It is well established that external calcium can have a marked effect on sodium fluxes in both marine and freshwater organisms (Potts & Fleming, 1971; Cuthbert & Maetz, 1972; Fleming, Nichols & Potts, 1974; Eddy, 1975; Carrier & Evans, 1976; Isaia & Masoni, 1976). In the few fish species which have so far been examined the effects of external calcium appear to be primarily on sodium efflux, which is reduced in the presence of calcium (Fleming et al. 1974; Eddy, 1975; Isaia & Masoni, 1976). The influence of calcium on sodium influx shows considerably more variation between species (Shaw, 1960; Sutcliffe, 1971; Eddy, 1975) making it difficult to propose a suitable mechanism to account for the observed effects.

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