Metabolic rate reduction as an overwintering strategy: just krill, or other species too?

J.J. Torres, J. Donnelly, J. Bellucci, M. Parker

Euphausia superba is clearly the dominant pelagic crustacean species on the Antarctic Peninsula Shelf, comprising up to 98% of the total micronekton/zooplankton biomass. It is the prey of choice for most species in the intermediate and upper trophic levels, and thus is the primary vehicle for moving phytoplankton energy up the trophic pyramid. Understanding E. superba's overwintering strategies is a central concern for understanding the shelf ecosystem as a whole. Although dominant, E. superba is a member of a community of about 20 common pelagic species in shelf waters, including several other crustaceans that must endure the same swings in primary production during the progression of the Antarctic seasons. It is important to decipher other species' mechanisms of dealing with the winter months, both to aid our understanding of how Euphausia superba overwinters and to increase our knowledge of the Antarctic marine system as a whole.

Results from our SO GLOBEC physiology studies show that Euphausia superba juveniles and adults exhibit a drop in metabolism of about 50% over the winter months when compared to summer rates. One question which is of primary interest is whether larvae show a similar drop in metabolism from fall to winter. Our results suggest a smaller but significant drop in metabolism in E. superba furcilia during the change from fall to winter. Due to their inability to accumulate a substantial energy depot, krill furcilia are probably constrained to feed. Does the same overwintering strategy apply to adults and larvae? We think yes. Our data also suggest that metabolism drops substantially in the euphausiids, Euphausia triacantha and Thysanoessa macrura. Copepods that we have worked up sufficient data to examine, Calanus propinquus and Metridia gerlachei, also show a drop. It appears that metabolic reduction is a widespread response to the Antarctic winter. What we need to unravel is whether the reduction is simple starvation, or photoperiod-related drop in metabolism that is mediated hormonally, similar to insect diapause.