Lipid biochemistry of Antarctic euphausiids - energetic adaptations and a critical appraisal of trophic biomarkers

The energetic and physiological condition of various ontogenetic stages of Antarctic euphausiids was examined at the onset of winter. Field and experimental data were used to evaluate the relative importance of alternative overwintering strategies for Euphausia superba. Furcilia III larvae had low lipid levels, mainly phospholipids (PL), high metabolic and feeding activities, with the main diet being diatoms. Hence, the larvae exhibited a business as usual strategy. A clearly deviating behaviour was observed in juvenile and adult E. superba, with high lipid depots, mainly as triacylglycerols and PL, and low metabolic and feeding rates as compared to summer values. These data indicated a compromise overwintering strategy between switch feeding and energy conservation for postlarval E. superba.Long-term feeding experiments were carried out to examine the effect of various diets on the lipids and stable isotopes of larval, juvenile, and adult E. superba. Total lipid content, lipid class, fatty acid and stable isotope compositions showed very little variation with the different feeding regimes in postlarval krill. The weak signal of the trophic marker fatty acids was attributed to the large lipid reserves buffering short-term variations in dietary lipid supply on the one hand, and inefficient utilisation of the offered food on the other hand. Furciliae were influenced more strongly by the fatty acid and stable isotope signatures of their food. Changes in fatty acid composition, combined with significant lipid build-up, could be induced experimentally and were also detected in field samples from different locations. The faster growth and the active food utilisation and assimilation of furciliae in conjunction with their lower lipid levels favour the incorporation and detection of trophic biomarkers.The lipid biochemistry of three further Antarctic euphausiids, E. frigida, E. triacantha and Thysanoessa macrura, was studied and discussed in comparison to E. superba.