Rythmes nutritionnels et organisation trophique d'une population de crustac�s p�lagiques (Euphausiacea)

Abstract

The nature of the food (animal, plant or mixed) and the fullness of the stomachs at different times of the day have been studied through dissection of 18620 specimens representative of almost all the euphausiid species of the Central and Western Tropical Pacific Ocean. Animal food predominates in 22 of the 28 species studied, while 12 can be considered as omnivorous; only 4 are mainly phytophagous. The trophic level of a given species is more or less the same in different zones, but scarcity of phytoplankton in oligotrophic tropical regions results in an increase of the trophic level of herbivorous and omnivorous species. There is no correlation between trophic level and vertical distribution of a species. Each species follows a clearly defined feeding rhythm, usually characteristic for each genus: nutrition most active by night in Euphausia, continuous in Thysanopoda, restricted to light hours in Stylocheiron, mainly from noon to midnight in Nematoscelis and Nematobrachion. As a rule, it is obvious that the smaller the daily vertical migration, the more pronounced the feeding rhythm: the range of fluctuations in fullness of stomachs over 24 h is weak or non-existent in migrating species, maximum in non-migrating ones. Nevertheless, daily vertical migration does not appear to be “advantageous” from the point of view of trophic efficiency: assuming that the stomachal transit is the same for all species (a speculative proposition), it is shown that the ratio “total biomass of species: food consumed during 24 h”, i.e., “biomass permanently available for the upper link: biomass eaten daily at the expense of the lower link” is 4 times higher in non-migrating than in migrating species. It is thus considered that daily vertical migration is an expensive manner to transfer energy from link to link, and therefore fulfills other functions, amongst which diffusion throughout the whole water column of the biomass produced in the upper levels is probably one of the most important.