Freshwater and Saltwater Fish

Why do some fish normally live in freshwater and others in saltwater? How can some fish adapt to both?
Aldo Palmisano is a research chemist at the Western Fisheries Research Center of the U.S. Geological Survey Biological Resources Division and is affiliated with the University of Washington in Seattle. Here is his answer.

The reason some fish normally live in freshwater and others live in seawater is that one or the other environment provides them with opportunities that have traditionally contributed to their survival. An obvious difference between the two habitats is salt concentration. Freshwater fish maintain the physiological mechanisms that permit them to concentrate salts within their bodies in a salt-deficient environment; marine fish, on the other hand, excrete excess salts in a hypertonic environment. Fish that live in both environments retain both mechanisms.
SALMONĀ and other so-called anadromous fish species spend portions of their lives in both fresh and saltwater. | |
Life began evolving several billion years ago in the oceans and since that time, living things have maintained an internal environment closely resembling the ionic composition of those primeval seas. Presumably, the ionic conditions in which life began are uniquely appropriate to its continuation. Laboratory studies support the view that the various chemical phenomena on which life depends--including the interactions of nucleic acids with each other and with proteins, the folding and performance of proteins such as enzymes, the functioning of intracellular machines such as ribosomes, and the maintenance of cellular compartments--are critically dependent on the ionic milieu in which the reactions take place.
Given time, ocean-dwelling creatures took advantage of untapped resources, such as relatively safe spawning habitats or new food sources, that were available to them only by colonizing other environments, like freshwater and land. Colonization was facilitated, if not...