Indian Journal of Geo Marine Sciences Vol. 47 (04), April 2018, pp. 846-853 Habitat heterogeneity determines structural properties of intertidal gastropod assemblages in a pristine tropical island ecosystem Vikas Pandey, Ganesh Thiruchitrambalam* & Kunal Satyam Department of Ocean Studies and Marine Biology, Pondicherry University, Brookshabad Campus, Port Blair-744112, Andaman & Nicobar Islands, India [E-mail: [email protected]] Received 16 August 2016; revised 17 November 2016 Community dynamics of gastropods were studied from four selected intertidal locations along an18km long coastal stretch on the east coast of south Andaman Islands. A total of 71 taxa belonging to 17 families and 3 orders were identified. While Nassariidae (31.15%) made maximal contribution to faunal abundance, Muricidae (11 species) and Conidae (9 species) were mainly responsible for changes in diversity patterns. In general, abundance and diversity exhibited a reversing trend. Most diverse area (H’=1.816±0.49 and d= 2.68±0.97) was found in the middle of the southeast coastal stretch. Multivariate analysis revealed the presence of location-specific gastropod assemblages spearheaded by Nassarius spp. and Nerita spp. The results yielded a potential positive correlation between locations and gastropod diversity along the entire coastal stretch. [Keywords: Gastropods assemblages, Intertidal diversity, Rocky shores, Andaman Islands.] Introduction salts that help in gastropod shell growth18, changes in Mollusca constitute the second largest Phylum of physico-chemical properties of the environment invertebrates after Arthropods, with an estimated 93,000 related to temperature, salinity, dissolved oxygen, living species and 70,000 known fossils1. Estimated pH, pollution, vegetation type and cover etc. influence species richness of gastropods (including undescribed the abundance and distribution of gastropods. species), the most species rich class of Mollusca2,3, Notwithstanding, structural complexity, heterogeneity varies from 47,0004 to 200,0005. The gastropods exhibit of habitats and availability of food is postulated the clear latitudinal variations in species richness6,7,8.They key factors that regulate growth and survival rates of can be used as of pollution bioindicators9 because they different species of gastropods19,20 ultimately may accumulate organic and metallic pollutants at higher result in in varying diversity and abundance concentrations10.While corallivorous gastropods (e.g. patterns21,22 across different habitats. Muricid Drupella) are important components structuring Globally, numerous works and field experiments reef communities11,12,13, certain gastropods like Trochus have been carried out to test the effect of habitat on niloticus and Turbo marmoratus have economic intertidal gastropods including the ones by Beck23,24 importance as food source for human consumption in and Olabarria and Chapman20. While Baums et al.15 coastal regions14. Gastropods play an important role in and Averbuj et al.25 studied the ecology of gastropods, shaping intertidal assemblages and controlling the Beasley et al.26, Tan and Clements27, Miloslavich et al.28, structure of intertidal communities as they can respond Chapman and Underwood29 and Carranza et al.30 to differences in microhabitats along a vertical gradient investigated taxonomic diversity and distribution of and change their behaviour in response to different intertidal gastropods. Yeung and Williams31 studied environmental conditions15. Periwinkles (a group of the behavioural response of gastropods. Apolinario et al.16 grazing gastropod) move down to middle intertidal zone studied the habitat selection and grazing pressure of and reduce the abundance of microalgae by feeding the periwinkle. upon them, but do not completely eliminate them16. In India, mangroves associated molluscs have been Survival of gastropods is regulated by various well documented32,33,34,35,36. New records of marine physico-chemical factors that affect their eco- gastropods37,38,39,40,41 strengthen our knowledge on the physiology17. In addition to the presence of calcium distributional patterns of gastropods in the Indian EEZ PANDAY et al.: STRUCTURAL PROPERTIES OF INTERTIDAL GASTROPOD 847 (Exclusive economic zone). Most recently, Trivedi regions. This location does not receive any fresh and Vachhrajani42 studied the distribution pattern of water input except rain. The sedimentary rocks on gastropods along Gujarat coast. The distribution of high water line are covered with microalgae that gastropods from Andaman and Nicobar islands was support high abundance of gastropods that feed on studied earlier by Das and Roy43, Apte44 and Subba them. During low tide a distance of almost 70m is 14 Rao recorded gastropods