ABSTRACT
Previous work on Gammarus duebeni from brackish and fresh waters established that a correlation exists between certain features of sodium regulation and the sodium concentration of the particular habitat occupied by a population. In populations from fresh water sodium losses in the urine are reduced and the transporting system at the body surface has a higher affinity for sodium ions (Sutcliffe, 1971). There is also some evidence to suggest that the sodium concentration in the blood tends to fall more rapidly in animals from brackish-water populations than in animals from freshwater populations when they are exposed to media containing less than 10 mM/1 sodium (Sutcliffe, 1967; Sutcliffe & Shaw, 1968). This investigation provides some further information on the regulation of sodium, chloride and potassium in the blood of animals acclimatized to low external concentrations. Chloride regulation at salinities above 2 % sea water was studied by Beadle & Cragg (1940 a), who also found that when G. duebeni was placed in distilled water the blood chloride concentration fell to a lower level in animals from a brackish-water locality compared with animals from a freshwater locality (Beadle & Cragg, 1940b).
