The bull shark has gained the ability to live in both fresh and salt water which allows it to inhabit almost all waters of the earth.  Even though bull sharks can live in fresh water, they must retain salt inside of their bodies.  If they did not, their cells would rupture and they would die.  Because of this almost all sharks other than the bull shark can not enter fresh water because their internal sodium concentration would become diluted. The kidneys of the shark recycle salt inside the body very efficiently with the help of glands located near the tails of the shark.  Even with this specialized recycling process, the body of the bull shark still is extremely hypertonic to the surrounding fresh water.  Osmosis causes the body to absorb large amounts of water which must be evacuated from the body by urination.  It is estimated that this shark urinates twenty times more when in fresh water than in salt, which would be assumed to put a massive strain on the kidneys.  It does however seem like the kidneys and the shark are not damaged from this extra strain since bull sharks has been documented to stay in freshwater for up to six years without negative effects to their health.  It is still unknown why only the bull shark developed the ability to live in both fresh and salt water, but it is believed that this ability gave its young a greater ability to escape predators.  It has been also hypothesized that this unique ability may have developed from bull sharks that were unable to compete for resources in salt water and needed this ability to survive. 



How sharks swim while sleeping:


It is unknown whether or not sharks sleep, because one has never been recorded sleeping.  All species of sharks rely on constant movement to push oxygenated water into their mouths and over their gills in order to survive.  Some sharks have been recorded lying motionless in caves, but their eyes always follow divers that see them.  It is believed that sharks shut down one hemisphere of there brain for short 2 to 3 minute stretches in order to rest.  Through experimentation on sharks it was discovered that the Central Pattern Generator which regulates swimming movements is not located in the brain, but in the spinal chord.  Because of this an unconscious shark can swim, allowing its gills to come in contact with oxygenated water.  



Movie Myth:


Many movies show sharks swimming with their dorsal fin out of the water although this rarely happens.  Most sharks rarely swim in shallow enough waters for their dorsal fin to be seen out of the water.  Bull sharks are however one species that does swim in shallow water and its dorsal fin can sometimes be seen out of water.  When the dorsal fin protrudes from the water, it means nothing else other than the shark is in shallow water.  It has nothing to do with an oncoming attack.