Sodium and chloride balance in the killifish Fundulus heteroclitus.

Abstract

Measurements have been made of the rates of influx and efflux of sodium ions in sea water, 40% sea water and fresh water in the killifish, Fundulus heteroclitus. Measurements have also been made of the rates of efflux of chloride and bromide ions in the same media. The rate of drinking has been measured using inulinlabeled sea water. In sea water the sodium influx averages 20 mM Na/kg./hr., of which more than half enters by diffusion, the rest by drinking. In fresh water the rate of influx is about 0.6 mM Na/kg./hr., practically all of which takes place by active transport. Adaptation to fresh water is accompanied by a great reduction in permeability to sodium and chloride ions. This fall in permeability takes place within a few minutes of transfer to fresh water but the increase of permeability on return to sea water takes many hours. The relationship between active uptake and external concentrations at low concentrations resembles that found in fresh-water crustaceans.