Lipids as trophic markers in Antarctic krill. II. Lipid composition of the body and digestive gland of Euphausia superba in controlled conditions

To examine the potential of lipids as dietary markers in Euphausia superba Dana, juvenile krill were fed 4 phytoplankton diets (diatoms, cryptomonads, dinoflagellates and thraustochytrids over a range of food concentrations) and 2 species of zooplankton (rotifers and Artemia sp.) for 20 d. Lipid content, lipid class and fatty acid (FA) composition were analysed for whole krill and digestive glands (DG). Changes detected in whole krill were almost entirely associated with the lipid variability observed in DG. With marked diet-induced changes, DG lipids were the major repository for the trophic signal in krill. DG lipid content increased in krill fed on phytoplankton due to the accumulation of polar lipid (0.5 mg per DG) and triacylglycerol (up to 1.7 mg per DG). Herbivorous diets induced distinct FA signatures in the DG, characterised by higher proportions of: 16:1 omega 7 and 20:5 omega 3 in diatom-fed krill; 18:1 omega 9 and 18:4 omega 3 in cryptomonad-fed krill; 18:1 omega 9, 18A omega 3 + 18:5 omega 3 and 22:6 omega 3 in dinoflagellate-fed krill; and 14:0 and 22:6 omega 3 in thraustochytrid-fed krill. Dietary FA markers increased in relative proportion (% total FA) in the DG with increasing concentration of each particular food. Krill fed rotifers or Artemia sp. showed relatively low DG lipid content, with only slightly higher proportions of 18:2 omega 6 than observed with phytoplankton diets. Zooplankton diets induced no significant difference in FA composition in relation to the species of zooplankton used as prey or to the algal type consumed by these prey.