Species migrating between marine and fresh waters need adaptations for both environments when in salt water they need to keep the bodily salt concentration on a level lower than the surroundings, and vice versa. Some other kinds of fish are, on the contrary, born in salt water, but live most of or parts of their adult lives in fresh water for instance the eels. These are known as anadromous fish, and include, for instance, salmon, trout, sea lamprey and three-spined stickleback. Many species of fish do reproduce in freshwater, but spend most of their adult lives in the sea. Sturgeon are found both in anadromous and fresh water stationary forms
They also have well developed kidneys to reclaim salts from body fluids before excretion. Their scales reduce water diffusion through the skin: freshwater fish that have lost too many scales will die. Their gills must be able to diffuse dissolved gases while keeping the salts in the body fluids inside. When dealing with ponds and lakes, one might use the same basic models of speciation as when studying island biogeography.įreshwater fish differ physiologically from saltwater fish in several respects.
This is primarily due to the rapid speciation that the scattered habitats make possible. To survive fresh water, the fish need a range of physiological adaptations.Ĥ1.24% of all known species of fish are found in fresh water. These environments differ from marine conditions in many ways, especially the difference in levels of salinity. Tench are common freshwater fish throughout temperate Eurasia.įreshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 1.05%.
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