Vertebrate & Evolution

Why can some fish live in freshwater, some in salt water, and some in both?

William A. Wurts

he various species of fish their blood is virtually identical. fresh water, fish had to replace salts found in , lakes, rivers Vertebrate blood has a of lost through diffusion to the water and streams have evolved approximately 9 grams per liter (a 0.9 and eliminate excess water absorbed Tover millions of years and percent salt solution). Almost 77 from the environment. Kidney have adapted to their preferred percent of the salts in blood are function had to be altered accordingly environments over long periods of sodium and chloride. The remainder for fish to survive in these different time. Fish are categorized according is made up primarily of bicarbonate, habitats. Eventually, the gills to their salinity tolerance. Fish that potassium and calcium. Sodium, developed the ability to excrete salts can tolerate only very narrow ranges potassium and calcium salts are in seawater and absorb salts from of salinity (such freshwater fish as critical for the normal function of fresh water. goldfish and such saltwater fish as heart, nerve and muscle tissue. In seawater, fish must drink salt tuna) are known as stenohaline If the salinity of water is water to replace lost fluids and then species. These fish die in waters diluted to approximately one quarter eliminate the excess salts. Their having a salinity that differs from that of its normal concentration, it has kidneys produce small volumes of in their natural environments. almost the same salinity as fish blood fluid containing high concentrations Fish that can tolerate a wide and contains similar proportions of of salt. Freshwater fish produce large range of salinity at some phase in sodium, potassium, calcium and volumes of dilute urine, which is low their life-cycle are called chloride. The similarities between the in salt. High concentrations of species. These fish, which include salt content of vertebrate blood and environmental calcium help reduce , eels, red drum, striped bass dilute seawater suggest a strong salt loss through the gills and body and flounder, can live or survive in evolutionary relationship among surfaces in freshwater environments. wide ranges of salinity, varying from vertebrates and with the primordial Less demand is placed on the kidneys fresh to brackish to marine waters. A oceans. to maintain stable concentrations of period of gradual adjustment or Indeed, it seems likely that blood salts in brackish or low salinity acclimation, though, may be needed vertebrate life evolved when the waters. for euryhaline fish to tolerate large oceans were approximately one Ultimately, fish adapted to or changes in salinity. quarter as salty as they are today. As inhabited marine, fresh or brackish It is believed that when the newly the oceans became saltier and water because each environment formed planet Earth cooled vertebrates evolved further, several offered some competitive advantage sufficiently, rain began to fall groups of vertebrates (birds, to the different species. For instance, continuously. This rainfall filled the mammals, reptiles and amphibians) it has been suggested that euryhaline first oceans with fresh water. It was left the oceans to inhabit the land fish are able to eliminate external the constant evaporation of water masses, carrying the seawater with parasites by moving to and from fresh from the oceans that then condensed them as their blood. They maintained and saltwaters. Habitats of differing to cause rainfall on the land masses, their blood salt concentrations by salinity offered new or more food, which in turn, caused the oceans to drinking freshwater and absorbing escape from predators and even become salty over several billion salts from food. thermal refuge (stable temperatures). years. As rain water washed over and But fish stayed in the aquatic ______through the soil, it dissolved many environment. To adapt, they had to minerals -- sodium, potassium and either remain in low salinity William A. Wurts, Ph.D. is a State calcium -- and carried them back to environments, such as bays and Specialist for Aquaculture at the oceans. , or they had to evolve Kentucky State University Vertebrate animals (fish, birds, mechanisms to replace water lost Cooperative Extension Program. mammals, amphibians and reptiles) through osmosis to the seawater and have a unique and common to remove salts absorbed from the http://www.ca.uky.edu/wkrec/ characteristic. The salt content of increasingly saline oceans. To inhabit Wurtspage.htm World Aquaculture 29(1) March 1998 65