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 so ...

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