Autotrophic microalgae are in general used as prey for copepods in laboratory experiments and in aquaculture mass culturing. We tested the suitability of using the osmotrophic thecate dinoflagellate Crypthecodinium cohnii as an alternative prey for the live prey organism for fish larvae, the planktonic calanoid copepod Acartia tonsa. We found that A. tonsa fed and transformed ingested C. cohnii into new production well, although the gross growth efficiency was somewhat lower (~22%) than those reported in the literature when fed the autotrophic microalgae Rhodomonas salina (> 36%). We also compared the egg hatching success of eggs produced by the copepod when fed C. cohnii and R. salina and found a slightly lower hatching in eggs produced based on C. cohnii (60%) than on R. salina (89%)-fed copepods. The dinoflagellate C. cohnii is reared in the dark in bioreactors where it can obtain by far higher cell concentrations and biomasses per unit time and volume than the autotrophic prey R. salina reared in photobioreactors. Biochemical composition among the two prey showed that the carbon and nitrogen content was not very different; however, their fatty acid content deviated. The total fatty acids were ~ 17% of the cell carbon in R. salina whereas ~ 9% of the total cell carbon in C. cohnii. Moreover, ~ 18% of the fatty acids were EPA in R. salina, whereas EPA was mostly absent in C. cohnii. In contrast, ~ 63% of the fatty acids were DHA in C. cohnii compared to 12% DHA in R. salina. The trade-off of switching to the heterotrophic dinoflagellate diet is that the copepod performance is about 40% lower. Still, we propose that eliminating light in the rearing of copepod feed makes C. cohnii an interesting alternative and an economical feasible feed worth pursuing in large-scale rearing of copepods.