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Question: Why are some sharks able to swim in freshwater when others can only swim in saltwater?
Every creature living on our planet has evolved special features, or adaptations, which enable it to survive in the environment it inhabits. A shark is a fish (a special kind of fish called a cartilaginous fish) and so it must be able to live in water and adapt to the specific challenges that type of environment presents.
For example, oxygen is more dilute in water than in air. Therefore fish have developed gills to extract oxygen from the water. Water is also denser than air so fish must have strong muscles to swim. But the problem that we are concerned with is something called osmosis. That is, water likes to move from places where it is more concentrated to places where it is less concentrated (water concentration is affected by salts; the more salt the less concentrated the water). Fish have salts in their bodies, and their skin is actually leaky. So the concentration of salt in the fish's body and the concentration of salt in the water it lives in determine which adaptations the fish needs to survive.
A fish living in salt water is generally less salty (water is more concentrated) than its environment. Therefore water leaks out of the fish and the fish must drink a lot of water to replace it. Because it drinks seawater, it needs to get rid of extra salt. This is accomplished through special cells (called chloride cells) at the base of the gills. A fish living in fresh water has the opposite problem. Because the fish is saltier than the surrounding water, water leaks into the fish. The fish compensates by excreting large quantities of urine. The gills and kidneys of freshwater fish extract salts from water and urine and return them to the bloodstream. They do this because salts also like to move to areas of lower concentrations, in this case from the fish into the surrounding water.
Sharks are different in that their bodies are a little bit saltier than seawater. Therefore they don't lose much water. The water that comes in through their mouth and gills is just enough to replace lost water. Such a salty fish would have great difficulty surviving in fresh water, and most sharks can't. However, two kinds of sharks, Bull sharks and River sharks, can survive for extended periods of time in freshwater. Bull sharks are commonly found in rivers and lakes and have been sighted as far up the Mississippi River as Illinois. River sharks are rare and primarily live in Asia and Australia. These animals actually reduce the concentration of salt in their bodies by excreting some through a rectal gland. They are still saltier than the water and produce about twenty times as much urine as when they are in the ocean. This would seem to be quite a strain on the animals' kidneys, but Bull sharks have been known to inhabit fresh water for up to six years with no adverse health effects.
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