Can salt water fish live in fresh water and vice versa?

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There are a number of fish that have this ability. Fish that live in the interface between fresh and salt water--such as many mangrove swamps, marine estuaries, and salt marshes--have the capacity to change their osmoregulatory functioning depending upon the salt concentration of the water. We would call these "brackish" water fish and they live in places where the salinity varies seasonally and sometimes more frequently. Many brackish water fish are found in the estuaries and drainages of tropical waters, but a some are found on and around the coasts of temperate waters, as well. These fish are not like anadromous fish (like salmon, striped bass, or eels) that spend part of their lives in freshwater (eggs to hatchlings to fingerlings), move to the sea to grow, and then return to the freshwater as mature adults to mate and lay eggs. Brackish water fish live almost entirely in the zone of changing salinity--although several species are known to move into the saltwater zones if there are rich coral reefs near by.

Adaptations of brackish water fish include adaptations in the renal (kidney) system, but especially in the gills. These fish often live in areas where sedimentation is a regular occurrence and the water can be quite turbid; laden with particulates that might clog the gills of any other fish. The gills of these fish are able to shed the particulates better, and are adapted to changes in osmoregulation by virtue of the salt concentration.