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The nutritional suitability of various algae and cyanobacteria, and the influence of food composition and concentration on development time, fecundity and body size of Eudiaptomus gracilis were studied. The phytoflagellates, Cryptomonas sp. and Rhodomonas minuta, were found to be suitable single food resources for complete juvenile development and for egg production. Uni-algal diets of the green flagellates, Chlamydomonas reinhardii and Chlamydomonas sphaeroides, were sufficient for naupliar, but not for copepodite, development. Cyanobacteria, diatoms and coccalean green algae did not support juvenile development. The lowest food concentration of Cryptomonas sp. on which complete development occurred was 0.1 mgC/l. Development time decreased, and body length increased, with rising food supply. Egg production occurred at a food concentration of 0.2 mg C/l and above, and dutch size increased with food density.
The addition of toxic bluegreens to a diet of Cryptomonas sp. resulted in a slightly delayed juvenile development compared to a diet containing only Cryptomonas, but no mortality was found which could be attributed to the presence of toxic bruegreens. The findings are discussed in relation to the importance of food availability for the ontogenetic development of Eudiaptomus gracilis and the interaction between cyclopoid and calanoid copepods. Nauplii are probably the developmental bottleneck within both groups, but threshold food concentrations are lower in calanoid than cyclopoid copepods. Therefore, calanoids should have an advantage over cyclopoids under conditions of food scarcity. The inability of cyclopoids to develop at food concentrations as low as those at which calanoids develop is expected to reduce the predation pressure on calanoid instars exerted by the carnivorous cyclopoid stages
