Flounder metamorphosis: its regulation by various hormones.

Metamorphosis in the flounder has often been compared with the transition of tadpoles into frogs. The dorsal fin rays of the Japanese flounder (Paralichthys olivaceus) elongate during prometamorphosis when thyroid hormone levels are low, and are resorbed during metamorphic climax when thyroid hormone levels are high. Using an in vitro system for the culture of the flounder fin rays, we have examined how various hormones affect the resorption process. Both thyroxine (T4) and triiodothyronine (T3) directly stimulated fin ray shortening, T3 being more potent than T4. Other hormones, such as prolactin, cortisol and sex steroids, did not directly affect the resorption process but modified the tissue's response to thyroid hormones. Similar observations were obtained from in vivo studies. We also monitored the changes in the whole body concentrations of various hormones during early development and metamorphosis, and related these with the thyroid hormone profiles in order to get a better picture of their interactions. The gaps in the present status of research on the role of thyroid hormones during metamorphosis in the Japanese flounder are also discussed.