FRESHWATER INPUT INFLUENCES TAXONOMIC AND BODY-SIZE VARIATION OF MARINE BIVALVE ASSEMBLAGE IN A TROPICAL ECOSYSTEM

The spatial variation in taxonomic composition and body-size of marine benthos is known to be influenced by both physical and biological components of its environment. The tropical marine ecosystem is thought to be heavily influenced by biological interactions. Tropical seas generally characterized by a near uniform temperature profile, however, often display substantial regional variation in physical oceanographic characters other than SST. We attempted to evaluate the effect of such factors on the recent marine bivalves in a tropical marine setting, namely the western coast of India. The western coastline, bordering the Arabian Sea is characterized by a wide latitudinal spread (8–22° N). Moreover, the northern coast (above 15° N) is substantially different from the southern coast in its oceanographic characters (such as oxygen concentration, freshwater input, shelf area, productivity).

We compared the diversity and morphology of bivalves between the northern and southern coast along the western margin of India using time-averaged recent bivalve specimens collected from 29 beach sites spanning over 14 latitudinal bins. Our results reveal the existence of two distinct ecoregions separated at 15ºN, each with a characteristic species composition. This compositional distinction extends at higher taxonomic level with Lucinidae, Pteriidae and Myidae being exclusively present in the north. A similar pattern is observed in the regional variation of the shape and body size of bivalves. The southern coast is characterized by a smaller body size in comparison to the north and the difference is significant for the infauna.

This compositional difference in oceanographic variables at 15ºN is driven by the monsoon induced seasonal freshwater influx from the Bay of Bengal that flows to the Arabian Sea and produces a low salinity water mass south of 15ºN. Because of the highly evaporative nature of the Arabian sea, the northern coast maintains a high and uniform salinity with minimal seasonal variation. The variation in taxonomic composition of bivalves is best explained by the regional salinity variation. The freshwater input in the south also modifies the seasonal nutrient profile , probably influencing the body size. The present study highlights the influence of oceanographic variables in controlling the regional variation of molluscan composition and morphology in the tropical ecosystem.