Is the fatty acid composition of Daphnia galeata determined by the fatty acid composition of the ingested diet?

1. The fatty acid (FA) composition of Daphnia galeata and their algal food was analysed and showed many similarities, however, some significant differences were also found in the relative abundance of the FA C16:4 omega 3 and docosahexaenoic acid (DHA). Their relative abundances were much lower in daphnids than in their algal diet. 2. When daphnids were fed three distinct emulsion particles with DHA: eicosapentaenoic acid (EPA) ratios of c. 0.7, 2 and 4, the final DHA: EPA ratio in the daphnids always favoured EPA. The increase of the food DHA : EPA ratio resulted in a minor increase of DHA (to c. 2%). Feeding the animals on emulsion particles with increasing ratios of DHA : EPA, caused a minor (c. 2%) increase of DHA level but EPA levels remained high (c. 10%). 3. When labelled with [C-14]linoleic acid and [C-14]linolenic acid daphnids showed low conversion of both essential FA into C20 polyunsaturated fatty acids (PUFAs). This low conversion activity might explain the importance of C20 PUFAs as dietary compounds in the food of Daphnia. 4. The results indicate the insignificance of DHA and C16:4 omega 3 for daphnids. As EPA can be derived from C18:3 omega 3 it is not strictly essential, although it might be a significant factor in food quality for Daphnia. [KEYWORDS: Insects; microalgae; metabolism; quality; lipids; magna