Why are there freshwater fish and saltwater fish?

At present, there are about 22000 kinds of fish in the world, which are distributed in almost all salt water or fresh water environments that have not been seriously polluted. Living in oceans, lakes, rivers and streams, these fish have experienced a long evolutionary period of millions of years and have been used to their different living environments. Different fish have different physiological mechanisms: freshwater fish live in salt deficient waters, so they need to accumulate salt into their bodies; On the contrary, saltwater fish live in a hypertonic environment, so they have to excrete the excess salt. Fish that can live in both fresh and salt water are more wonderful. They have two physiological mechanisms: salt accumulation and salt excretion!

In fact, fish are classified according to salt tolerance. Fish that can only live in waters with a narrow salt range are called narrow salt fish; Freshwater fish such as goldfish and sea fish such as tuna belong to this kind of fish. Fish that can survive in waters with different salinity are called widely salted fish, such as salmon, eel, eye spotted Pseudosciaena leucocephala produced along the Atlantic coast of North America. They can migrate from freshwater areas to slightly salty waters or from slightly salty waters to very salty waters – of course, if the salinity changes greatly, they need a period of adaptation

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