J. Cuzin-Roudy, E. Albessard, P. Virtue, P. Mayzaud
Abstract:
Krill molt as adult, which implies a share of resources between growth and reproduction. Species like the Northern krill, Meganyctiphanes norvegica, and the Antarctic krill, Euphausia superba, are adapted to contrasted climatic and environmental conditions, and share the same basic pattern for reproductive physiology. Reproduction is seasonal and limited to the period of the year when food resources are at a maximum level. Maturation of egg batches is cyclical and several spawning periods are possible for individual female krill, while in male krill, sperm production is permanent during the whole reproduction season. Female krill release their eggs during the premolt of "spawning molt cycles", which alternate with "vitellogenic molt cycles" for lipidic yolk accumulation in successive egg batches. Reproducing krill molt more frequently than sexually resting krill, intermolt period is shorter and temperature dependent.
Mature female krill have a total lipid content twice as high as males, mostly due to accumulation in ovary, but also in the fat body and hemolymph. In Northern krill, the most abundant lipid fractions are polar lipid and triacylglycerol, the later being absent from reproducing male gonad and fat body. Triacylglycerol is believed to be a pure energy source and polar lipids are essential for membrane development in embryos. The fatty acid content and composition of the triacylglycerol and polar lipid fractions in female Northern krill compared to male krill show differences related to both reproductive and dietary processes. Higher levels of polyunsaturated fatty acids (PUFA) in the polar lipid fraction were found in reproductive females compared to males. During the non reproductive season, the converse of this situation was found, indicating the specific role PUFA and other fatty acids play in growth and egg production.
In both krill species, the fat body development is seasonal and in parallel to ovarian development. Histology shows that lipids are associated with the yolk platelets of mature oocytes and also with the fat body cell membranes and blood lacunae. Thus, lipid synthesis and storage during vitellogenesis appear closely linked with fat body activity. The hemolymph certainly plays an important role in transporting dietary lipids metabolized in the digestive gland. In males, as well as in non-reproducing females, a higher percentage of lipids are found in the digestive gland and hemolymph than in gonad and fat body.
Adaptive processes linked to reproduction in krill were studied comparatively in 3 populations of the northern krill: Clyde Sea (W-Scotland), Kattegat (E-Denmark), Mediterranean (Ligurian Sea), and in Antarctic krill. They are expressed as an adjustment of the various phases of ovarian development in parallel with the timing of growth and molt preparation and will be discussed as reproductive strategies developed by krill in response to various environmental conditions.
(LOBEPM, Observatoire Oceanologique, UMPC-CNRS-INSU, BP 28-06230 Villefranche sur mer, France)