| Summary: | Environmental (temperature, salinity) and nutritional (DHA, EPA, ArA, vitamin A, phospholipids, iodine) factors during larvae rearing largely dictate the successful transformation of larvae to juveniles during metamorphosis which, in turn, determines juvenile quality. Studies on Atlantic halibut, turbot and Japanese flounder report higher metamorphic success, in terms of pigmen- tation, eye migration and general development, when copepods, rather than enriched Artemia, were fed to larvae. Copepods have higher levels of vitamin A, which is required for the synthe- sis of rhodopsin in the retina, a critical visual pigment in the rods necessary for vision at low light intensities. Deficient rhodopsin affects neural transmission from the retina via the central nervous system that triggers pituitary production of melanophore stimulating hormone leading to reduced melanin synthesis and pigmentation deficiency. DHA, an abundant PUFA in copepods, is also vital to vision as it provides the membrane fluidity necessary for rhodopsin to function when stim- ulated by light. The essential fatty acids EPA and ArA are more involved in eicosanoid synthe- sis. These highly potent metabolites are thought to regulate the mechanisms involved in the release of melanophore stimulating hormone and pigmentation. Thyroid hormones play a major role in regulating many developmental processes that occur during metamorphosis. Immersing different age marine fish larvae into various concentrations of thyroid hormone has been shown to synchronize and shorten the duration of metamorphosis in a dose dependent manner in species such as grouper. However, the effect of this immersion on survival varied with the type of thyroid hormone, dose and timing of application. In some species, such as the European seabass (Dicentrarchus labrax), females grow up to 40% faster than males. However, when this species is intensively cultured, masculinization can result in a 70-90% male population. A num- ber of studies have shown that manipulating temperature and salinity during larviculture can result in higher quality juveniles, i.e., a higher percent of faster growing females.
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