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 (31.15%) made maximal contribution to faunal abundance, Muricidae (11 ) 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 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 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 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 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 Abbott and Dance46). After this, the Vasidae, Ovulidae, Vanikoridae and Patellidae had specimens were preserved in 7% formalin solution. In only one species each. addition to biological data, environmental parameters Spatially, Neritidae and Muricidae were recorded viz. salinity and dissolved oxygen were estimated in higher abundance at St.C (9.75% and 3.20% following standard methodologies (Grasshoff et al.47 respectively) and St.A (18% and 4% respectively). whereas air and water temperature was measured Species viz. Nerita albicilla Linnaeus, 1758, using a hand held thermometer (±0.1⁰C)and pH using N. chamaeleon Linnaeus, 1758, Thalessa virgata a microprocessor pH meter (Model 1010 Deep (Dillwyn, 1817), Menathais tuberosa (Roding, 1798), Vision, accuracy 0.01 pH). mendicaria (Linnaeus, 1758), Vasum turbinellus Linnaeus, 1758, Conus ebraeus Linnaeus, Data analyses 1758 and Turbo crassus Wood, 1828 were the most Univariate diversity measures (e.g., Species richness, commonly encountered species across entire study Margalef’s diversity, Pielou’s evenness, Shannon- area. Among the 71 gastropod taxa identified, Wiener diversity index) and the non-parametric distribution of the most abundant taxa Nassarius multivariate statistical procedures probing spatio- globossus (Quoy & Gaimard, 1833) was restricted to temporal patterns in gastropod distribution were the southernmost sampling location (Appendix 1). carried out as described in PRIMER v6.1 statistical The mean abundance (N) was higher at St.D package48. Non-metric multi-dimensional scaling (38.7 ind/m2; coefficient variation, CV: 0.57) (nMDS) based on Bray-Curtis similarity matrix of followed by St.A (32.3 ind/m2; CV: 0.53). Despite the square root transformed gastropod abundance data. lowest density of gastropods (11.2 ind/m2; CV: 0.56), Percentage of variability in sample ordination patterns species richness (S) was significantly higher at St. B along nMDS axes were examined using Principal (33 species, CV: 0.52) followed by St. C (32 species, Coordinate analysis (PCO) and those species that CV: 0.48). Lowest number of species was recorded exhibited Pearson correlation value of >0.65 were from St.A (13 species, CV: 0.37). superimposed. Individual species responsible for Similar to species richness (S), Margalef’s observed assemblage patterns were identified through diversity (d) and Shannon-Wiener diversity index (H') their contribution to within group similarity and or also exhibited highest diversity at St.B (2.684, CV: between group dissimilarity, using the routine 0.36 and 1.816, CV: 0.27 respectively) and lowest at SIMPER (Similarity Percentage) analysis49. St.D (2.153, CV: 0.55 and 1.526, CV: 0.43). Intertidal PANDAY et al.: STRUCTURAL PROPERTIES OF INTERTIDAL GASTROPOD 849

zone of St.B has a mixture of rocky flats, crevices and revealed higher affinity of Nassarius globossus, coralline turfs, and these can act as a trap for food and N. pullus (Linnaeus, 1758), N. luridus (Gould, 1849), sediment. In addition, St.B harbours good growth of N. albescens (A. Adams, 1852), Conus eburneus algae, seagrasses and seaweeds and since a majority Hwass in Bruguiere, 1792, C. virgo Linnaeus, 1758 of gastropods are grazers, they find this location and Trochus sp. to St.D whereas Vasum turbinellus suitable for survival. On contrary, being a sandy shore and Trochus ochroleucus Gmelin, 1791 beelonged to St.D exhibited least diversity. Pielou’s evenness index St.B and Nerita polita Linnaeus, 1758, N. chamaeleon, (J') revealed highest value at St. B (0.952, CV: 0.03) N. albicilla, Lunella cinerea (Born, 1778), Scutellastra and the lowest at St.D (0.731, CV: 0.27) (Table-1). flexuosa (Quoy & Gaimard, 1834) and Clypeomorus sp. The average value of air temperature, water to St.A and St.C. temperature, salinity, dissolved oxygen and pH Similarity percentage analysis (SIMPER) during the study were 29.62 ⁰C, 30.45 ⁰C, 34.5, confirmed that gastropod assemblage in St.D is 4.22 mg/l and 8.59 respectively. In general, significantly different from other locations. The fact intermittent exposure to extreme environmental that dissimilarity in gastropod assemblage was conditions do affect macrobenthos distribution in the intertidal region. But during this study there was no significant correlation between measured background variables and species composition or distribution patterns.

