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SYNOPSIS. We have conducted three hatchery-scale experiments designed to examine the actions and interactions of cortisoi and thyroid hormones in the development of a larval marine fish. Survival among controls varied significantly between the 3 replicate experiments. The threadfin (Polydactylus sexfilis) consistently responds to 1 hr posthatch immersion in a combination of triiodothyronine and cortisoi (T 3 + F) with accelerated gut development and increased survival compared with untreated controls (C). Survival among larvae treated with T, or F separately was significantly improved over controls in one of the three experiments. The frequency of spinal deformities was reduced by cortisoi treatment, alone or in combination with T 3 . Growth did not vary with treatment, except that variance in larval length was reduced in (T, + F) vs. C in all 3 experiments. A hormoneinduced increase in uniformity could lead to reduced cannibalism, which is a problem in the culture of threadnns. These results suggest interactive hormonal regulation of developmental processes, working within the context of other biological variables.

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