Contrasts in the hypo-osmoregulatory abilities of a freshwater and an anadromous population of inconnu

Juvenile freshwater and anadromous inconnu Stenodus leucichthys regulated plasma ions following a direct transfer from fresh to brackish water (10–15‰), but suffered osmoregulatory collapse with 100% mortality in 48 h when directly transferred to 25‰ salinity. Acclimation to brackish water for 2 weeks improved hypo-osmoregulatory capacity in both populations, with acclimated fish showing smaller increases in blood plasma ion concentrations, higher Na⁺-K⁺ ATPase activity, and lower mortality than non-acclimated fish following transfer to 25‰ sea water. Anadromous inconnu maintained pre-treatment plasma ion levels during acclimation, whereas these levels increased during acclimation in freshwater inconnu. Juvenile anadromous inconnu are therefore able to adapt physiologically to sea water of at least 25‰, if brackish-water acclimation is available, but freshwater inconnu have diminished saltwater tolerance, relative to the anadromous form.