Medium-term acclimation of feeding and of digestive and metabolic enzyme activity in the neritic copepod Acartia clausi. I. Evidence from laboratory experiments

Abstract

The time-relationship between food supply, intermediate metabolism (ETS, GDH activities) and nutritional processes (ingestion, digestive enzyme activities) was established experimentally for the small neritic copepod Acartia clausi. In the first 3 experiments, food-limiting conditions (suboptimal growth) were imposed and the food supply simulated: (1) different concentrations of a single pulse of phytoplankton; (2) different declining phytoplankton concentrations; or (3) a double pulse of phytoplankton. In all 3 cases positive acclimation of ingestion rates and digestive enzyme activities (amylase, laminarinase and trypsin) was observed. Trypsin appeared to have a low threshold of sensitivity to food changes suggesting that the copepods may enhance their potential for protein digestion at low food concentrations. In the fourth experiment we confirmed that under saturating food concentrations, copepods showed increased food intake and intermediate metabolism but no acclimation of their digestive enzyme systems. Except during starvation, day-to-day patterns of ETS and GDH activities did not covary with food quantity, although ETS varied with food quality and food type. Contradictions between previously published and present results are discussed and confirm that the time acclimation hypothesis of Mayzaud & Poulet (1978; Limnol. Oceanogr. 23: 1144-1154) applies only for food-limiting conditions. Under the present experimental conditions, A. clausi appeared to require 48 h to significantly acclimate to changes in food ration exceeding a threshold of 1.5 to 2-mu-gC ind.-1. The time acclimation hypothesis should be revised to emphasize that nutrition is likely controlled by feedback mechanisms which operate to meet the metabolic requirements of individuals and that acclimation of the different aspects of nutrition (ingestion and digestion) take place over different time scales