MEDIUM TERM TIME ACCLIMATION OF FEEDING AND DIGESTIVE ENZYME-ACTIVITY IN MARINE COPEPODS - INFLUENCE OF FOOD CONCENTRATION AND COPEPOD SPECIES

Inter-species differences in the acclimation strategy of the nutritional processes (ingestion, digestive enzyme activity) of female and copepodite stage V copepods were established experimentally for 3 different species: Acartia clausi, Eurytemora herdmani, and Drepanopus pectinatus. In all experiments food was supplied over 4 to 7 d at 2 limiting concentrations of the cultured diatom Skeletonema costatum. The first 3 experiments confirmed earlier results suggesting that under food limiting conditions, Acartia-type copepods (low level of energy reserve, high turnover of its biomass) displayed positive acclimation of both ingestion rate and digestive system. Under the experimental conditions used, both A. clausi and E. herdmani required 48 h to significantly acclimate their digestive activity to changes in food ration exceeding 1 mug C copepod-1. Smaller but sustained differences in food ration could result in similar behaviour but seem to require a longer time period to show significant acclimation. Despite larger food rations, experiments with the lipid-rich species D. pectinatus failed to show any acclimation of digestive enzyme activities, except for individuals collected before a summer bloom which displayed partial positive acclimation of their trypsin activity. These inter-species differences agree with the hypothesis that internal control of the acclimation process is governed by the metabolic requirement of the animals. In addition, the differential response of D. pectinatus collected before and after a phytoplankton bloom suggests an important role for past feeding conditions (physiological memory), thus explaining the variability in responses between natural field conditions and laboratory conditions. Physiological adaptation is proposed as a mechanism which minimizes the effect of stressful trophic conditions.