Fatty acids, vitamin A and thyroid hormone have all been shown to affect development of flatfish larvae and they are ligands to nuclear receptors that participate in the control of development. Our hypothesis was that one of these factors or an interaction between them may be the cause of abnormal development of flatfish larvae. Atlantic halibut larvae were fed either DHA-elcoenriched Artemia or copepods from first feeding. In fish that had been fed Artemia, only 7% had normal pigmentation and 10% normal eye migration. The numbers for fish fed copepods were 68% and 88%, respectively. Malpigmented fish fed Artemia were depigmented, while those fed copepods had ambicoloration. The differences in development were probably nutrient dependent, since all other conditions were similar for the two groups. Larvae fed copepods had markedly higher body levels of docosahexanoic acid (DHA, 22:6n–3) and eicosapentaenoic acid (EPA, 20:5n–3) and lower levels of arachidonic acid (ARA, 20:4 n–6) than larvae fed Artemia. The DHA/EPA ratio was similar in the two groups, but the EPA/ARA ratio was more than four times higher in larvae fed copepods than in larvae fed Artemia. Larvae fed copepods had higher body levels of total retinol than larvae fed Artemia, but the difference was due to higher levels of the storage forms, retinyl esters, whereas the levels of free retinol and retinal were similar in the two groups. The level of iodine was 700 times higher in copepods than in Artemia and 3–4 times higher in larvae fed copepods than in larvae fed Artemia. There was a significantly higher level of T4 in larvae fed copepods during the “window of opportunity”, 15–30 days after first feeding. In an experiment where Atlantic halibut larvae were fed Artemia enriched in iodine up to the levels found in copepods, there was a significant effect on the body level of iodine and a non-significant tendency of higher levels of thyroid hormone, but no effect on pigmentation or eye migration. It is concluded that Artemia probably offers a sufficient access to vitamin A precursors to meet the larval requirement. More research should be done to elucidate possible effects of iodine on development of Atlantic halibut larvae. Fatty acid composition is still the most likely candidate for causing abnormal development in Atlantic halibut larvae.
Flatfish larvae fed rotifers and Artemia often develop malpigmentation, resulting in juveniles with their ocular side partially or totally white and sometimes with ocular pigmentation on the blind side. This also applies to Atlantic halibut larvae, which in addition often gets impaired eye migration, where the right eye fails partly or totally to migrate to the ocular side during metamorphosis. These developmental errors are seldom seen in larvae fed copepods, the main prey item in the wild, and different nutrient composition of Artemia and copepods is a possible explanation for the differences in development. We have therefore looked at differences in nutrient composition in copepods and Artemia and in larvae fed them and studied morphological and physiological responses in the larvae to the two feed organisms. The studies are based on literature describing effects of nutrition on metamorphosis in flatfish, and more generally, effects of nutrients on vertebrate development.