Metamorphosis in the summer flounder, Paralichthys dentatus: Thyroidal status influences gill mitochondria-rich cells

Metamorphosis in the summer flounder (Paralichthys dentatus) is mediated by thyroid hormones (TH) and is accompanied by changes in gill mitochondria-r ich cells (MRCs) and in salinity tolerance. Altered thyroid status during l arval development and metamorphosis in this species influences salinity tol erance, though the influence of any hormone on MRCs of larval marine teleos ts is not known. This study characterized the effect of altered thyroid sta tus on MRC intracellular membranes, mitochondria size and ultrastructure, i mmunoreactive (ir)-Na+,K+-ATPase, and cell size and density during metamorp hosis in summer flounder. Inhibition of metamorphosis with thiourea (30 ppm ) (TU, an inhibitor of TH synthesis) inhibited changes in MRCs, producing l arge "larval" type MRCs with weak reactivity to osmium: large, electron-luc ent mitochondria; and weak ir-Na+,K+-ATPase. Replacement of TH with TU + th yroxine-Na salt (100 ppb) rescued the fish from developmental inhibition, p roducing smaller "juvenile" type MRCs with strong reactivity to osmium; sma ller, electron-opaque mitochondria; and strong ir-Na+, K+-ATPase. The findi ngs suggest that TH are necessary for MRCs to change from larval to juvenil e form during metamorphosis. (C) 2000 Academic Press.