MARTINEZ, A.-S.; TOULLEC, J.-Y.; SHILLITO, B.; CHARMANTIER-DAURES, M. *; CHARMANTIER, G.: Osmotic and ionic regulation in the hydrothermal vent crab Bythograea thermydron

This study investigates the salinity tolerance and the ability to osmoregulate and ionoregulate of a brachyuran crab endemic to the deep sea hydrothermal vent habitat, Bythograea thermydron. Crabs were collected during the HOPE 99 mission from hydrothermal vents on the East Pacific Rise (EPR) on the 13�N and 9�N sites, at a depth of approximately 2500 m. Salinities of 32-34 ppt were measured in the seawater surrounding the captured specimens. We found that this species is a marine stenohaline osmoconformer with tolerable salinities ranging between approximately 31 and 41 ppt. The time of osmotic adaptation after a sudden decrease in external salinity was approximately 15-24h, which is particularly short for an adult brachyuran crab. In the range of tolerable salinities, it exhibited an iso-osmotic regulation, which was not affected by changes in hydrostatic pressure, and an iso-ionic regulation for Na and Cl. The hemolymph Ca concentration was slightly hyper-regulated, K concentration was slightly hyper-hypo-regulated and Mg concentration was strongly hypo-regulated. These findings probably reflect a high permeability of the teguments of these crabs to water and ions. In addition, the presence of ionocytes was not detected through electron microscopy, and immunocytochemistry revealed the absence of Na-K ATPase in all the structures (gills, epipodites and branchiostegites) of the branchial chambers. No histological difference was noted between anterior and posterior gills. These results lead to the hypothesis that B. thermydron, as a stenohaline osmoconformer, lives in a habitat of stable seawater salinity.