Metamorphosis in the summer flounder Paralichthys dentatus: Changes in gill mitochondria-rich cells

Salinity tolerance changes during larval development and metamorphosis in t he summer flounder (Paralichthys dentatus) and other teleosts. The physiolo gical mechanisms responsible for osmoregulation during these early stages o f development are not well understood. This study characterized changes in ultrastructure, intracellular membranes and immunoreactive Na+/K+-ATPase of mitochondria-rich cells (MRCs) in the gills of summer flounder during meta morphosis. Gill ultrastructure at the start of metamorphosis revealed only one type of MRC, which had weak reactivity to osmium and lacked a well-defi ned apical pit. In juveniles, two types of MRCs were observed: light-staini ng MRCs (LMRCs) with weak reactivity to osmium, and dark-staining MRCs (DMR Cs) with strong reactivity to osmium and positioned adjacent to LMRCs. Comp ared with MRCs at the start of metamorphosis, the mitochondria of juvenile MRCs appeared smaller, with more transverse cristae and electron-dense matr ices. Changes in MRCs during metamorphosis were also accompanied by increas ed immunoreactive Na+/K+-ATPase. These findings suggest that gill MRCs deve lop during the metamorphosis of summer flounder as the gill takes on an inc reasingly important osmoregulatory role.