Regulation of water and some ions in Gammarids (Amphipods). I. Gammarus duebeni Lilljeborg from brackish water and fresh water.

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 (1940c), 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, 19406). If differences do exist in the osmoregulatory mechanisms of G. duebeni from different habitats then it might be supposed that these would be reflected in the total content of water and ions in animals exposed to a range of external salinities. This was found to be the case in Mesidotea entomon (Croghan & Lockwood, 1968). Particular attention was given to salinities representing the range found in natural fresh waters up to 2% sea water. Another point of interest concerning G. duebeni from freshwater populations is its inability to reproduce in standing fresh water in the laboratory, although it will do so in running fresh water (Hynes, 1954). The effect of stirring the medium on ion regulation was therefore examined, but over a period of a few days only.