Gill chloride cell morphology and Na+,K+-ATPase activity were examined in cultured Japanese eels (Anguilla japonica) acclimated to fresh water (FW) or seawater (SW), and in yellow and silver eels caught in wild stocks. Gill Na+,K+-ATPase activity was higher in SW-acclimated cultured eels than in FW eels. Chloride cells were detected in both filament and lamellar epithelia by immunocytochemical staining using anti-Na+,K+-ATPase serum. The filament chloride cells were more abundant and larger in SW eels than in FW eels, whereas there was no apparent difference in lamellar chloride cells. In wild-caught eels, gill Na+,K+-ATPase activity increased as they developed from yellow to silver eels. Filament chloride cells in silver eels were more active than those in yellow eels. In contrast, lamellar chloride cells, which were frequently observed in yellow eels, had decreased in number or disappeared in silver eels. These findings suggest that chloride cells in the filament are responsible for excretion of excess salt in hyperosmotic environments, and that lamellar chloride cells may play a significant role in hypoosmotic environments, presumably acting as sites of ion uptake. The excellent euryhalinity of the Japanese eel may be due, at least in part, to the presence of functionally different types of chloride cells. The activation of filament chloride cells in silver eels in FW, together with the increase in Na+,K+-ATPase activity, could be interpreted as a preadaptive response to forthcoming entry into SW.
Swimming depth of migrating silver eels Anguilla japonica released at seamounts of the West Mariana Ridge, their estimated spawning sites