Abstract
The tissue concentration of L-thyroxine (T4) was measured by radioimmunoassay for whole bodies of laboratory-reared black sea bream larvae from 12 to 38 days after hatching. T4 level was less than 2ng/g body wet weight in early postlarvae. It increased markedly from 1.7ng/g on day 19 to 7.0ng/g on day 24, and then gradually decreased to 2.4ng/g by day 33, followed by the second elevation to 5.0ng/g on day 38. The first surge, occurring on day 24, corresponded to the notochord postflexion phase (Fukuhara's I stage) at which average standard length was 6.1mm. Previous field research on the early life history of this species suggests that this phase corresponds to the critical phase when the larvae migrate from coastal waters to near-shore nursery areas. Thus, it seems possible that thyroxine is involved in habitat transfer during the early ontogeny of the black sea bream.
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