All fresh-water animals, whether they are invertebrates or vertebrates, are hypertonic to the surrounding water because the body fluids are moasre concentrated than the surrounding water.
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Therefore, there is a tendency for the osmotic inflow of water inside their body and the outward diffusion of salts (== inorganic ions from their body surface which is not completely impermeable.
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If this process goes on without some restoration mechanisms, the useful salts of the body will be lost and it will result into haemodilution.
This problem has been solved in different animals differently. The problem of excess of water has been solved in two ways: by storing the excess water that enters t!2!he body through exposed areas, or by pumping it out.
The latter is the more effective way. The problem of salts loss from the body ha -s been solved by the development of active transport systems designed to bring salts into the body through specialized regions of the gills, skin, rectum, or other regions of the gastro-intestinal tract.
Procerodes (Gunda) ulvae, a triclad flatworm living in the tidal zone of estzuaries of small streams and fresh-water Hydra are the best exampulies in which water storage has been demonstrated.
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They have storage vacuoles in their body by which they are able to retain the excess of water for many hours and after that they disintegrate entirely.
The fresh water protozoans such as Amoeba, Euglena and Paramecium have solved the problem of osmoregulation by their contractile vacuoles which expell the excess of water.
These vacuoles are formed by coalition of smaller vacuoles near the nucleus and are disintegrated to remove excess of water at the outer surface of the body.
All other fresh water organisms except the coelenterates and sponges, to remove excess water from the body, have developed kidneys, or other excretory organs capable of producing copious urine, hypotonic to body fluids.
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The kidneys of amphibian, most bony fishes and elasmobranchs have very small nephrons and large glomeruli which are adapted for filtering out and eliminating the excess water in the form of dilute (hypotonic) urine.
It has been suggested that the renal systems of fresh-water organisms usually contain cells that can remove sugar, salts and other useful materials from the urine and restore them to body fluids.
To maintain the high osmotic concentration of the body fluids than that of the surrounding water, work must be done in order to absorb salts against the concentration gradient, that is, active transport must be involved.
Fresh-water fishes and some crustaceans have special type of cells in their gills which can reabsorb salts from the environment and thus carrying out the function.
In general, the fresh water organisms show the following adaptations for osmoregulation:
(i) They possess semipermeable areas for respiratory and digestive purposes.
(ii) The rest of the body remains covered by a protective and impermeable covering.
(iii) They possess excretory organs like kidneys and nephridia that are capable to secrete very dilute (hypotonic) urine.
(iv) They possess special type of cells either in the gills, or in gut, or in some parts of the kidney that can remove salts from the environment and restore them to body fluids to maintain high osmotic concentration of them.