ANALYZING MORPHOLOGICAL DIFFERENCES BETWEEN REEF- AND SOFT SUBSTRATE-HOSTED RECENT MARINE BIVALVES FROM INDIA: A MORPHOSPACE PERSPECTIVE

Spatial changes in substrate conditions through time is thought to be one of the most important driving factors influencing the temporal increase in Phanerozoic morphological and ecological diversities of benthic marine invertebrates, given that substrate determines varied biological and ecological activities. Marine bivalves, one of the most common benthic invertebrates of the Phanerozoic, show remarkable morphological variations and associated ecological adaptations, which have helped them in successfully colonizing diverse substrates, and has given them an evolutionary edge over other benthic groups. However, only few studies have quantified these substrate-related morphological variations and correlated them with the associated adaptations. To analyze these aspects, here we compared shape variation of 191 marine bivalve species between two locations of eastern India – Andaman and Nicobar Islands (having coral reef substrate) and West Bengal coast (having soft substrate). Since adaptations required to live on coral substrate are completely different from those required for life in soft sediments, we hypothesized that bivalves associated with these two substrates will have distinct morphologies. To visualize this, morphospaces were created using Principal Coordinates Analysis (PCoA) and Canonical Variates Analysis (CVA), and the results from these ordination analyses were statistically compared using multivariate analysis of variance (MANOVA) and Kruskal-Wallis tests. Our results show statistically significant substrate-specific segregation among bivalves living in these two distinct substrates: elongated, bysally attached epifauna and borers of reefs plot separately from semicircular to elongate burrowing forms of soft substrates. These results suggest that substrate has a profound effect on morphological variation, and different morphologies are directly related to the different adaptive strategies. From a macroecological perspective, the results imply that episodic changes in substrate conditions, which have happened during the Phanerozoic, could have promoted localized as well as global increase in ecological and morphological diversifications in different clades.