Comparative feeding on chlorophyll-rich versus remaining organic matter in bivalve shellfish

Abstract

Filter feeding was compared in the blue mussel Mytilus edulis, Mediterranean mussel Mytilus galloprovincialis, Pacific oyster Crassostrea gigas, Chinese pleated oyster Crassostrea plicatula, Chinese scallop Chlamys farreri,Manila clam Tapes phillipinarum, razor clam Sinonvacula constricta, and blood cockle Tegillarca granosa over 95 dietary conditions across nine locations in Europe and China. Conditions included natural seawater enhanced, on occasion, with variable additions of microalgal monoculture and/or natural silt collected from the surface of nearby mudflats, thereby ensuring that diets ranged from oligotrophic to hypereutrophic (>325 μg chlorophyll a/L) and highly turbid (>1,250 mg total particulate matter/L). Building on past insights into differential particle processing, we resolved the availability, filtration, and ingestion of organic matter rich in chlorophyll a, and which matter is known to be ingested selectively, from all remaining organics, such as detritus, bacteria, protozoans, and/or colloids. The forms of fitted relations that best described feeding responses were different for chlorophyll-rich versus remaining organics, that difference being consistent in all eight bivalve species, including when measured in the same species across different locations. Throughout, positive linear relations between net ingestion of chlorophyll-rich organic matter and the availability of that chlorophyll-rich organic matter indicated that each species ingested constant proportions of the organics associated with chlorophyll a, with no limitation evident to such ingestion over our experimental range. Alternatively, ingestion of the remaining organics by each species increased as saturating functions of the availability of those remaining organics alone. Differences were evident between species both in the proportions of available chlorophyll-rich organic matter that were ingested and in saturated ingestion rates for the remaining organics. Comparison of those differences suggests an evolutionary trade-off among species between their relative abilities to process chlorophyll-rich versus remaining organics. Differences also suggest congeneric adaptation to natural habitat, impressing a need for careful appraisal of each and every bivalve species. Significant costs of particle sorting, digestion, and absorption, evidenced as metabolically derived material egested and assayed within feces, were greatest in species with higher saturated capacities for the ingestion of remaining organics. Last, we describe how, compared with predictions based on chlorophyll a or total particulate organic matter alone, our approach resolving coincident differential availabilities and processing of chlorophyll-rich versus remaining organic matter has improved the accuracy of growth simulations when applying single calibrations of the generic shellfish model ShellSIM across coastal habitats that contrast in seston composition.