Diet of the Antarctic krill ( Euphausia superba Dana): II. Selective grazing in mixed phytoplankton assemblages

Diatoms, prymnesiophytes, and cryptophytes are the major taxonomic classes of phytoplankton available to Antarctic krill as primary food resources. However, the relative contribution of each class to the diet of krill is unknown. In this study, selectivity for different phytoplankton taxa by Euphausia superba was examined during laboratory experiments with E. superba grazing on mixed phytoplankton assemblages from the wild and from laboratory cultures. Clearance rates and changes in relative concentrations of diatoms, prymnesiophytes and cryptophytes were measured by analysis of taxon-specific accessory photopigments with high performance liquid chromatography (HPLC). E. superba grazed diatoms at higher rates than Phaeocystis and cryptophytes. Grazing was negligible in cryptophyte-dominated assemblages. Increases of up to 369% in 19′-hexanoyloxyfucoxanthin (HF)/fucoxanthin (FUCO) (prymnesiophyte/diatom) ratios indicated a high level of selectivity for diatoms over Phaeocystis. This selectivity occurred even for Phaeocystis colonies of similar size to diatoms, and thus cannot be entirely attributed to differential sieving efficiency based on particle size. The results suggest that krill actively select diatoms in phytoplankton mixtures.