Résumé
This study investigates the salinity tolerance and the pattern of osmotic and ionic regulation of Bytho-graea thermydron Williams, 1980, a brachyuran crab en-demic to the deep-sea hydrothermal vent habitat. Salinities of 33‰-35‰ were measured in the seawater surrounding the captured specimens. B. thermydron is a marine steno-haline osmoconformer, which tolerates salinities ranging between about 31‰ and 42‰. The time of osmotic adaptation after a sudden decrease in external salinity is about 15-24 h, which is relatively short for a brachyuran crab. In the range of tolerable salinities, it exhibits an iso-osmotic regulation, which is not affected by changes in hydrostatic pressure, and an iso-ionic regulation for Na ϩ and Cl Ϫ. The hemolymph Ca 2ϩ concentration is slightly hyper-regulated, K ϩ concentration is slightly hyper-hypo-regulated, and Mg 2ϩ concentration is strongly hypo-regulated. These findings probably reflect a high permeability of the teguments to water and ions. In addition to limited information about salinity around hydrothermal vents, these results lead to the hypothesis that B. thermydron lives in a habitat of stable seawater salinity. The osmoconformity of this species is briefly discussed in relation to its potential phylogeny.