Reproductive strategies of the Mediterranean krill,Meganyctiphanes norvegicaand the Antarctic krill,Euphausia superba(Crustacea: Euphausiacea)

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Abstract
Reproductive strategy was studied in two euphausiid species. Euphausia superba occurs in large aggregations south of the Antarctic Convergence, living in extreme climatic conditions. In the Mediterranean Sea, populations of Meganyctiphanes norvegica experience moderate seasonal variations and a stable temperature regime (13°C). Both species release their eggs in the water column and have similar basic patterns of development. The early seasonal development of the gonads of E. superba (especially ovarian previtellogenesis) is associated with the ice-edge in spring. Egg production is strictly limited to the summer (December, January and February), but multiple spawns are then produced by individual females through a succession of short vitellogenic cycles. Both males and females undergo a sexual regression in winter when only basic activity of the gonads (gametogenesis) is maintained. Ovarian development of M. norvegica starts in January (previtellogenesis) in the Ligurian Sea before the spring bloom. Eggs are produced from February to May by successive vitellogenic cycles. Gonadal activity is reduced during summer and autumn. This timing is different from the northern populations of the species that rather spawn in spring and summer. In both species the seasonal variability in food availability is tempered by storage of carbohydrates and lipids in the fat body for subsequent yolk accumulation in the eggs. These two krill species appear to have adapted their reproductive cycle to different habitats by using the flexibility of the physiological cycles involved in gonad development, a strategy that allows them to enhance fecundity and to tune the reproductive effort with food availability for the offspring.