Most sharks are happy to swim around, splash and play , and tear giant hunks of blubber out of marine mammals in a salt water environment. Actually, even the ones that dine on small fish prefer to keep to the ocean and avoid freshwater. This is because sharks have evolved to live in balance with a salty surrounding. To do this, their bodies are designed to maintain proper osmotic pressure. For any cell membrane or animal skin surface, there is a balance that must be maintained between the “solutions inside’ and the “solutions outside.” If there is an imbalance between the two, water will flow to the area of its lowest concentration. Sharks have fairly salty interiors so they are at home in the ocean. Urea in their systems allows them to absorb the salty water, and their kidneys produce a great deal of urine to get rid of the excess salt that comes with the water. This is called osmoregulation.
Some few sharks, however, can control their osmoregulation as they swim into brackish or even fresh water. Bull sharks and the Australian speartooth shark can do this trick wherein their kidneys automatically remove less salt and more urea from their systems as they become surrounded by less salty water. In freshwater they increase their urine flow to throw off the excess water that is trying to balance the osmotic pressure. If not for this, freshwater would move continuously into their bodies until they more or less exploded or at least bloated to the point of death.
With their ability to adjust their osmoregulation, bull sharks have the means to hang around in estuaries and even to swim up rivers. They have been found more than 2,000 miles up the Amazon, as well as into the Ganges, and to St. Louis on the Mississippi! On rare occasions they have even been sighted up the Illinois River to Lake Michigan. There is a population of bull sharks in Lake Nicaragua, 120 miles up the San Juan River from the Caribbean. It was once thought that they were a specially-adapted separate species—that since the lake had once been part of the Pacific and tectonic forces had isolated it over time, that the sharks which had gotten trapped in it gradually evolved to tolerate the ever-freshening water. Tagging, however, proved instead that they travel from sea to lake and back when they feel like it, staying in the freshwater environment for periods of two weeks or more. Unfortunately for us, bull sharks are rather aggressive and prefer murky and shallow waters, so some rivers, estuaries and even lakes may bear some caution for swimmers. Bull sharks are the most common offender in shallow water shark/human contacts. The speartooth shark is infrequently seen and is even better adapted to freshwater. No specimens have ever been taken in a marine environment.
Why do just a few species have this superpower to move from salt to fresh at will? Scientists aren’t entirely sure, but it may afford some safety for the younger sharks to live their small-sized youth away from the main population of other sharks until they are bigger, or allow them to access alternate feeding grounds at any time of their lives. Whatever the reason, it takes a good bit of energy to produce and operate this enhanced physical system, so apparently it is worth the extra effort for a select few species.