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.
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 necessitated, by geological events, such as the movements and collisions of land masses (plate tectonics) and volcanic activity, which served to isolate portions of very similar populations of a single species from one another. Such geological change forced some populations to either adapt or face extinction. Time and natural selection due to physical and environmental variation worked in concert with isolation to foster adaptations. In some cases, these adaptations became permanent and led to species differentiation.
One important aspect of environmental variation is the ionic composition of bodies of water utilized as habitat. Chloride cells in the gills of marine fish produce an enzyme, called gill Na+/K+ ATPase, that enables them to rid their plasma of excess salt, which builds up when they drink seawater. They use the enzyme to pump sodium out of their gills at the cost of energy. Additionally, their kidneys selectively filter out divalent ions, which they then excrete. An alternative set of physiological mechanisms allows freshwater fish to concentrate salts to compensate for their low salinity environment. They produce very dilute, copious urine (up to a third of their body weight a day) to rid themselves of excess water, while conducting active uptake of ions at the gill.
Certainly, other adaptations contributed to the capability of isolated populations to adapt more fully to their circumstances. With different sets of predator and prey organisms present in the differing habitats, and different physical ranges available to them, behavioral changes would be required; perhaps a smaller or larger body size or body part would be favored. The accumulation of these kinds of physiological, behavioral and physical changes ultimately led to new species. Isolation may have forced them to conserve their newly developed adaptations among their own descendants, rather than distribute them more broadly. For some, the rift eventually became complete and there could no longer be any cross-breeding between populations that once interbred.
Not unreasonably, there were multiple instances of colonization of the freshwater environment by seawater species of fish; some were more or less complete. The ability to escape an environment may have been seasonal, or periodic in some other way, or intermittent, and the ability to osmoregulate in freshwater need not have excluded the capacity to revert to a seawater mode of osmoregulation, as long as the capacity could be utilized by a substantial portion of the population, and selected for, rather than simply lost.
Salmon spend a relatively short time in freshwater before developing the capacity to osmoregulate in seawater, where they live for the majority of their lives. Some species of salmon, like pink salmon, migrate to sea as soon as they emerge from the gravel as free-swimming juveniles. Others, such as sockeye and coho and some chinook salmon, remain in freshwater for one or two years or more before the urge to migrate downstream overcomes them, in a sequence of physiological and physical events that coincides with the development of their capacity to osmoregulate in seawater. So the different species of salmon exploit different aspects of the freshwater environment, but evidently they all enjoy better life prospects if they are spawned in a freshwater habitat and spend their adult lives in seawater.
Other related species, like trout, are physiologically less tolerant of salty water. Most have permanently adapted to life in freshwater. They have probably also lost characteristics (e.g., mating behaviors) that might enable them to lead a successful life in the marine environment. For reasons that may relate to their geographic distribution, the characteristics that once made life in seawater natural to them eventually became excess baggage and fell into disuse and disrepair.
William A. Wurts is an aquaculture specialist in Kentucky State University's cooperative extension program. He provides additional insight on fish evolution and physiology.
The various species of fish found in oceans, lakes, rivers and streams have evolved over millions of years and have adapted to their preferred environments over long periods of time. Fish are categorized according to their salinity tolerance. Fish that can tolerate only very narrow ranges of salinity (such freshwater fish as goldfish and such sea water fish as tuna) are known as stenohaline species. These fish die in waters having a salinity that differs from that in their natural environments.
Fish that can tolerate a wide range of salinity at some phase in their life-cycle are called euryhaline species. These fish, which include salmon, eels, red drum, striped bass and flounder, can live or survive in wide ranges of salinity, varying from fresh to brackish to marine waters. A period of gradual adjustment or acclimation, though, may be needed for euryhaline fish to tolerate large changes in salinity.
It is believed that when the newly formed planet Earth cooled sufficiently, rain began to fall continuously. This rainfall filled the first oceans with freshwater. It was the constant evaporation of water from the oceans that then condensed to cause rainfall on the land masses, which in turn, caused the oceans to become salty over several billion years. As rain water washed over and through the soil, it dissolved many minerals--sodium, potassium and calcium-- and carried them back to the oceans.
Vertebrate animals (fish, birds, mammals, amphibians and reptiles) have a unique and common characteristic. The salt content of their blood is virtually identical. Vertebrate blood has a salinity of approximately 9 grams per liter (a 0.9 percent salt solution). Almost 77 percent of the salts in blood are sodium and chloride. The remainder is made up primarily of bicarbonate, potassium and calcium. Sodium, potassium and calcium salts are critical for the normal function of heart, nerve and muscle tissue.
If the salinity of ocean water is diluted to approximately one quarter of its normal concentration, it has almost the same salinity as fish blood and contains similar proportions of sodium, potassium, calcium and chloride. The similarities between the salt content of vertebrate blood and dilute seawater suggest a strong evolutionary relationship among vertebrates and with the primordial oceans.
Indeed, it seems likely that vertebrate life evolved when the oceans were approximately one quarter as salty as they are today. As the oceans became saltier and vertebrates evolved further, several groups of vertebrates (birds, mammals, reptiles and amphibians) left the oceans to inhabit the land masses, carrying the seawater with them as their blood. They maintained their blood salt concentrations by drinking freshwater and absorbing salts from food.
But fish stayed in the aquatic environment. To adapt, they had to either remain in low salinity environments, such as bays and estuaries, or they had to evolve mechanisms to replace water lost through osmosis to the seawater and to remove salts absorbed from the increasingly saline oceans. To inhabit freshwater, fish had to replace salts lost through diffusion to the water and eliminate excess water absorbed from the environment. Kidney function had to be altered accordingly for fish to survive in these different habitats.
In seawater, fish must drink salt water to replace lost fluids and then eliminate the excess salts. Their kidneys produce small volumes of fluid containing high concentrations of salt. Freshwater fish produce large volumes of dilute urine, which is low in salt. Less demand is placed on the kidneys to maintain stable concentrations of blood salts in brackish or low salinity waters.
Ultimately, fish adapted to or inhabited marine, fresh or brackish water because each environment offered some competitive advantage to the different species. For instance, it has been suggested that euryhaline fish are able to eliminate external parasites by moving to and from fresh and saltwaters. Habitats of differing salinity offered new or more food, escape from predators and even thermal refuge (stable temperatures).
Steven K. Webster, marine science advisor to the Monterey Bay Aquarium in California adds some perspective on fish that move between salt and freshwater.
The approximately 22,000 species of fishes alive today live in virtually all sorts of marine and aquatic habitats that are not unduly toxic. Some, including salmon, lampreys, shad, sturgeon and striped bass, move between freshwater bodies and the ocean at least once in their lives to spawn. Many of these anadromous species do so annually, finding conditions needed for reproduction in one realm and those needed for feeding and growth in the other.
These fishes have to switch over their salt balance physiology when they move from fresh to saltwater and back again. They typically make these adjustments in a brackish estuarine environment--which lies on the way between salt water and freshwater habitats.