Are There Freshwater sharks?
Whether it exist true freshwater sharks are still somewhat of an open question. There is no doubt that there exist sharks that live in freshwater but most of these species are really marine sharks that are able to migrate up into freshwater bodies and live out their entire lives there. An example of such a shark is the bull shark that is notorious for wandering into freshwater and that has been found over 4200 kilomaters / 2,600 miles up into 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 does however show that even if bull sharks can live for many years in Lake Nicaragua they doesn’t breed here and 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 they can’t reproduce in freshwater.
The bull sharks and other sharks ability to wander between marine and freshwater has for a long time been a mystery. Especially since sharks even in saltwater has a higher salinity in their body then the surrounding water. Recent research made in Lake Nicaragua has shed some light on this mystery. It seems that sharks are able to reduce their bodily sodium and chloride levels by excreting the excess salts via a rectal gland and thereby reducing the amount of bodily sodium and chloride by 33%. They 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 the osmosis causes there body to absorb a large amount of water. It is believed that they sharks deals with this by extracting this water from their bodies by excretion urine. The massive amounts of water that are absorbed into their bodies by osmosis will result in that the shark produces up to 20 times more urine than they would in freshwater. Scientist believes that the kidneys regulate this water excretion which would but massive strain on them. It does however seem like the kidneys and the sharks doesn’t take damage 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.
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 - True freshwater 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 makes it likely that more species might be found.
River sharks are primarily found in Australia and Asia. The River shark group contain the
Ganges shark (Glyphis gangeticu ) which is found in parts of India and Pakistan. The Speartooth Shark (Glyphis glyphis) which can be found in Queensland, Australia, as well as on Borneo and New Guinea. It is however likely that the Speartooth shark in the future will be divided into several species.
The Irrawaddy River Shark (Glyphis siamensis) is the last of the describied river sharks species and 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 belong to the river shark group.
There are also 3 (possible 4 since specimens recently where collected from an area where no river shark species is know to exist in borne) 2 of these species can be found in the Australia in river systems in the northern territory. Primarily in, Adelaide River and alligator River. The last (possible last two) species has been found in borne.
There have recently been some very young specimens of river sharks collected in freshwater, which suggests that these sharks might be able to reproduce in freshwater and therefore be able to be called true freshwater sharks. The river sharks species have never been collected in marine water (they have been found in slightly brackish water) which further suggests that these might be true freshwater sharks.
It is however hard to learn more about these sharks as they are critically endangered and only found in a small number of habitats but we will hopefully be able to safe these species for posterity and find out whether they really are true freshwater sharks that can live there entire life and reproduce in freshwater. The future might even prove that they like the freshwater stingrays of the amazons no longer are able to survive in marine water.
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