Diet of the eastern oyster, Crassostrea virginica, growing in a eutrophic tributary of Chesapeake Bay, Maryland, USA

Abstract

A 2-year study was undertaken to understand feeding preferences of the eastern oyster Crassostrea virginica in the eutrophic Rhode River, a tributary of Chesapeake Bay, Maryland, USA. A subset of experimentally suspended oysters was collected monthly and environmental parameters were simultaneously measured. Oysters were measured in height to determine growth, and the phytoplankton in their gut were examined both microscopically and using indicator pigments and compared with phytoplankton abundance and composition in the water column. Growth was higher in the second year of the study when flow was lower and salinity higher. Food selectivity was calculated using a modified electivity index (Ei), which relates phytoplankton composition in the gut to that in the water. Oysters appeared to preferentially graze–or at least preferentially retain in the gut–various (unidentified) flagellates, Ochrophyta (diatoms) and Myzozoa (dinoflagellates), and appeared to generally reject cyanobacteria, especially picocyanobacteria, from their diet. The Myzozoa included several common harmful algal bloom taxa, including Prorocentum minimum (=P. cordatum) and Heterocapsa rotundatum, that can detrimentally affect oyster growth. Reductions in eutrophication will likely be beneficial for oyster diets if such reductions result in fewer dinoflagellate blooms and in picocyanobacteria abundance during the critical feeding summer months.