Osmoregulation and evolutionary patterns in coastal terrestrial amphipods (Talitridae)

The osmoregulatory responses and survival of 3 species of landhoppers (Austrotroides maritimus, Keratroides rex, and Tasmanorchestia annulata) confined to the coastal zone of western Tasmania and 2 species from inland forests (Mysticotalitrus tasmaniae and M. cryptus) are described over a range of external concentrations, from tap water to full strength sea water (approximately 1,000 mOsm·kg-1). Austrotroides maritimus and K. rex showed a hyper-iso-osmotic osmoregulatory pattern (maintaining the hemolymph between 362-1,080 mOsm·kg-1 and 412-1,045 mOsm·kg-1, respectively, over a concentration range of 40-1,045 mOsm·kg-1), whereas T. annulata showed a trend toward a hyper-hypo-osmoregulatory pattern (maintaining the hemolymph between 423-948 mOsm·kg-1 over the same concentration range). Mysticotalitrus tasmaniae and M. cryptus, the inland species, also showed a hyper-iso-osmotic regulatory pattern. Among the coastal species, there were significant interspecies differences in hemolymph osmotic concentration at dilute external concentrations and at the most concentrated external concentration examined. Tasmanorchestia annulata showed 100% survival over the full range of salinities examined, A. maritimus showed increased mortality on more dilute media, and K. rex showed increased mortality on more concentrated media. Silver-staining studies did not reveal evidence for the existence of extrabranchial sites of ion exchange in the 3 species. The data support the hypothesis that, of these coastal landhoppers, A. maritimus, at least, is physiologically limited to the coastal habitat by a dependence on the supply of ions from sea spray.