FLOUNDER METAMORPHOSIS - ITS REGULATION BY VARIOUS HORMONES

Metamorphosis in the flounder has often been compared with the transit ion of tadpoles into frogs. The dorsal fin rays of the Japanese flound er (Paralichthys olivaceus) elongate during prometamorphosis when thyr oid hormone levels are low, and are resorbed during metamorphic climax when thyroid hormone levels are high. Using an in vitro system for th e culture of the flounder fin rays, we have examined how various hormo nes affect the resorption process. Both thyroxine (T4) and triiodothyr onine (T3) directly stimulated fin ray shortening, T3 being more poten t than T4. Other hormones, such as prolactin, cortisol and sex steroid s, did not directly affect the resorption process but modified the tis sue'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 metam orphosis, and related these with the thyroid hormone profiles in order to get a better picture of their interactions. The gaps in the presen t status of research on the role of thyroid hormones during metamorpho sis in the Japanese flounder are also discussed.