Metamorphosis in the summer flounder (Paralichthys dentatus): stage-specific developmental response to altered thyroid status.

The summer flounder (Paralichthys dentatus) undergoes a true metamorphosis during which the bilaterally symmetrical larva transforms into an asymmetrical juvenile. This study addresses the influence of manipulating thyroid status on larval development and growth. Development was assessed by categorizing larvae in stages based on the position of the translocating eye and growth was assessed as a change in total length. Larvae were raised in seawater or in seawater containing thyroxine-sodium salt (100 ppb, T4) or thiourea (30 ppm, TU). Ambient T4 raised whole-animal T4 concentration almost threefold, whereas treatment with TU reduced whole-animal T4 concentration by 95%. The whole-animal T4 concentrations of untreated larvae increased during metamorphic climax (MC) and were related to developmental stage rather than age. Altering thyroid status of larvae in different stages had different consequences. T4 treatment of late premetamorphic (late pre-M) larvae accelerated their rate of development to prometamorphosis (pro-M), early MC, and mid MC, whereas TU treatment at this stage caused developmental stasis in early MC. T4 treatment of pro-M larvae accelerated development to early MC only and, unlike the response of late pre-M larvae, some of the pro-M larvae treated with TU slowly developed to late MC. TU treatment could inhibit completion of metamorphosis in early MC and mid MC, but not in late MC. Thyroid status of larvae had no effect on growth. However, starting at an age when most untreated and T4-treated larvae were in late MC, they began to grow longer than TU-treated larvae in developmental stasis at early MC and mid MC. Thus, T4 is necessary and sufficient for metamorphosis in summer flounder and, at this single dose, has a more pronounced effect on development at earlier stages.