Compare the osmoregulatory challenges
of freshwater and marine animals.
Body tissues in a saltwater fish contain less salt than the water in which it lives. Because the saltier environment in the outside water draws water from body tissues, a saltwater fish constantly loses water through its skin and gills. To compensate and prevent dehydration, a saltwater fish drinks large quantities of saltwater, produces very little urine and secretes salt from this water through its gills.In contrast, body tissues in a freshwater fish contain more salt than the water in which it lives. As a result, water continually flows into the body of a freshwater fish through its skin and gills and the fish has no reason to take in additional water by drinking. Freshwater fish avoid an excess of water in body tissues by producing large amounts of urine.