The scheduling of spawning with the moult cycle in Northern krill (Crustacea : Euphausiacea): a strategy for allocating lipids to reproduction

Euphausiids moult and grow throughout their life, which implies sharing of resources between growth and reproduction for adult krill. In the Northern krill, Meganyctiphanes norvegica (M. Sars), female krill produce eggs cycli cally. Spawning moult cycles alternate with vitellogenic moult cycles for l ipid yolk accumulation. Histology shows that lipids are associated with the R cells of the digestive gland in both sexes, with the yolk platelets of m ature oocytes and with the fat body cell membranes and blood lacunae in rep roducing females. Mature female krill can have a total lipid content twice as high as males, mostly due to accumulation in the ovary, the fat body and the haemolymph. In contrast, in males, as well as in non-reproducing femal es, the highest percentage of lipids is found in the digestive gland and th e haemolymph. In Meganyctiphanes norvegica, the most abundant lipid fractio ns are polar lipids and triglycerides, the latter being relatively low in r eproducing female gonad and fat body. Triglycerides are believed to be a pu re energy source and polar lipids are essential for membrane development in embryos. The fatty acid content and composition of the triglyceride and po lar lipid fractions in females are different from males, related to both re productive and dietary processes. Higher levels of polyunsaturated fatty ac ids (PUFA) in the polar lipid fraction were found in reproductive females. During the non-reproductive season, the converse was found, indicating the specific role PUFA and other fatty acids play in growth and egg production. Adaptive processes linked to reproduction were studied comparatively in th ree populations of the Northern krill - Clyde Sea (W, Scotland), Kattegat ( E, Denmark), Ligurian Sea (Mediterranean) - all differing considerably in c limatic and trophic conditions. Such adjustments in lipid synthesis and sto rage are viewed as reproductive strategies developed by the Northern krill in response to different environmental conditions.