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

Calanus finmarchicus C5 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. finmarchicus C5s 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. Calanus finmarchicus is a dominant component of the zooplankton in the North Atlantic. As with many members of the genus Calanus, the life cycle of C. finmarchicus includes an overwintering stage characterized by high levels of stored lipids and reduced metabolic expenditures during the C5 copepodite stage, resembling in many respects the diapause stage of insects (reviewed in Hirche 1996). In the Gulf of Maine, C. finmarchicus typically goes through two generations beginning in late December–January when overwintering C5s mature to adulthood and ending in the summer months as the C5 copepodites enter dormancy and sink to deep (300–400 m) waters (Meise and O’Reilly 1996; Miller et al. 2000). Overwintering C. finmarchicus C5s display distinct physiological characteristics. Activity levels are low and oxygen consumption is reduced (Hirche 1983; Ingvarsdóttir et al. 1999), lowering overall energy demand. During overwintering the digestive system is greatly reduced, with reduced gut epithelium and low digestive enzyme activities (Hallberg and Hirche 1980; Hirche 1983). Lipid storage is pronounced and may account for as much as 50% of body volume (Miller et al. 2000). 1 Corresponding author ([email protected]).