Life-stage-specific differences in exploitation of food mixtures: diet mixing enhances copepod egg production but not juvenile development

Development, egg production and hatching success of the calanoid copepods Temora longicornis and Pseudocalanus elongatus were measured in food mixtures to test their ability to obtain a complete nutrition by combining different nutritionally poor food species. In all the food mixtures used, the copepods failed to moult past the first copepodite stage, and the mortality was high. In sharp contrast, mixing two nutritionally poor food species often resulted in egg production which was not significantly different from nutritionally high quality food, although hatching success in many mixtures was low. Whereas egg production was significantly correlated with particulate organic nitrogen in the diet, and independent of the highly unsaturated fatty acids (HUFAs), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), hatching increased with increasing DHA and EPA concentration. Growth and juvenile mortality were, however, independent of either nitrogen or HUFAs in the diet. Our results show that adult copepods are effective in combining their nutrition from several food sources, whereas juveniles are not. We suggest that there are species- and life-stage-specific differences in nutritional requirements and/or in the ability to digest and/or assimilate essential nutrients from food mixtures, which may significantly contribute to the success of copepod populations in nature.