NEW DISTRIBUTIONAL RECORDS OF MOLLUSCA FROM SUNDERBAN BIOSPHERE RESERVE, INDIA

a bstract : Sunderban Biosphere Reserve (SBR) is the largest existing mangrove ecosystem in the world harbouring 177 species of Mollusca. During several surveys in Sunderban Biosphere Reserve to record the faunal diversity, five molluscan species, namely Dosinia tumida (Gray, 1838), Babylonia spirata (Linnaeus, 1758), Babylonia zeylanica (Bruguière, 1789), Indothais gradata (Jonas, 1846), Architectonica perdix (Hinds, 1844) have been identified as new records from the region. Also, D. tumida and B. zeylanica have been identified as new for the faunal diversity of West Bengal. Another one, I. gradata is being recorded from India by this communication for the first time. The recorded molluscan diversity from Indian Sunderban and these present findings imply that malacofaunal study from SBR need to be reassessed for an updated malacofaunal diversity status for implementing better conservation tactics in SBR.


INTRODUCTION
India has a coastline of more than seven thousand kilometres and the heterogeneity of the eco-geography from this long coastal stretch has given a rich species diversity.The state of West Bengal alone covers a 157 km long coastline among the northern part of Bay of Bengal and it is one of the biogeography hotspots, Sunderban Biosphere Reserve (SBR) is a crown jewel for its species richness in this coastal estuarine region (danda et al. 2017).The oldest and largest existing single mangrove belt is Sunderban, recognised as one of the most productive ecosystems of the world covering an area of 4,264 km 2 in the Indian region.The confluence of river Brahmaputra and Ganga gave rise to numerous islands, creeks and riverine channels, thus forming the mangrove network of Sunderban Biosphere Reserve.Following the immense importance of this mangrove ecosystem, United Nations Educational, Scientific, and Cultural Organization (UNESCO) declared the Indian portion of Sunderban a World Heritage Site since 1987 and SBR has been designated its status as a biosphere reserve under the UNESCO Man and the Biosphere program in 2001.The diverse habitat of Sunderban encompasses a diverse landscape, from mudflats, sandy and muddy beaches, small estuarine channels to vast intertidal mangrove belt (chandra et al. 2017(chandra et al. , Fsi 2017)).
Molluscs are one of the most important group of invertebrates in terms of species richness, biomass and abundance covering almost every possible habitat except for the aerial (Kabir et al. 2014).Mangroves are one of the most productive ecosystems harbouring a large diversity of molluscs, and their species composition varies with several factors of the ecosystem such as exploitation, disturbance, and rehabilitation of mangrove trees (for the protection of mangrove belt along the coastline).Molluscs can thus be used as a major indicator to assess the ecosystem health (sKilleter & warren 2000).Interms of food chain roles and ecosystem services, molluscs are a major source of food for several birds and fishes; they act in conversion of organic detritus, which is one of the bases for carbon sequestration in a mangrove ecosystem.The algal component of estuaries, which is a major storage of carbon, forms part of carbon sink and carbon chain with the help of the molluscs feeding on these algal components (mitra et al. 2013).Several factors like habitat and shore line alteration, uncontrolled exploitation of natural resources, commercialisation of shells, pollution from various sources, are acting as destroyers of malacofaunal diversity of Sunderban, which could be a major factor in future for the survivability of Sunderban Biosphere Reserve (danda et al. 2017).
Research on the molluscs of lower Bengal started with the work of stoliczKa (1869), followed by study of brackish water molluscs of nevill (1880), and later contributions by annandale (1907,1922) laid the foundation for the study of molluscan fauna in this area.Presently 5,249 species of Mollusca have been reported from the Indian Subcontinent and a total of 301 species has been recorded from the coastal and estuarine part of the state of West Bengal (tudu et al. 2018a(tudu et al. , banerJee et al. 2022)).The last updated work from SBR by Zoological Survey of India and World Wildlife fund in 2017, recorded 177 molluscan species from this region (chandra et al. 2017, danda et al. 2017).Thus, this present work gives an account of five new molluscan records from SBR; three new records from the state of West Bengal and one new record from India.
After collection, live specimens were fixed in 10% formalin (in river/marine water) solution and later preserved in 70% ethanol for long term preservation.Shells were washed in fresh water after collection and dried in the sun.Identification and classification used here is based on rao (2003,2017) and WoRMS (http://www.marinespecies.org/).All length measurements were taken to nearest 0.1 mm using electronic callipers.Specimens are deposited in the National Zoological Collections of the Sunderban Regional Centre of the Zoological Survey of India.

