Sediment Organic Matter Content Drives Bivalve Density in Tropical Oligotrophic Seagrass Ecosystem

Seagrass ecosystems are storehouses of both autochthonous and allochthonous organic matter due to their presence at the land and sea interface. This organic matter acts as a food source and plays an important role in biodiversity assemblages within seagrass ecosystems. The present study assessed the density of the bivalve (Cardita calyculata) along a gradient of sediment Organic Matter (OM %) in the mono-specific seagrass meadows of Thalassia hemprichii to those in adjacent bare sediments in the Andaman and Nicobar Islands (ANI) of India. The present study also assessed the correlation between sediment OM with Cardita's density and seagrass morphometric traits.C. calyculata density was 2-fold higher in T. hemprichii meadows (647.7±27.0 ind. m^-2) than in the bare sediment (306±14.3 ind. m^-2). Bivalve abundance correlated positively with OM in the seagrass meadows but showed no correlation in the unvegetated areas. The root branching of seagrass was higher, leading to increased C. calyculata density, suggesting a strong correlation of seagrass meadows with bivalves in nutrient-rich environments. The correlations between C. calyculata density and various plant traits were positive. Our study suggests that in the oligotrophic coastal ecosystems of the ANI, the positive influence of sediment OM of seagrass meadows on C. calyculata density is significant. Further studies looking at various interactions between seagrass and C. calyculata are essential to establish the precise interrelationship between seagrass and the biodiversity of associated faunal assemblages.