Utilization of detritus and bacteria as food sources by two bivalve suspension-feeders, the oyster

The concentration and composition of suspended particulate food available to estuarine suspension-feeding bivalve molluscs varies temporarily and spatially. Non-algal food sources may be important to suspension-feeders when algal concentrations are seasonally low or where there are high concentrations of suspended detrital material and bacteria, as found withln marshes. We carried out a series of laboratory experiments and field measurements to determine to what extent 2 common estuarine bivalve molluscs, the oyster Crassostrea virgin~ca and the ribbed mussel Geukensia demissa, could utilize cellulose and bacteria from Canary Creek marsh. Delaware, USA. Endogenously produced extracellular cellulases of the oyster depolymerized ingested cellulose to soluble oligomers. Subsequent intracellular cleavage of the oligomers to glucose was limited. The oyster absorbed carbon from refractory cellulosic material with an efficiency of only 3 %. In contrast, the ribbed mussel absorbed carbon from the same cellulosic material with an efficiency of 9 % and thls increased to 14 % if mussels were subjected to a 6 h exposure/6 h submergence cycle, a typical exposure regime for this intertidal species. We estimated that suspended cellulosic carbon in Canary Creek marsh during summer could supply 0.7 O/O and 8.6 % of the respiratory carbon requirements of subtidal oysters and intertidal mussels, respectively. In laboratory feeding experiments, colonization of refractory cellulosic food material by cellulolytic bacteria isolated from the marsh resulted in the oyster indirectly assimilating cellulosic carbon with an efficiency of 10 %. The oyster was able to filter free, unattached bacteria from suspension with an efficiency of only 5.0 %, compared with an efficiency of 15.8 % for the ribbed mussel. We estimated that both unattached and attached bacteria combined in Canary Creek marsh during summer provide only 5.5 % of the oysters' metabolic carbon requirements but could provide 31.0 % of an intertidal mussel's metabolic carbon requirements. Experiments with 1 5 ~ labelled bacteria indicated that attached bacteria associated with the breakdown of cellulosic material could mediate the flow of dissolved inorganic nitrogen from seawater to the oyster We estimated that unattached and attached bacteria in Canary Creek marsh during summer could contribute 26.7 % and 70.6 % of the metabolic nitrogen requirements of subtidal oysters and intertidal mussels, respectively. These results indicate that in thls marsh, utilization of bacteria as a food source could make a significant contribution during the summer to the nitrogen requirements of the oyster and to the carbon and nitrogen requirements of the mussel. However, cellulosic detritus and bacteria do not appear to fully meet the requirements of these bivalve species for carbon and nitrogen and utilization of other food sources is required, such as phytoplankton, nanozooplankton or non-cellulosic particulate and dissolved organic matter O Inter-Research/Printed in F. R. Germany Mar. Ecol. Prog. Ser. 58: 299-310, 1990