Dosinia tumida (Gray, 1838)
Arthemis tumida gray 1838 Conservation Status Not Evaluated (IUCN 2022).Material examined: ZSI/SbRC/KN6455, 2 samples, total length (53-60 mm), Dhanchi Island, Sunderban Biosphere Reserve (21°37'43.96"N,88°26'48.31"E),12 March 2020, Coll: s. banerJee.Description.Shell is properly orbicular and maximum distance between ventral ligament and es-cutcheon is 5 mm.Escutcheon is narrow and its shell is smooth.Umbo is prominent and obliquely erected to the anterior end.Lunule is black coloured and shaped like a heart.Internal part of the shell is smooth and the muscle scar is almost the same size as the shell (Figs 3-4).Habitat.Only two samples were collected from the intertidal zone of a sandy beach near an estuarine zone.This beach is protected area of Sunderban and no human interference is observed in the area.The area of the island nearby is protected with mangrove vegetation.The present study marks the initial documentation of the species in West Bengal and within the Sunderban Biosphere Reserve.While this species is quite popular in Indian fisheries trade, with both Babylonia species being exported to Thailand, Singapore and Malaysia, it is also increasingly sold in India as whelk meat (mohan 2007).Both of the Babylonia species are quite common along India's eastern coast, and these shells are widely used in the region to make shell ornaments.Locals used to gather Babylonia species for this purpose on sandy beaches along the east coast, but these new records of B. spirata and B. zeylanica from Sunderban may reflect its low frequency of occurrence as it is an estuarine site.

Williams et D. Reid, 2013
Indothais gradata (Jonas, 1846) Purpura (Cuma) gradata Jonas 1846 (Original description not documented) Purpura trigona reeve 1846 Conservation Status: Not Evaluated (IUCN 2022).Material examined: ZSI/SbRC/KN6023, 3 specimens, total length (26.3 mm, 24.2 mm, 27.1 mm), Jamboo Island, Sunderban Biosphere Reserve (21°34'39.56"N,88°11'33.83"E),29 December 2019, Coll: A. sen.Description.The identified shells of Indothais gradata are highly variable in size and shape.The carina has a projection.The structure which separates the last whorl from the others is just below the carina of the second last whorl.The side of the last whorl is angled directly from the carina towards the anterior canal (30 degrees), creating a triangular-shaped aperture.Bottom of surface whorl is smooth.Spire convex shaped with small thorny ridges.Shell is slightly dark and characterised by the colour of the greyish internal aperture.Outer lip is thick and rounded.Siphonal tube is well-developed.Shell colouration is brownish white (Figs 9-10).Distribution.India -Maharashtra (questionable occurrence), Elsewhere -Thailand, Japan, Singapore, Malaysia, Japan, Vietnam, Australia (gbiF 2022b).Habitat.A live specimen has been collected from the southern part of Jamboo Island, which is in core area of SBR.No human activities are allowed in this region.The specimen was collected from a mudflat, the southern-side of the area is Bay of Bengal and mangrove vegetation covers the northern part of the area.The species was collected with a sample of Indothais lacera (Born, 1778), which has a wide distribution in SBR.Remarks.Globally 12 species are recorded under the Indothais genus, but only two were known from the Indian region, I. lacera and I. blanfordi (Melvill, 1893).Both the species are reported along the whole coastline of India (chandra et al. 2020).Both of these species are widely distributed in SBR (chandra et al. 2017(chandra et al. , tudu & balaKrishnan 2018)).I. lacera can be easily distinguished from other two species by the presence of strongly elevated shouldered body whorls, which is angulated without any elevation for other two.The colouration patterns of I. gradata and I. lacera are totally different.I. gradata has longitudinal rows of brownish bands over the whole shell whereas I. lacera has fine brownish vertical lines arranged in pair over the sculpture.I. gradata shell is quite thin and arrangement of denticles on the outer lip is inconspicuous compared to the other two.Studying the global occurrence pattern of I. gradata revealed that, this species has been mostly observed from the eastern, southeastern Indian Ocean and eastern Pacific Ocean.A single record of I. gradata has been registered in GBIF database from western Indian Ocean from Mumbai, India and from Bangladesh.The collection of the species is mentioned by the US Academy of Natural Sciences, and the Australian Museum.Other collection details and timing are not registered for the Indian specimen and as for the GBIF information source, the details of these collection has been flagged with questionable collection locality and institution details ("Continent

