HERBIVOROUS OR OMNIVOROUS - ON THE SIGNIFICANCE OF LIPID COMPOSITIONS AS TROPHIC MARKERS IN ANTARCTIC COPEPODS

Three dominant Antarctic copepods, Calanoides acutus, Rhincalanus gigas and Metridia gerlachei (copepodite stages V and females), were collected during summer (January/February) in the southern Weddell Sea south of 70-degrees-S. Detailed analyses of their lipid and fatty acid/alcohol compositions were carried out. The trophic positions of these copepods were elucidated by means of the lipid compositions (''marker lipids''). High amounts of wax esters were found in C. acutus (92% of total lipids) and in R. gigas (84-86%). The level of wax esters in M. gerlachei was relatively low (27-42%), while the accumulation of triacylglycerols tended to be higher (19-22%). Characteristic lipid components of C. acutus were the long-chain monounsaturated fatty acids and fatty alcohols 20:1 (n-9) and 22:1 (n-11). These components together with elevated amounts of the 18:4 (n-3) and, to a lesser degree, of the 16:1 (n-7) fatty acids, typical of phytoplankton lipids, indicate herbivorous feeding for C. acutus. Other abundant fatty acids were 20:5 (n-3) and 22:6 (n-3). The fatty acid composition of M. gerlachei was characterized by very high amounts of these 22:6 and 20:5 acids. Other important fatty acids were 18:1 (n-9) and 16:0, but only small amounts of 16:1 (n-7) and 18:4 (n-3) occurred. In contrast to C. acutus the fatty alcohols of M. gerlachei consisted almost exclusively of the short-chain components 14:0 and 16:0. M. gerlachei is known as an omnivorous species, which was clearly reflected by its lipid and fatty acid/alcohol pattern. Few data are available on the feeding of R. gigas, but it is usually described as an herbivorous small-particle feeder. R. gigas showed fatty acid/alcohol characteristics typical of either C. acutus or M. gerlachei. Higher amounts of the 16:1 (n-7) and 18:4 (n-3) fatty acids suggest herbivorous feeding, whereas the dominance of short-chain alcohols (14:0 and 16:0) resembled the lipid pattern found in the omnivorous M. gerlachei. Hence, the lipid composition of R. gigas showed an intermediate pattern, which implies a tendency towards an opportunistic feeding mode, positioned somewhere between the other two species.