Osmotic Regulation in Marine and Fresh-Water Gammarids (Amphipoda)

The relationship of the osmotic concentration of the blood to that of the external medium is described for four species of gammarid. In media of various concentrations each species regulates its blood concentration in a manner that reflects its natural habitat. The marine species, Marinogammarus finmarchicus and Gammarus oceanicus, regulate their blood concentration at the highest level; the brackish-water species, G. tigrinus, regulates at a lower level; and the fresh-water species, G. fasciatus, regulates at the lowest level. Moreover, M. finmarchicus and G. oceanicus die in fresh water; G. fasciatus dies in full-strength sea water; but G. tigrinus survives both in fresh water and in sea water up to at least 1.5 molal. 2. The rate of urine production in G. fasciatus and G. oceanicus is proportional to the osmotic gradient between blood and medium, indicating that urine formation represents elimination of osmotically absorbed water. The coefficient of proportionality is smaller in G. fasciatus (5.9% of body weight per hour per molal gradient) than in G. oceanicus (10.5%), indicating that the latter species is more permeable to water. 3. The urine of G. oceanicus is nearly isotonic to the blood in all media. The urine of G. fasciatus is much more dilute than the blood. 4. The differences in flow and in concentration of urine combine to give G. oceanicus a much greater rate of urinary salt loss than G. fasciatus. 5. The osmotic gradient maintained by each species varies in a way that indicates the animals have an ionic uptake mechanism which is gradually activated as the salinity is lowered. In all except the most dilute media, it appears that the mechanism is more completely activated and takes up salt more rapidly in G. oceanicus than in G. fasciatus.