Metamorphosis in the summer flounder, Paralichthys dentatus: Thyroidal status influences salinity tolerance

Metamorphosis in the summer flounder (Paralichthys dentatus) is controlled by thyroid hormones (TH) and takes place as the larvae move from a salinity of about 35 parts per thousand (ppt) in the ocean to salinity ranging from 0-35 ppt in estuaries. Historically, the role of TH in juvenile and adult teleost osmoregulation has been ambiguous, and it is not known if TH influe nces larval teleost osmoregulatory development. This study addresses the in fluence of thyroxine (T4) on the development of tolerance to low (5 ppt) an d high salinity (45 and 50 ppt) as determined by salinity tolerance tests. In untreated larvae, tolerance to both low and high salinity was high durin g early premetamorphosis (early pre-M) and decreased or was very low from l ate prometamorphosis (late pro-M) through mid-metamorphic climax (mid-MC). Salinity tolerance in creased 2-3-fold during late MC when whole-animal T4 levels are highest, and reached maximum tolerance at the juvenile stage. Th e early induction of metamorphosis by exposing larvae in pre-M to exogenous T4 reduced tolerance to low salinity during early and mid-MG, though toler ance of fish that had developed into juveniles was not impaired. In contras t, T4 increased high salinity tolerance during early and mid-MG, and the ju venile stage. This T4-induced heterochrony in salinity tolerance with regar ds to developmental stage suggests that the effects of T4 on salinity toler ance may be uncoupled from accelerated metamorphosis. Treatment of larvae w ith thiourea (TU, an inhibitor of T4 synthesis) inhibited metamorphosis and reduced tolerance to high salinity, but did not affect tolerance to low sa linity Reduced tolerance to high salinity by TU was only partially countera cted by T4 treatment, suggesting that TU also affects hypoosmoregulatory ac tivity by an extrathyroidal mechanism. Our findings suggest that in the sum mer flounder T4 plays a more important role in the development of hypoosmor egulatory ability than hyperosmoregulatory ability. J. Exp. Zool. 284..414- 424, 1999. (C) 1999 Wiley-Liss.