Molluscan diversity of coastal Karnataka, India and role of physicochemical parameters on their diversity

The intertidal zone is unique in terms of its ecological conditions and faunal assemblages. Molluscs form a key link in the food chain and have a high socioeconomic value for coastal fisheries. Intertidal habitats of Karnataka are modified by anthropogenic disturbances which threaten the animals living there including molluscs. The present study is designed to know the diversity of molluscs of Karnataka coast, India and to understand the factors influencing variations in molluscan diversity. Thirteen intertidal habitats of coastal Karnataka were selected for the study of which six sites were visited on monthly basis from February 2017 to January 2018. Molluscan shells were collected from these sites using quadrates of 0.25m2 from fixed intervals. The other sites were visited once during the study and molluscs were collected using quadrates and by handpicking. Molluscs were brought to the laboratory, preserved in 70% alcohol, and identified using the standard identification keys. Abiotic parameters such as air temperature, water temperature, wave frequency were noted in the field. Environmental variable such as precipitation was studied. Physico-chemical parameters of water (salinity, electrical conductivity, pH of water) were analysed using standard methods. A total of 36 species of molluscs were recorded from the intertidal habitats of coastal Karnataka. It includes 21 species of gastropods and 15 species of bivalves. Bullia melanoides, Saccostrea cucullata and Echinolittorina leucostica were found throughout the study. Abranda myeformis, Mactra luzonica, Sunetta sp, Turricula tornata fulminata were the rare shells found on the beaches of coastal Karnataka. The abundance of molluscs was observed post-monsoon in all the sites. Species richness of molluscs was more in post-monsoon in all the intertidal areas except beaches of Kaup and Kota-Padukere. Shannon-Weiner index and Simpsons index were more in post-monsoon in the beaches of Someshwar, Panambur, Sasihithlu, and Gangolli whereas diversity indices were highest in intertidal areas of Kaup and Kota-Padukere in premonsoon. Kruskal–Wallis test showed the significant differences between abundance (P = 0.001) and species richness (P = 0.002) of molluscs with the seasons. During the study salinity, electrical conductivity, and pH of water ranged between 21.7–33.66ppt, 41.42–57.90 mS/m, and 7.36–8.47 respectively. Average values of air temperature, water temperature, and wave frequency varied between 26.500C–30.250C, 27.750C–30.250C, and 6–9 between the seasons. However, no significant correlation was found between these parameters and species diversity. The number of molluscs was more when the wave frequency was between 6–7. A significant negative correlation of wave frequency with molluscan abundance was observed which shows that reduced wave-action increases the molluscan diversity. A significant correlation between the electrical conductivity of water with species richness indicates that this is the important physico-chemical parameter responsible for the distribution of molluscan species on the coast. Wave frequency and electrical conductivity are the parameters are responsible for molluscan diversity in the intertidal zones. Thus the littoral zones of the Karnataka coast provide suitable habitats for molluscs.