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Some fish drink the water they swim in, while others absorb it through osmosis. It just depends on whether the fish lives in freshwater or saltwater. The MadSci Network explains that fish need to keep a certain concentration of salt in their bodies. Marine fish need to worry about preventing water loss, because they already get plenty of salt. They drink sea water, and their gills process the water but remove the salt. In fact, some saltwater fish lack properly functioning kidneys, in order to help avoid losing too much water. Freshwater fish have the opposite problem: They need more salt. They absorb water through
their skin and have effective ways of excreting excess liquid to maintain the salt they need. Some fish have developed the ability to exist comfortably in both freshwater and saltwater -- the salmon and bull shark are notable examples. Bull sharks have been found far up the Mississippi and Amazon rivers, much to the surprise of people who believe sharks are found only in the salty, salty sea.
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