Early larval development and metamorphosis in the summer flounder: changesin per cent whole-body water content and effects of altered thyroid status

At the end of premetamorphosis, summer flounder Paralichthys dentatus larva e had 84.1% whole-body water content (WBW), which decreased to the lowest l evels (81.5%) at the start of metamorphic climax (MC). During mid- and late MC, %WBW was slightly higher (82.1%) then returned to the lowest levels at the juvenile stage. In fish treated with thyroxine (T-4-Na salt, 100 ng ml (-1)) beginning at premetamophosis, %WBW never differed from controls of th e same age throughout metamorphosis, despite an earlier start of metamorphi c climax and transitional settling behaviour. This suggests that thyroid ho rmones do not mediate the drop in %WBW which accompanies natural metamorpho sis. Thiourea(TU, 30 mu g ml(-1)) treatment of fish over the same period in duced a developmental stasis in early MC which was accompanied by initially higher %WBW than controls at 33 days post-hatch, followed by a progressive decrease to abnormally low %WBW by 42 and 45 days post-hatch. Since concur rent treatment with TU + T-4 rescued the fish from both the TU-induced deve lopmental stasis and abnormally low %WBW, these findings suggest that thyro id hormones, or thyroid hormone-mediated developmental progression, are nec essary for regulating %WBW. (C) 1999 The Fisheries Society of the British I sles.