from coastal regions of exposed. St.B is a vast and rather flat exposed area of India. Although most of these studies provided an about 150 m from supra littoral zone to low water account on the taxonomical composition and line, during lowest low tide. This area has a patch of distribution of gastropods from Indian waters, none of poorly developed mangroves towards the south. The them provided details on gastropod assemblage intertidal zone has mixed substratum of rocky with patterns and/or the potential influence of habitat patches of coral reefs, and the area is mainly heterogeneity on them. Therefore, the aim of the characterized by boulders. St.C is located about 3km present study is to describe the role of habitat south to the Burmanallah and exposed intertidal area heterogeneity on structural properties of intertidal covers a distance about 80m. This area is relatively gastropods along south Andaman Islands. We flat with a combination of rocky, sandy and hypothesised that location-specific differences in the mud mixed substrata. Intertidal region is large and habitat will determine spatial patterns of abundance, sustains many life forms like crustaceans, molluscs, diversity and structure of intertidal gastropod assemblages along this pristine coastal environment. Materials and Methods Study Area Andaman and Nicobar group of islands (6°45′ N and 13°45′ N latitude and 92°12′ E and 93°57′ E longitude) are located in the south-eastern Bay of Bengal with a coast line of about 1962 km. The Andaman archipelago is broadly divided into three regions viz. North Andaman, middle Andaman and South Andaman. The climate of these islands is typically tropical, with hot and humid conditions. The annual temperature ranges between 18°C and 36°C, with slight increase from Northern Andaman group of islands. The Andaman & Nicobar (A&N) Islands are exposed to heavy rainfall of about 3100 mm/year, between May and December45. Majority of rainfall occurs during late May to early October (southwest monsoon), while a weaker spell of northeast monsoon brings rain during November to December. As most of the islands are covered with dense vegetation, the coastal waters are much influenced by heavy rainwater influx from forest hinterlands. In the present study four locations: Science Centre in the north (St.A), Burmanallah in the middle (St.B), Kodiyaghat in the south (St.C) and, Chidiyatapu in the southernmost tip (St.D) that represent different heterogeneity of intertidal habitats along the east coast of south Andaman Islands were selected (Fig. 1). At all locations, samples were collected during low tides. St.A is characterised by rocky substratum, with Fig. 1—Map showing the study locations along the east coast of sand covering the supra littoral zone at certain South Andaman. 848 INDIAN J. MAR. SCI., VOL. 47, NO. 04, APRIL 2018 nudibranchs and seaweeds. St.D is the southernmost Results point of the south Andaman Islands, during low tide, A total of 2472 individuals of gastropods, exposes a distance of about 100m. The coastal zone is belonging to 71 taxa, 32 genera, 17 families and endowed with extensive rocky outcrops, sandy 3 orders were recorded during the study. beaches and mangrove vegetation. Corals, seaweeds Neogastropoda that exhibited maximal abundance and seagrasses are common in this region. This area is constituted 53% of total gastropod population, characterised by sandy bottom with small to medium followed by Archaeogastropoda (35%) and size pebbles throughout. Mesogastropoda (12%). Amongst the families, maximal contribution was made by Nassariidae (31%) Methodology followed by Neritidae (28%) and Muricidae (10%). The gastropod samples were collected from the Together these 3 families constituted about 70% of eastern coastal region of south Andaman during total species from the study area. Neogastropoda January 2014 to June 2014. Samples were collected showed the highest species richness (32 species) monthly during lowest low tide using a quadrat of followed by Mesogastropoda (23 species) and 1 m2. The quadrat was randomly placed on the shore Archaeogastropoda (16 species). Whereas among the and the number of gastropods inside the quadrat were families, Muricidae (11 species) exhibited the highest recorded. From each locations four replicate samples species richness followed by Conidae (9 species), were collected. Species which posed difficulties in Cypraeidae, Strombidae, and Trochidae showing identification were brought to the laboratory in live 7 species each and, Nassariidae (6 species). Families condition, where they were identified to the lowest viz., Buccinidae, Mitridae and Ranellidae were possible taxon following standard identification keys represented by 2 species each whereas Fasciolariidae, (Subba Rao14 and