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 chloride 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.
River Sharks - True freshwater sharks?
As mentioned above, there are no known instances of bull sharks breeding in freshwater.
There are however other sharks that might be able to live their entire life in freshwater without having to migrate to marine water to reproduce. Most of these species are found among the river sharks.
River sharks are the popular name for shark species from the genus Glyphis. This group of sharks that looks similar to whaler sharks (Carcharhinus) contains a total of 6 known species. All species are very rare and half of them are still waiting to be scientifically described and given a name. The fact that so little is known about these species and that they earlier often have been misidentified as freshwater bull sharks make it likely that even more species may be found in the future.
River sharks are primarily found in Australia and Asia. The river shark group is currently comprised of the Ganges shark (Glyphis gangeticu) which is found in parts of India and Pakistan, the Speartooth Shark (Glyphis glyphis) that lives in New Guinea, on Borneo, and in Queensland, Australia, and finally the Irrawaddy River Shark (Glyphis siamensis). The Speartooth shark will however most likely be divided into several species in the future, since the different populations are quite dissimilar to each other.
The abovementioned Irrawaddy River Shark (Glyphis siamensis) can be found in waters in the area surrounding Rangoon (Myanmar/Burma). This species was until recently believed to be a variety of bull shark but has since been declared a separate species belonging to the river shark group.
There are also three not yet described species of river shark. (Possibly four, since specimens now have been collected from a part of Borneo where no river shark species are known to exist.) Two of these species can be found in Australia in river systems located in the Northern Territory, primarily in Adelaide River and Alligator River. The last (possibly last two) species has been found on Borneo.
Some very young specimens of river sharks have quite recently been collected from freshwater and this which suggests that these sharks might be able to reproduce in freshwater, which would earn them the label true freshwater sharks. In addition to this, river shark species have never been collected from marine waters (although they have been found in slightly brackish water) which further suggests that these might actually be true freshwater sharks.
It is difficult to obtain more information about the river sharks as they are critically endangered and only found in a small number of habitats, but we will hopefully be able to save these species from extinction and find out if they really are true freshwater sharks that can live there entire life and reproduce in freshwater. Future research might even show us that they, just like the freshwater stingrays of the Amazon, have lost the ability to survive in marine waters.
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