Physiological characteristics of lipid-rich "fat" and lipid-poor "thin" morphotypes of individual Calanus finmarchicus C5 copepodites in nearshore Gulf of Maine

Abstract

Calanus finmarchicusC5 copepodites may be found in a range of body morphologies from small, lipid-poor (‘‘thin’’) individuals to larger, lipid-rich (‘‘fat’’) individuals, which are often differentiated by depth. In order to assess the physiological status of these animals, C5s collected from nearshore Gulf of Maine were visually sorted by these criteria and assayed for activity of four enzymes characteristic of different physiological processes: citrate synthase (CS, Krebs cycle); glutamate dehydrogenase (GDH, protein catabolism); hydroxyacyl CoA dehydrogenase (HOAD, fatty acid catabolism); and laminarinase (digestive enzyme), as well as protein content. The lipid content of a smaller subset of animals was determined by image analysis of the lipid storage sac. Visual differences among copepods were reflected in their physiological characteristics. Thin C5s had less protein and lipid, and higher citrate synthase and laminarinase activity, than fat C5s. There was no difference in GDH or HOAD activity between the two groups. However, in both groups HOAD was negatively correlated with laminarinase activity, indicating that digestive activity is inversely related to capacity for b-oxidation of lipids. In contrast, C. finmarchicusC5s collected from 200 m in the Gulf of Maine as well as those raised in tanks had lower CS and laminarinase activity than the nearshore C5s, whereas HOAD activity was comparable with that of the fat nearshore C5s. These results suggest that metabolic responses are being induced in C5s advected into nearshore waters.