This is for bio
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They absorb water that passes through their gills,to get the nurturance they need to keep them in balance.
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Each species will tend to have their own method of maintaining the appropriate equilibrium. Most simply they actively excrete freshwater to stop it from completely diluting. This active secretion can be done by contractile vacuoles or other similar processes. Basically they can't rely on passive diffusion or they would fill up with water and be imbalanced, so they use active transport.