Are There Freshwater sharks?
Whether true freshwater sharks exist or not is still somewhat of an open question. There is no doubt that there are sharks that live in freshwater, but most of these species are really marine sharks that are able to migrate up into freshwater and live out their entire lives there. An example of such a shark is the bull shark which is notorious for wandering into freshwater and has been found over 4200 kilometers / 2,600 miles upstream in rivers such as the Amazon. Bull sharks also live in Lake Nicaragua and these sharks are often referred to as true freshwater sharks. Recent studies do however show that even if bull sharks can live for many years in Lake Nicaragua, they do not breed there and they need to return to the ocean to breed. The fact that the bull shark can live for prolonged periods in freshwater doesn’t really make it a freshwater fish as long as it can’t reproduce in freshwater. (Having to migrate from freshwater to saltwater or vice verse to breed is however not unheard of among fishes; the famous salmons runs do for instance occur when salmons migrate from the ocean to freshwater streams to spawn.)
The ability of bull sharks and some other shark species to wander between marine and freshwater has been a mystery for a long time, especially since sharks even in saltwater have a higher salinity in their body then the surrounding water. Recent research carried out in Lake Nicaragua has shed some light on this mystery; sharks seem to be able to reduce their bodily sodium and chloride levels by excreting the excess salts via a rectal gland and thereby reduce the amount of bodily sodium and
[color=black ! important][font=Verdana,Helvetica][color=black ! important][font=Verdana,Helvetica]chloride[/font][/font][/color][/color] by 33%. They can also reduce the amount of urea in their body by 50% in the same way. This process makes them more adapted to freshwater, but they still have a much higher salinity in their body than the surrounding water which means that osmosis makes the body absorb a large amount of water. Sharks are believed to deal with this by extracting excess water from their bodies in the form of urine. The massive amounts of water that are absorbed into the body through osmosis will result in the shark producing up to 20 times more urine in freshwater than they would in saltwater. Scientists believe that the kidneys regulate this water excretion, and living in freshwater is probably putting a massive strain on a shark's kidneys. The kidneys – or the rest of the shark – do however not seem to be damaged by this extra strain since bull sharks have been known to stay in freshwater for up to six years without scientists being able to detect any negative effects to their health.
