia), Chromadorida (Chromadorita, Spilophorella), Desmodorida (Spirinia, Zalonema) and perhaps within a few further groups. The Mononchida presents plenty of material to examine the functional context for a terminal respectively a subterminal opening.
As a rule, the reduction of caudal glands and loss of a spinneret apparatus offers the organismic licence for a sharply pointed tail tip (like in several lineages within Mononchida) or for an almost flagellate tail (like in the Enoplida Bathyeurystomina, Filoncholaimus or Parironus). Otherwise, examples like Trefusia (Enoplida) or Terschellingia (Monhysterida) show that caudal glands with extremely long ducts can be compatible with very long filiform tails, though these are exceptions. Very often and in various taxa the spinneret is flanked by two to six short terminal setae (rarely longer than 9 μm) which should play a sensory role in the attachment process. The spinneret tube itself bears rather small setae in Chromadorida e.g., species of Ethmolaimus [223], Chromadorina, and Punctodora [33]. In Pseudocella species (Leptosomatidae) very tiny setae irregularly encircle the gland pore [224]. Observations of small papillae associated with the spinneret are remarkable. Meyl [94] described two short papillae laterally of the spinneret in Deontolaimus papillatus (Camacolaimidae) (Fig. 10C). In Domorganus macronephriticus (Plectida) tiny processes were visible around the spinneret [225] as well as “setiform projections” in some species of Caribplectus [92] and irregularly arranged setose papillae on the spinneret tube of Plectus magadani [226]. At the opening of the caudal glands of Miconchus bulbicaudatus (Mononchida) the tail tip shows a bulbous outgrowth with knob-like papillae encircling its base [227]. At first SEM studies have indicated that in Tobrilus sp. (Triplonchida) as well as in Plectus zelli (Plectida) the base of spinneret tube was surrounded by ten minute, about 1 µm long papillae [228, 229]. Though of the same number, these papillae between very distantly related taxa cannot be homologous. These few examples vouch for a comparative SEM study of the tail terminus of nematodes.
In rare cases, the method of anchoring with aid of adhesive glands is mechanically supplemented by certain structures. This way we interpret the about 10 µm long appendages of a bifurcated tail tip in some species of the enoplid Halalaimus [230], a crooked spinneret in few species of Plectus [226], the two small thorns subventral on the spinneret tube of Plectus decens and about four prongs at the tail tip in Caribplectus magdalenae (Fig. 10G) [108]. By this means secondary anchor structures possibly could replace stepwise the adhesive glands in lineages finally resulting in the loss of caudal glands. However, we have no indications that such steady substitutions occurred, possibly within Chronogaster (also considered by Holovachov O [69]). So far, we can see, at all times caudal glands were lost first and special terminal structures for anchoring with the tail end like mucros, hooks, three spines etc. evolved later. Because they originated again and again - especially in several lineages of Secernentea – there exists a large variety of such structures, worth to be documented and analysed comparatively.
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