Multivariate analyses of gastropod community attributes Multivariate analyses such as non-metric multidimensional scaling yielded four distinct sample groups. Among the twenty four monthly mean samples used in this analysis, Group-I comprised samples collected from St.D (n=6) characterised by sandy substratum, Groups-II & III consisted of six Fig. 2—nMDS plot showing the presence of locations-specific samples each from St.A and St.C, characterised rocky gastropod assemblages along intertidal habitat of eastern south substratum and Group-IV consist of samples from Andaman Island. Sample codes A, B, C and D refer to sampling St.B, characteristic of mixed substratum of rock with locations. patches of reefs and boulders (Fig. 2). The PCO ordination based on species association patterns revealed the importance of the first two principal coordinates accounting for 67.2% of total variation in species distribution patterns (Fig. 3). The first PCO axis showed a clear separation of St.D samples from the remaining locations whereas second axis separated St.A, St.B and St.C from each other. Taxa that exhibited highest correlation (r >0.65) with assemblage patterns revealed the importance of species belong to the families Nassariidae and Neritidae. Species superimposed as vector lines

Table 1—Diversity indices and CV values for the gastropod at different locations Locations d J' H'(loge) St.A 1.951; 0.33 0.826; 0.12 1.61; 0.26 Fig. 3—Two-dimensional Principal Coordinates Analysis (PCO) St.B 2.684; 0.36 0.952; 0.03 1.816; 0.27 showing superimposed vectors for important gastropod species St.C 2.544; 0.43 0.874; 0.13 1.800; 0.31 (that contributing to r >0.65) that shaped species assemblage St.D 2.153; 0.55 0.731; 0.27 1.526; 0.43 patterns in the study area. 850 INDIAN J. MAR. SCI., VOL. 47, NO. 04, APRIL 2018

maximal with St.B (97% average dissimilarity) This area offers a very heterogeneous habitat for may underline the substratum specific gastropod gastropods as it contains rocks, coralline turfs, patches distribution patterns in the study area. As mentioned of soft bottom substratum and mangroves. The earlier, St.D contains sandy substratum with pebbles coralline turfs and other biogenic structures are whereas St.B is a rocky habitat with coralline turfs. known to trap the sediment and food for On the other hand, attributable to the likeness in gastropods20,51. In addition, coralline turfs and habitat type, maximal similarity was evident between macrophytes provide different microhabitats, which the gastropod assemblages of St.C and St.A. Amongst may offer refuge from predation, in addition, the four locations (i.e. St.A, St.B, St.C and St.D), macrophytes are important primary producers in SIMPER analysis revealed that maximum average coastal ecosystems, and are therefore a potential food dissimilarity (97.01%) between St.B and St.D resource for the associated animals52. These whereas the minimum dissimilarity was noticed heterogeneous habitats can also provide shelter for between St.A and St.C (55.32%). The species which predators and thereby increasing the overall diversity. characterized St.A were Nerita albicilla, N. Thus, availability of a variety of ecological niches chamaeleon, Scutellastra flexuosa, Clypeomorus makes St.B one of the richer gastropod localities in batillariaeformis Habe & Kosuge, 1966, Clypeomorus the south Andaman Islands. Gastropods at this sp.1, Clypeomorus sp.2, Thalessa virgata and Lunella location show more similarity to those reported at cinerea whereas for St.B Latirolagena smaragdulus St.A and St.C because of their similarity in habitat (Linnaeus, 1758), Vasum turbinellus, Conus ebraeus structural complexity. On the other hand, lack of and Trochus ochroleucus were characteristic species. substratum complexity at the southernmost location St.C was characterized by only two species Trochus (St.D) resulted in the lowest gastropod diversity maculatus Linnaeus, 1758 and Nerita polita whereas recorded from the entire study area. St.D was characterized by Nassariidae family and As evident from this study, habitat complexity main characteristic species for this location were seems to be an important variable for the growth and Nassarius globosus, N. pullus, N. graniferus (Kiener, survival of gastropods. Olabarria and Chapman20 who 1834), N. graphiterus (Hombron & Jacquinot, 1848), experimentally tested this concept on Eatonina Nassarius gemmuliferus (A. Adams, 1852) and rubrilabiata by combining coralline turfs with and Nassarius sp. without sediment revealed faster growth rates in the presence of sediment and, greater mortality in its Discussion absence. In this study, we have also noticed habitat Quantitative studies on the gastropod assemblages specificity among certain species of gastropods. around the Andaman Islands, despite their ecological Nassarius globossus, numerically most dominant taxa importance, are little known. The present study is during the study, was profusely found in the pebble based on gastropod samples collected from 4 laden sandy substratum at St.D but altogether absent locations along the eastern coastline of the South from other locations. The reason for this restricted Andaman Islands. It is not surprising that majority of distribution may be the differences in habitat the molluscan fauna reported by Subba Rao14 in the structure, as the habitat complexity has a strong ‘Indian Seashells’ is based on collections from influence on the abundance and diversity of Andaman and Nicobar islands. gastropods21,22. Species diversity is one of the most important traits During this study, Neritidae - a group of of a biotic community, because diversity is related to herbivorous, euryhaline snails that feed on algae by productivity and stability of the ecosystem50. In this scraping them off from the substratum27, usually study we observed a direct relationship between inhabit the middle to upper intertidal zones - habitat types, faunal density and diversity. Locations dominated the St.A and St.C locations. These two comprising greater diversity included locations with locations are similar in terms of habitat structure to a high substratum complexity, indicating that higher great extent, which is also reflected in their species structurally complex habitat increases the potential composition and gastropod assemblage patterns. The niche exploitation by various species of gastropods. rocky substratum at these locations support good During this study, maximum diversity, species richness macroalgal growth which in turn provide food and and evenness were recorded from Burmanallah (St.B). habitat for variety of associated mobile , PANDAY et al.: STRUCTURAL PROPERTIES OF INTERTIDAL GASTROPOD 851

