Feeding physiology of two bivalves under laboratory and field conditions in response to variable food concentrations

In response to uncertainties about whether laboratory feeding rate measurements of bivalves reflect those occurring in situ, this study compares bivalve feeding rates experimentally determined both in the laboratory and in situ for a mussel (Mytilus chilensis) and a clam (Mulinia edulis), species exposed to a wide range of seston concentrations. Artificially prepared seston was administered in the laboratory using a mixture of Isochrysis galbana and sediment at concentrations ranging from 2 to 200 mg 1(-1). Field measurements were carried out at Yaldad Bay, southern Chile, during 2 tidal cycles in March and April 1999 with naturally occurring seston ranging from 3 to 665 mg 1(-1). We found that: (1) Clearance rate (CR) and selection efficiency (SE) were similar in laboratory and in situ conditions, but the rates of filtration (FR), ingestion (IR) and absorption (AR), and absorption efficiency (AE) were significantly higher in the laboratory than in the field. This was attributable to the greater retention efficiency and digestibility of I. galbana compared to the naturally occurring microalgae in the seston, and also to the greater acclimation time of specimens to the laboratory diet. (2) The responses to the changes in seston concentration (TPM) were similar under both conditions and validated previous results; CR decreased exponentially with increasing TPM while the FR, IR and AR increased. At high seston concentrations (200 mg 1(-1) in the laboratory and 665 mg 1(-1) in situ), almost all the feeding rates decreased, showing an impairment of food processing under these conditions. (3) The resuspended sediment in the intertidal zone of Yaldad Bay, especially under the influence of southerly winds and near the lowest tidal level, is an important food source for M. edulis and M. chilensis populations, allowing continued growth during the winter season when primary productivity is very low.