Field estimates of feeding rate for Gammarus pseudolimnaeus (Crustacea: Amphipoda) in the Credit River, Ontario

Abstract

SUMMARY. The feeding rate of G. pseudolimnaeus was measured monthly for 7 months in the field by monitoring the decline in weight of gut contents when the amphipod was starved. This decline was modelled by an exponential regression of weight on time. As the amphipods appeared to be continuous feeders, feeding rate was calculated by multiplying the dry weight of a full gut by the specific rate of emptying, i.e., the slope of the exponential regression. Specific rate of emptying was independent of animal size, but increased with temperature. Therefore, food has a longer period in which to be digested at low temperatures, which suggests that assimilation efficiency may increase.However, the assimilation efficiency of amphipods feeding on decaying maple leaves in the laboratory was only 10% and did not vary with temperature. Ingestion and egestion rates were measured in the laboratory by weighing amounts eaten and defecated. The turnover time of the contents of a full gut in the laboratory often agreed very well with turnover time measured in the field, i.e., the reciprocal of the specific rate of emptying, thus confirming the use of an exponential regression.