Hemolymph osmotic, ionic status, and branchial Na⁺/K⁺-ATPase activity under varying environmental conditions in the intertidal grapsid crab, Gaetice depressusd

Osmo- and ionoregulatory abilities were examined in the intertidal grapsid crab, Gaetice depressus, transferred from normal seawater (30 ppt) to low (10 ppt) or high (50 ppt) salinities for 2 and 10 days, in addition to animals kept out of water for 2 days. The results of the hemolymph osmotic and ionic status indicate that G. depressus is able to adapt for more than 10 days in these salinities and for 2 days under terrestrial conditions. Especially, the free Ca²⁺ concentration was relatively maintained compared with concentrations of monovalent ions and osmolality values in 10 and 50 ppt, partly using the complexed calcium (total minus free calcium) as an internal reserve in the hemolymph. In 10 ppt, complexed calcium disappeared from the hemolymph after 10 days, indicating that all the hemolymph calcium was ionized. In 50 ppt, free Ca²⁺ was regulated to lower levels than concentrations in the medium, while total calcium increased to higher levels after 2 days. Examination of Na⁺/K⁺-ATPase activity, which has been implicated in ion transport in many crustaceans, revealed that induction of high Na⁺/K⁺-ATPase activity varies among the posterior gills in response to salinities. Ten-ppt salinity induces activity in two of the posterior gills (gill numbers 6 and 7, eight in total), albeit with differing degrees of response. In contrast, 50-ppt salinity stimulates the activity primarily in gill number 8, suggesting that this gill may be associated specifically with ion excretion in G. depressus. As a euryhaline amphibious crab, this abundant species around Japan will serve as a model to study the osmotic/ionic regulatory mechanisms which operate in crustaceans.