OSMOTIC REGULATION IN MARINE AND FRESH-WATER GAMMARIDS (AMPHIPODA)

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Abstract
Hypertonic regulation of the blood concentration (determined by freezing point depression) is described in 4 gammarid species. In those sea-water dilutions in which they regulate, the marine species Gammarus oceanicus and Marinogammarus finmarchicus maintain the blood between 0.65 and 0.85 molal %in equivalents of ideal solute); the brackish-water G. tigrinus maintains it between 0.55 and 0.65 molal; and the fresh-water G. fasciatus maintains it between 0.35 and 0.50 molal. Rate of urine flow, directly measured in 2 species, is proportional to osmotic gradient between blood and medium, indicating that osmotically absorbed water is eliminated as urine. Rate of urine flow, expressed as percent of body weight per hour per molal gradient, is 10.5% in G. oceanicus and 5.9% in G. fasciatus; hence the latter is less permeable" to water. Urine of G. oceanicus is isotonic to blood; that of G. fasciatus is hypotonic. Despite losing salt at a greater rate, G. oceanicus maintains a greater osmotic gradient than G. fasciatus in intermediate external concentrations; hence its rate of salt uptake must also be greater. In each species, the uptake mechanism appears to be activated gradually, as the external concentration is lowered.

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