Effects of algal diet on digestive enzyme activity in Calanus helgolandicus

Adult female Calanus helgolandicus were transferred, immediately after collection in the English Channel in June 1984, to two unialgal diets one of which, the flagellate Cryptomonas maculata, was rich in starch, while the other, the diatom Thalassiosira weissflogii, contained no starch. The activity of the digestive enzymes amylase, trypsin and laminarinase was measured in these two populations under foodsaturating conditions over an acclimation period of eight days. Ingestion rates were measured on a daily basis and the results confirmed, together with a constant level of body protein, that the experimental conditions were above the incipient limiting concentration. In the long-term (4 to 8 d), the activities of all three enzymes were significantly elevated in the C. maculata-fed copepods, whereas ingestion rates were lower than those on T. weissflogii. These results observed under food-saturating conditions indicate a compensatory mechanism between digestive enzymes and the substrate ingested. They are consistent with previous work on Artemia sp. suggesting that the rate of assimilation is a function of the digestive enzyme activity and ingestion rate. Enzyme activity exhibited differing shortterm responses (<48 h) on transfer to the two algal diets, which are interpreted in relation to the in situ activity of the field population.Communicated by J. Mauchline, Oban