Water Uptake and Loss in Relation to the Salinity of The Medium in the Amphipod Crustacean Gammarus Duebeni

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Abstract
1. The water fluxes across the body surface and the rate of urine production have been studied in the euryhaline amphipod Gammarus duebeni. 2. Urine flow rates (fPOs) have been determined from measurements of loss of [131I]sodium diatrizoate from the body, and the expected urine flow (fPdiff) has been calculated from determinations of the osmotic gradient between blood and medium and the flux of tritiated water. 3. For animals in 2% and 40% sea water the ratio fPOs/fPdiff are 1.16 and 1.44 respectively, and thus approximate fairly closely to unity. This implies that in these media the water subsequently excreted as urine enters the body by osmosis and that there is little interference with the free diffusion of water at the body surface due to passage through long pores or across unstirred layers. 4. In sea water the ratio fPOs/fPdiff is normally (assuming an osmotic gradient of 10 m-osmoles) almost twice unity but urine production is approximately halved when the animals are exposed to sea water saturated with an inhibitor of active sodium uptake (thionine). 5. It is suggested that there is a prima facie case for assuming that part of the fluid subsequently excreted by this species, when in sea water, is taken into the body initially by a process dependent upon active ion transport.