Osmotic Regulation in Freshwater Animals

Abstract

IT has recently been shown that both brackishand fresh-water animals can actively absorb ions from the external medium against a considerable concentration gradient. Most fresh-water animals can do so from solutions as dilute as normal fresh-waters¹. Krogh has suggested that in some cases, during periods of enforced starvation when salts are therefore not available from food, this mechanism is of survival value in maintaining the normal ionic and osmotic concentration of the blood¹. In a recent paper² on the euryhaline crab Carcinus mœnas, Webb has gone further than this in assuming that “the active absorption of salts forms the basis of the osmoregulatory mechanism, not only in freshwater forms, but also in brackish water forms, including the euryhaline Crustacea”. Without criticizing his conclusions as applied to Carcinus, we wish to point out that some experiments which we have carried out on brackish and freshwater species of Gammarus suggest that active absorption of ions from the medium is not of such universal importance as is implied in the above sentence.