including meiofauna, macrofauna and fish52 therefore also meet their energy expenditure by feeding on their Neritidae being herbivore thrive under these mucus. A study by Yeung and Williams31 on the condition. These findings are in agreement with foraging behaviour of Nerita yoldii revealed that observations by Miloslavich et al.28 showing individuals of this species foraged when they are dominance of herbivore gastropods at locations with awashed, or emersed, migrating down-shore to forage higher macroalgal biomass. and up-shore to refuges. Muricidae was another important family Conclusions encountered during this study, especially at the Present study reveals that habitat complexity is one St.A and St.C locations. These are carnivore of the major but not sole- variable for describing gastropods which feed upon other gastropods, the assemblages of gastropods. Diversity and richness barnacle, crustaceans14 and the rock oyster Saccostrea of gastropods were higher at structurally complex cuccullata53. The family shows a preference to occupy habitats. The study also reveals gastropods mid shore in the barnacle and oyster belts as it feed assemblages and their preferred habitat, Family upon them54. Saccostrea cuccullata (rock oyster), a Nassariidae preferred soft bottom habitat and was common species in the mid intertidal zone of the abundant at St.D and altogether absent from other Indo-Pacific region55, experiences a strong predation locations. Whereas Neritidae and Muricidae were by muricid gastropods53. Supporting these found on almost all the locations. observations, we found high abundance of barnacles, rock oysters and, algal mats (to be grazed upon by Acknowledgement herbivorous gastropods) in locations where muricids Authors are grateful to the authorities of were abundant. On the other hand, barnacles and Pondicherry University for providing the necessary rock oysters were less abundant along St.B and facilities to carry out this work and, for the University St.D coast line consequently resulting in lower fellowship to Vikas Pandey and Kunal Satyam. abundance of muricidae. Apart from availability of food, abiotic factors such References 1 Brusca, R.C., and Brusca, G. J., Invertebrates, (Sinauer as nutrients, habitat structure, light, salinity and Associates, Sunderland, Massachusetts) 2003, pp. 936. environmental stressors such as wave action and 2 Bouchet, P., The magnitude of marine biodiversity, desiccation56 affect the intertidal organisms. To in: The Exploration of Marine Biodiversity: Scientific and survive under these harsh condition, intertidal Technological Challenges, edited by Duarte, C.M., (Fundacion BBVA, Madrid, Spain) 2006, pp. 32-64. organisms have adaptations, such as behavioural, 3 Rigby, P.R., Iken. 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