Osmoregulation by the Killifish, Fundulus heteroclitus in Fresh Water at Temperatures near Freezing

Abstract

OSMOREGULATORY failure has often been cited as the cause of death in fish exposed to low temperatures1–4. Moreover, even in those which survive quite well in the cold, concentrations of sodium and chloride in the serum are known to increase at low temperatures in marine species^5,6 and to decrease at low temperatures in freshwater species^7,8. Studies on the euryhaline killifish, Fundulus heteroclitus, adapted to saltwater showed that acclimatization to 2° C or −1.5° C increased serum osmolality of sodium and chloride when compared with controls at 10° C or 20° C (ref. 9). These data were interpreted to mean that, as in other fish, osmoregulatory ability was less efficient in cold-acclimatized F. heteroclitus. If this was so, then for fish adapted to freshwater the serum osmolality of sodium and chloride would be expected to decrease when the temperature was lowered. The preliminary data reported here demonstrate that this is the case.