A Note on Occurrence of Four Sea Cucumbers (Echinodermata: Holothuriidae) from Vellar Estuary, Southeast of India

Introduction

Almost 1400 holothurian species were reported from various parts of World. In Indian waters, about 200 species which belongs to 6 orders and 16 families are recognized. Holothurians are primarily distributed in the Indian coastal states in scattered numbers as well as prominent resources along the Andaman and Nicobar Islands, Lakshadweep, the Gulf of Kutch and around 39 species reported from the Gulf of Mannar and Palk Bay [1]. One of the most important non-finfish sources of income for coastal populations in the Indian Ocean, Southeast Asia, and the Pacific is sea cucumber fishing. Over the past two to three decades, sea cucumber fisheries have increased in capture and value around the world [2, 3, 4]. Overexploitation of this resource is a result of poor fishing management and certain ecological characteristics. Sea cucumbers play a pivotal role in sustaining the marine ecosystem by effectively consuming decomposing organic matter and subsequently converting it into usable and recyclable nutrients, which ultimately benefit other forms of marine life [5]. The feeding and excretion of sea cucumbers also contribute to the elevation of the alkalinity of the seawater, thus protecting the ocean from the effects of acidification. James DB [6, 7, 8, 9] has given for the first time an account of the holothurian resources of India chiefly based on the intensive surveys along the Gulf of Mannar and Palk Bay and the Andaman and Nicobar Islands.

The estuarine oyster beds have acted as a dam on the strong current along the river shoreline, providing some protection against coastal erosion and creating a shallow bay with elevated sediment levels [10].

The juveniles of the species were found in an intertidal zone, which is characterized by its abundance of algae and sparse seagrass beds. It is possible that sea cucumbers may appear to congregate in shallow grass beds prior to migrating to deeper water as they reach sexual age. During our survey on estuarine faunal diversity [2, 5, 11, 12, 13, 14] of Tamil Nadu coast, we recorded the four species of sea cucumbers for first time from oyster and seagrass beds of the Vellar estuary, Southeast India.

Material and methods

Weekly field survey on intertidal estuarine faunal

composition was conducted at Vellar estuary, Southeast India over a period of 16 months, from April 2022 to August 2023. Faunal surveys were conducted during the low and inter-tide. Visual surveys were used to describe different biotopes of the estuary and composition of substrate in terms of percent of sand, small stones, seagrasses, algae, rocks, creeks, dead and live oysters.

Discussion

This report is the first attempt to confirm the sea cucumber species in the mud flats of Vellar Estuary. The nature of bottom sediment composition is the most important factor which determine the sea cucumber habitat selection [12]. It has been noted that sea cucumber habitat preferences vary between species and even during life stages within the same species [12].

Holothurians are often referred to as the sea earthworms. They are responsible for the large-scale movement and mixing of the substrate and the recycling of the detrital mass into animal tissue as well as nitrogenous waste that can be digested by algae or sea grass [12, 15, 16]. The mudflats are the most suitable habitat for sea cucumbers due to the fact that they feed on detritus and consume organic matter in the mud [12, 15].

Based on the perception of Vellar estuary clam or oyster hand pickers and inland fishers divulges that the status of sea cucumber population has extremely sparse in estuaries as well as along the coromandel coast as they are ubiquitous only along the Gulf of Mannar and Palk Bay region [13]. The inland fishers of Vellar estuary are aware that the fishing of holothuroids is illegal and punishable in India, and therefore they are engaged in sustainable fishing of molluscs, crustaceans and other finfishes [10].

Raghunathan C, et al. [13] reviewed echinoderms from India and concluded that there are nearly 200 species of holothurians in the seas around India. This occurrence of holothurians in Vellar estuary reported in this study might be incidental, possibly introduced through salinity variations, clarified by the proximity of high salinities in the sampling location during the summer and pre monsoon seasons.

Conclusion

India is home to a population of approximately 200 species of sea cucumbers. All of these species are protected from collection, trade, or any form of exploitation under the Wildlife Protection Act, 1972. Furthermore, above two species, Holothuria fuscogilva and H. nobilis, have been listed in Appendix II to CITES since 2023, which governs their international trade.

Distribution of sea cucumbers in estuarine muds flats is rare and the study we have reported the occurrence of four holothurian in Vellar estuary might be due to the favourable environment conditions. Possibly, the present report of four holothurians in Vellar estuary confirms and expands its range towards the estuaries of coromandel coast of India. We suggest further studies required for better understanding of ecology and conservation of sea cucumbers in estuarine contexts.

Acknowledgements

The authors are thankful to the Director and Dean of the Faculty of Marine Sciences, Annamalai University, for providing necessary facilities.

Conflict of Interest

All the authors confirmed that the content of this manuscript has no conflict of interest.