Did you know that bull sharks have been sighted nearly 2000 miles from the ocean in fresh water lakes and rivers like the Amazon? They are able to survive due to their unique ability to self regulate the amount of salt stored in their body. Check out the information below for a more detailed description of this process!
In sharks, the normal mechanism of osmoregulation in a marine (salt) environment is the high concentration of urea and other biological solvents in their blood and the removal of excess salt from their bloodstream through urine. The former allows them to absorb water from their marine surroundings, while the latter rids them of the salt they continually absorb. These tasks are primarily controlled by the kidneys.
In most sharks, these adaptations cannot be changed. If they are put into a freshwater environment, they will absorb too much water relative to their concentration of bodily solvents and lose too much salt to stay alive. In other words, most sharks cannot survive in fresh water because they are not capable of adapting to it.
Bull sharks are unusual because they can adapt readily to fresh water because they can adapt their process of osmogregulation. The kidneys of bull sharks, (and to a lesser extent several other types of sharks) can be gradually adjusted to suit the salinity of the water they are in. When moved gradually into freshwater (perhaps by migrating from the ocean, to an estuary, and then upriver), bull sharks' kidney's remove less salt and more urea from the bloodstream through urination, essentially reversing the normal marine shark method of osmoregulation. This adaptation allows bull sharks to live entirely in estuaries or freshwater.
Information courtesy of #sharksavers
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