Convergent food sources: Sympatric Magallana gigas and Mytilus edulis select similar phytoplankton groups via morphological and nutritional value

Understanding the feeding selectivity of suspension-feeding bivalves for phytoplankton of varying sizes and nutrient compositions is a key research focus in bivalve ecophysiology. To examine feeding preferences of wild oysters ( Magallana gigas ) and mussels ( Mytilus edulis ), we compared the phytoplankton composition and indicator pigments in their stomach contents with those in the surrounding seawater, using fatty acids (FAs) in bivalve tissue as biomarkers of food sources. From November 2018 to October 2019, bivalves and seawater samples were collected monthly from the shore of Lu Xun Park (Qingdao, China). Oysters and mussels selectively consumed similar food sources, with the proportions of diatoms/fucoxanthin and dinoflagellates/peridinin alternating in their stomach contents. This dietary shift was further supported by FA composition in bivalve tissue. Both bivalves consistently rejected dominant diatoms species (e.g., Skeletonema costatum and Nitzschia longissima ) while preferentially grazed—or at least retained— rare phytoplankton taxa ( Pinnularia sp. 2, Paralia spp., Coscinodiscus spp., Dinophysis spp., Prorocentrum spp., dinoflagellate cysts and Dictyocha spp.). Haptophytes, cryptophytes, chlorophytes, and picoplankton (prasinophytes and Synechococcus ) were ingested in small amounts. Phytoplankton size, shape, and nutritional value likely play key roles in bivalve feeding selectivity. The selective feeding strategies of coastal bivalves revealed in this study serve as a key mechanism in maintaining the stability of nearshore ecosystems, offering significant reference value for sustainable fisheries. • Oysters and mussels co-occurring in the same habitat exhibit comparable phytoplankton feeding selectivity. • Bivalves selectively retain low-abundance taxa while rejecting some dominant species. • The foraging strategy of bivalves likely driven by particle size, shape, and nutritional value. • Bivalves mainly consumed diatoms and dinoflagellates, with small flagellates and cyanobacteria contribute less to their diet. • Seasonal shifts in diatom-dinoflagellate ratios in stomach contents matched tissue fatty acid profiles.