MORPHOLOGICAL DISPARITY IS STRUCTURED AND SEGREGATED BY THE LOCAL SUBSTRATE CONDITIONS AND LIFE HABITS: AN EXAMPLE FROM INDIAN RECENT MARINE BIVALVES

Temporal evolution of substrate has been instrumental in influencing the Phanerozoic diversification of marine organisms as substrate determines life habits related to mobility and stability. These substrate-related life habit preferences are further reflected in the morphological attributes of marine invertebrates. While these disparity patterns are usually documented over macroevolutionary scale, morphology variation within ecological scale is relatively less explored. Here, we study the influence of the nature of the substrate and related life habits (i.e., motility and tiering) on the morphological disparity of Recent marine bivalves from several coastal locations of India offering diverse substrates for bivalves: hard in coral reefs and rocky shores; soft in sand and mud-dominated tidal flats and beaches; and woody in mangroves and seagrass. To analyze the influence of substrate and life habits on shell shape, mean shapes of different substrates mentioned above, and life habit subgroups (byssate, cemented, burrower, facultative swimmer, recliner; surficial epifaunal, semi-infaunal, shallow and deep burrower and borer) were compared using PERMANOVA and ANOSIM. Centroids were calculated to assess median shape variation among all subgroups, and the Mean Nearest Neighbor Distances (MNND) were used to quantify morphodensity within each subgroup. Since different families may show morphological convergence to cohabit in/on the same substrate or having similar life habit, MNND values, supplemented by PERMANOVA and ANOSIM on PCA scores, were used to test morphological convergence among families sharing specific substrate or life habit subgroup. Our results show that the mean shapes of bivalve families are different for different substrate and life habit subgroups, suggesting that disparity is structured by substrate affinity and life habits of taxa. Within the soft substrate, families show strong morphological convergence, indicated by the low among-family MNND, and statistically similar (PERMANOVA and ANOSIM, p > 0.05) mean shapes, which is not observed in other subgroups. All these indicate that substrate and life habits determine the spatial patterns in disparity, culminating in morphological segregation across families even at shorter temporal resolution.