LBG OR LOCAL ENVIRONMENT: WHAT GUIDES THE DISTRIBUTION OF MARINE BIVALVES OF INDIAN COASTS?

While the causal mechanism of diversity and distribution of marine invertebrates remains highly debated, the pattern of richness change with latitude - a latitudinal biodiversity gradient (LBG), has been established as one of the strongest pattern of global marine diversity. Local environmental variables can also play a key role in shaping the diversity patterns. The relative effect of LBG and local environment within tropical region remains unclear.

Indian coastline accommodates over 15 latitudinal bins (8° N up-to 23° N). Within the same latitudinal range, India has two strikingly different coastlines in terms of local environmental parameters (such as freshwater mixing, sediment influx, circulation pattern, primary productivity, shelf area). Comparing the marine bivalve diversity between eastern and western coast of India, we are trying to evaluate the relative effect of latitudinal gradient and local environment. Using the occurrence data of bivalves from a pre-existing database, Biosearch, we compared the diversity and community structure of the two coasts. The rarefied species richness is much higher in east coast compared to the west coast. Species diversity does not show any significant consistent gradient for any of the coast for actual occurrence; however, range-though species richness for west coast shows a significant positive correlation between latitude and diversity – a trend contrary to the prediction of LBG. Similar trends are found when the analysis is performed for dominant five families. The community composition of the coasts did not yield any unique pattern; rather, coastal character dominantly dictate the species associations irrespective of their coastal affiliation. Southern coast of high carbonate, low siliciclastic input shows a characteristic fauna dominated by borers like Martesia striata. Areas with varying salinity and mangrove shows another type of bivalve fauna dominated by semi-infaunal species like Meretrix meretrix. The last one is open sandy shore to shores with high riverine input; this fauna is dominated by various species of Donax. The present study demonstrates that the species richness of Recent marine bivalves within tropics is largely driven by local conditions rather than factors that control large-scale patterns such as LBG.