LIPIDS AND LIFE-CYCLE STRATEGY OF A HYPOLIMNETIC COPEPOD IN LAKE-MICHIGAN

Concentration and composition of lipids varied seasonally in adult female Diaptomus sicilis, a hypolimnetic suspension-feeding calanoid copepod, in Lake Michigan. Triacyl-glycerol (TG) was the predominant storage lipid; its level remained high (33-40% of dry wt) from June through January. During the period of reproduction from February through May, TG dropped steeply to a low of 9%. The abrupt increase in TG concentration from May to June was probably caused by the recruitment of non-reproducing adult females. Throughout summer and fall ovaries remained unripe. Feeding experiments at satiating food concentrations at 6-degrees and 20-degrees-C suggest that low temperature and the hypolimnion was a major reason that ovaries did not ripen. Stored TG served as a potential buffer to starvation, although starvation conditions did not occur. By remaining in the hypolimnion rather than in the epilimnion, D. sicilis is assured a relatively stable supply of algae, low metabolic rate, and escape from predation. These advantages may have to be weighed against a reproductive bottleneck of low temperature.