Laboratory studies of assimilation and egestion of algal lipid by Antarctic krill — methods and initial results

Immature Antarctic krill (Euphausia superba Dana) (length 44–48 mm), which had been maintained in an aquarium, were fed dense suspensions of algal culture, either the haptophyte Isochrysis or the diatom Thalassiosira. Following acclimation to this regime, the animals were transferred to identical supplies of the same algal food, pre-labelled with 14C bicarbonate for 16 h. Faecal pellets were collected individually in each experiment. Following transfer to the radiolabelled food, radioactivity appeared in faecal pellets after 30 min for Isochrysis and 55 min for Thalassiosira. Fitting a first-order kinetic model to the time-course of appearance of radiolabel in faecal pellets, showed that gut turnover (mean residence time of material in the gut) was rapid for krill fed on Isochrysis (turnover time = 47 min) whilst krill took longer to process Thalassiosira (turnover time = 256 min). Uptake rate of radiolabel by the two algae differed by an order of magnitude and was inversely related to cell size. Removal of the two algal species by the krill also differed when expressed as either chanages in radiolabel or biomass but overall, similar amounts of lipid were ingested from the two food sources (817 and 615 ng· ml−1 over 12 h for Isochrysis and Thalassiosira, respectively). Assimilation efficiencies for the two algae, calculated on the basis of radiolabelled and total-lipid ingestion and egestion ranged from 63 to 86%. Most of the radioactivity in the fatty acids of Isochrysis was incorporated into C18 compounds, but this distribution did not correspond to the overall mass composition of algal fatty acids. Saturates, 16:1, 18:4 and 20:5 were labelled to a lesser extent. Radiolabelled fatty acids incorporated into krill gut and somatic tissue differed in composition from the original algal fatty acid pool. In particular, there was higher activity in saturates, in 16:3 and 18:4, whilst 18:1, 18:2 and 18:3 were labelled to a lower extent in krill. Overall, fatty acids showed lower specific activity in krill than in algae, except for 16:3. The distribution of radioactivity in faecal pellets showed a character more strongly related to that of the algal food supply than that of the krill, although the high proportion of label in saturates (predominantly 16:0) was derived from the animals. Much of the ingested fatty acids were subsequently catabolised. Taking all the fatty acids, 69% appeared to be lost by this route, indicating the major importance of fatty acids as an energy source for krill. However, some individual fatty acids were conserved, for example saturates and 18:4. In the latter case, biosynthesis from 18:3 is implied. Overall, the study indicates the importance of constructing detailed budgets in understanding the character and dynamics of feeding by Zooplankton and suggests that control of grazing by quality as well as quantity of the food supply may be a crucial aspect of understanding ocean carbon cycling