Kazutoshi WATANABE * and Juro YAMADA*

Osmoregulation of Eriocheir japonieU8 in media of varying salinities was inves-tigated. The crabs exhibited a hyper- and hyposmoregulatory ability. The osmotic concentration of the hemolymph was isosmotic when they were acclimated to 80% seawater. The hemolymph sodium concentration showed the same change as did the hemolymph osmotic concentration. The specific activity of Na+-K+-ATPase in the anterior gills was higher in crabs acclimated in 120 % seawater than those maintained in freshwater, while the activity in the posterior gills was higher in freshwater carbs than in 120 % seawater crabs. It was suggested that salt extrusive mechanism may be localized in the anterior gills and a salt absorptive mechanism in the posterior gills. Many crab species that inhabit waters of changeable salinity have the ability more or less to regulate their hemolymph osmotic concentration. Sodium is one of the major osmotically active components in the hemolymph and its regulation is important for the osmoregulation. It has been known that crab gills are capable of active abosrption1)2)3)4) or extrusion5) of sodium ions, and that Na+-K +-ATPase activity in the gills plays an important role in the process of osmoregulation in crabs6)7)8)9) • Since Eriocheir japonious lives in freshwater and undergoes a dramatic osmotic change during the spawning migration from freshwater to seawater, this crab may be a strong osmoregulator. This study was designed to investigate the ability of osmoregulation in E. japonious. The osmotic and sodium concentrations of hemolymph were analysed by acclimating the crabs to salt depleted or loaded condi-tions. Changes of the gill Na+-K+-ATPase activity were examined under the extreme osmotic conditions