Combined application of cortisol and triiodothyronine in the culture of larval marine finfish

Newly-hatched larvae of Pacific threadfin (or 'moi'; Polydactylus sexfilis) exposed to triiodthyronine (T-3; 2.6 p.p.m.) and cortisol (0.1 p.p.m.) by immersion for 1 h exhibited earlier development than untreated controls and larvae exposed to the individual hormones. The onset of initial gut formation and initial somatic pigmentation were accelerated by this treatment. Survival to 29 days of age was also improved (P<0.05, Experiment 1; P<0.005, Experiment 2) in hormone-treatment groups as compared with untreated controls. Our ongoing experiments address two working hypotheses: (1) advanced digestive function induced by T-3 and cortisol imparts improvements in food utilization during the critical time of first-feeding and (2) uniform growth following hormone treatment leads to reduced rates of cannibalism. These results appear to be consistent with established development-promoting interactions of T-3 and cortisol in Japanese flounder, amphibians, and other vertebrates. These observations suggest that hormone therapy may have practical utility in the culture of marine larvae.