Modelling short-term responsive adjustments in particle clearance rate among bivalve suspension-feeders: separate unimodal effects of seston volume and composition in the scallop Chlamys farreri

Abstract

Suspension-feeding behaviour in the scallop Chlamys farreri of 72±1 mm shell length was studied in response to wide variations in the amount and composition of suspended seston. Clearance rates (CR; l h −1) with which scallops removed all particles larger than 3.9 μm equivalent spherical diameter varied over an order of magnitude, initially increasing to maxima in separate unimodal relations with different measures of dietary abundance that included total particulate volume (VOL; mm 3 l −1), total particulate mass (TPM; mg l −1), particulate organic mass (POM; mg l −1), particulate organic carbon (POC; mg l −1) and chlorophyll a (CHL; μg l −1), before declining with further increases in seston concentration. Most variance in CR was associated with VOL (47%), compared with TPM (29%), POM (32%), POC (29%), PON (27%) and CHL (20%). This suggests morphological limits to feeding behaviour, as adjustments in CR were primarily dependent upon total seston volume, rather than any gravimetric measure of abundance. Nevertheless, stepwise regression indicated that VOL and CHL together accounted for as much as 58% of the variance in CR; more than for VOL alone ( p<0.001), and more than twice that explained by gravimetric measures of TPM, POM or CHL. The associated relation was best described by a combination of two unimodal curves, predicting that CR increased with food availability to a maximum of 7.1 l h −1 g −1 when VOL was 2.0 mm 3 l −1 and CHL was 5.3 μg l −1, before declining with further increases in either VOL or CHL. Response curves differed according to particle type. Most evidently, compared with silt-enriched diets, natural seawater contained about the same total chlorophyll l −1, but with much lower average volume, with the result that maximal CR was significantly higher in natural seawater. These findings establish highly flexible regulation of CR. They also indicate that to understand regulatory responses in feeding to a mixed suspension of different particles, it is necessary to measure the separate significant influences of particle volume and seston composition, when the range of experimental conditions must span the lower extreme of naturally occurring concentrations. We impress how our findings help to reconcile hitherto apparent differences between previous reports of bivalve suspension-feeding behaviour.