Explain In Terms Of Organisms Explain The Word Salinity (salt-concentration)?

The salinity of the water is an important factor for aquatic organisms. These organisms may be adapted for life in very salty water such as the sea or in streams and freshwater ponds where the salt content is very low. Aquatic organisms are therefore confronted with osmotic problems.

The salt concentration of the cytoplasmic contents of freshwater organisms is usually higher than that of the surrounding water so that water tends to enter these organisms by osmosis. The cells of freshwater plants are prevented from bursting by their rigid cellulose walls. Protozoa, such as Amoeba, have contractile vacuoles to remove water that enters them by osmosis. As the salt content of the surrounding freshwater is very low, the fish makes up for the lost salt from its food and also from the gills which are adapted to absorb dissolved salts by active transport from the surrounding water.

Animals living in sea water tend to lose water by osmosis as the sea water contains a higher concentration of salts than the tissue cells.Most aquatic organisms are so highly specialized that they can survive only in their own environment. E.g. many freshwater animals cannot survice in sea water and vice versa.

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