DISCUSSION AND CONCLUSION
The vast island network of Sunderban lies at the northernmost part of Bay of Bengal and SBR has always been a focus for biodiversity research and conservation due to the present threats and never-ending erosion of the mangrove ecosystem.Studies of malacofaunal diversity are vital in the quest to understand the mechanisms of different ecological processes in mangrove based estuarine ecosystems.While there are records from the late 18th century onwards, it is remarkable that more intensive surveys between 1900 and 2022 have revealed the presence of only 177 species within the SBR, considering that more than 5,000 species of Mollusca are recorded from India as a whole (tudu et al. 2018a(tudu et al. , 2018b(tudu et al. , banerJee et al. 2022)).
It follows that molluscs in the Sunderban are seriously under-recorded.The inaccessibility of the core area, and a focus on common and easily recognised species no doubt account for this in part.The records reported here come from core areas of the SBR, from mud flats, sandy beaches, and from trawl netting in estuaries.Much effort and long-term studies would be needed to establish the true extent of molluscan diversity in the region.It is important here to emphasise the key role played by molluscs in maintaining ecosystem health, a role that cannot be filled by other organisms.Molluscs store and release calcium, essential for vertebrate bone formation; they are also important elements in the carbon cycle.Bivalves, capable of tolerating a wide range of salinities, can control the abundance of plankton, and molluscs more generally, can accumulate various pollutants that are indicators of ecosystem health (rumisha et al. 2012).The microhabitat requirements of some species mean that their loss could impact a whole community (danda et al. 2017).
The lack of comprehensive knowledge thus prevents well-considered conservation measures.Uncommon species, of low mobility and highly specific requirements act as essential bioindicators of change or ecosystem function.We hope that this study, revealing five species previously unknown in the Sunderban, will act as a catalyst for further work to reveal the true molluscan diversity of the region, and enable conservation strategies to have a more secure basis.This is urgently needed.

Class Gastropoda Cuvier, 1795 Subclass Caenogastropoda Cox, 1960 Order Neogastropoda Wenz, 1938 Family Babyloniidae Kuroda, Habe & Oyama, 1971 Genus Babylonia Schlüter, 1838 Babylonia spirata (Linnaeus, 1758)
(chandra et al. 2017duetal.(2018a) recorded a single species of Dosinia, namely D. areolata from the Digha coast of West Bengal.Thus, this is the first record of D. tumida from Sunderban Biosphere Reserve and the state of West Bengal.D. areolate has a lanceolate ovate (egg) shaped lunule with a small and sharp umbo, which is hardly protruding whereas D. tumida does not have these characters.Escutcheon of D. areolate is wider than D. tumida.The discovery of D. tumida on Dhanchi island demonstrates the existence of Dosinia in the Sunderban, but because the collection site is not a human habituated island and there have been relatively few surveys, it is possible that this has led to the species' neglect in the region.Description.Specimen with a broad and ovately conical shell, shorter than in other Babylonia sp.Spire with a prominent and pointed end.Sutural canal is prominently wide and deep, visible with an angular keel shaped shoulder.Body whorl overall round-ed but moderately flattened between shoulder and upper periphery.Shell aperture dimension is half of shell height.Periostracum is thin and brownish coloured.Umbilicus is wide open.Base shell colour is off-white with dark brown blotches arranged over the shell in spiral bands.Size and shape of these blotches are variable in nature (Figs 5-6).Distribution.India -Tamil Nadu, Kerala, Orissa, Gujrat, Maharashtra, Goa, Karnataka, Andhrapradesh, West Bengal (mohan 2007, chelladurai et al. 2013, tudu et al. 2018a, chandra et al. 2020); Elsewhere -Indo-West Pacific (palomares & pauly 2022).Habitat.One shell sample was collected from the intertidal sandy beaches of Sagar Island, which is a mostly estuarine site.Three live specimens were collected from a fishing boat near Kakdwip harbor.This fishing boat was fishing inside the estuarine region of Matla River.The fishermen regarded them as trash.Remarks.B. spirata has a wide distribution around the Indian coast line and this species is widely popular for its usage in preparation of cultural artifacts and shell ornaments.This species is recorded in the state of West Bengal only in a single document and its specific collection locality is not known (chandra et al. 2020).The species was never reported from the SBR and thus this record provides the first proof of this species from SBR(chandra et al. 2017(chandra et al.  , tudu  & balaKrishnan 2018)).