Rhincalanus gigas and Calanus simillimus: lipid storage patterns of two species of copepod in the seasonally ice-free zone of the Southern Ocean

The lipid and hydrocarbon composition of two species of Antarctic copepod, Rhincalanus gigas and Calanus simillimus, was investigated at two contrasting sites. Differences in the quantity of total lipid between sites were pronounced for R. gigas; females from a station near South Georgia where a bloom was in progress contained similar to 8 times as much as those sampled in post-bloom waters in the Polar Frontal Zone (PFZ) some 450 km further north. In contrast, differences between sites for C. simillimus were less pronounced. The main lipid class for R. gigas was wax ester and for C. simillimus triacylglycerol. This fundamental difference is thought to reflect varying life-history patterns suggesting that C. simillimus may not undergo periods of pronounced food shortage. Pristane, a metabolite of phytol derived through the degradation of chlorophyll, was present in both species at both sites, indicating recent feeding activity, but the lack of the polyene diatom marker C-21:6 in C. simillimus at the PFZ station suggested that it was largely feeding on other microplankton. Fatty acid analysis of C. simillimus offered further evidence that this species was omnivorous and that R. gigas was predominantly herbivorous. 16:0 and 16:1 generally accounted for slightly >50% of total fatty acids in both species; however, 16:0 was proportionately more abundant in C. simillimus and 16:1 in R. gigas. The 16:1/16:0 ratio is usually >1 in diatoms, suggesting that the diet of C. simillimus contained items of prey other than diatoms, a fact confirmed in grazing experiments.