Feeding selectivity and gametogenic cycle of Crassostrea madrasensis in relation to seasonal plankton and environmental variability in the southeast coast of the Bay of Bengal, Bangladesh

The feeding behaviour of the Indian backwater oyster ( Crassostrea madrasensis ), a key estuarine filter-feeding bivalve native to the dynamic coastal waters of the Bay of Bengal, is intricately linked to a complex interplay of external environmental conditions and internal physiological processes. This study investigated the selective feeding behaviour of C. madrasensis over a complete annual cycle (July 2023–June 2024) along the southeast coast of Bangladesh, employing an integrated multivariate approach to elucidate how seasonal variations in plankton assemblages and environmental conditions influence feeding selectivity and the reproductive cycle of C. madrasensis . Various multivariate analyses revealed that water quality parameters, plankton abundance, and ingested gut plankton exhibited significant seasonal variability, driven by monsoonal hydrodynamics and nutrient influx. Environmental parameters such as temperature and turbidity were negatively associated, while chlorophyll-a and dissolved nutrients were positively correlated with both plankton availability and ingestion. Analysis of gut contents revealed that C. madrasensis selectively ingested specific plankton genera/groups, with clear seasonal positive selection for 17 genera/groups, including Amphidinium (17.5 % of total ingestion), Skeletonema (15.1 % of total ingestion), Coscinodiscus (12.0 % of total ingestion), and Cyclotella (10.8 % of total ingestion) comprised the highest proportions. Cluster analyses showed peak ingestion during the post-monsoon and late autumn months, coinciding with elevated chlorophyll-a concentrations and nutrient levels in the water column. The seasonal shifts in feeding selectivity corresponded with environmental changes and gonadal development phases, indicating an adaptive response to both external ecological factors and internal physiological cues. During the post-monsoon gametogenic phase, multivariate patterns suggest greater reliance on ingested food to support gamete maturation. In contrast, patterns observed during the pre-monsoon period are consistent with increased use of stored reserves, although continued feeding on smaller planktonic cells cannot be ruled out. These findings from these broad datasets provide valuable insights into the ecological adaptability of C. madrasensis , informing sustainable oyster farming strategies and enhancing resilience in coastal aquaculture systems in monsoon-influenced tropical environments like the Bay of Bengal. • Integrated multivariate analysis is employed to identify eco-physiological drivers of selective feeding in Crassostrea madrasensis ensis. • Seasonal plankton dynamics were shaped mainly by monsoonal hydrodynamics, nutrients, and environmental parameters. • Feeding selectivity was closely tied to environmental changes and gonadal development across reproductive phases. • C. madrasensis adopts a dual energy acquisition strategy, being opportunistic during the post-monsoon and conservative during pre-monsoon gametogenesis. • The findings can contribute to the sustainable expansion of bivalve farming in monsoon-driven tropical coastal regions.