By Blaise Jones
Sharks are incredibly ecologically-flexible animals. They can be found in every sea in the world, ranging from pole to pole. However, there are some places sharks do not frequent. Freshwater lake and river systems are relatively free from large predatory fish that could threaten humans. However, as has been proven time and time again, sharks are nothing if not adaptable and that means freshwater rivers and other ponds can be home to some specific sharks.
While modern science still struggles with many questions when it comes to sharks, we were even more unknowledgeable in our past. At the turn of the 20th century sharks were a subject of speculation and uncertainty. People knew they existed, but didn’t know all that much about them. And they certainly weren’t afraid of them.
All that changed during the summer of 1916, when over the course of 10 days five people were attacked, four of whom died. While the attacks in and of themselves were shocking enough, what really grasped everyone’s spines in an icy grip was the fact that three of these attacks occurred in a creek, 30 miles from the coast.
Not only did this event serve as the start of America’s fearful fascination with sharks, it also was one of the first confirmed recordings of sharks in freshwater. While the species in question is still in debate, with speculation ranging from bull sharks to great whites to even porbeagles, what is certain that sharks can venture into fresh water.
A Fresh Perspective
Sharks are primarily oceanic animals, but many species do spend time in areas of lesser salinity. Sharks often give birth to their pups in shallow inshore waters, often in estuaries. Not only do these locations offer plenty of food and places to hide from predators to the shark pups, but the lower salinity helps keep bigger sharks away. As the pups grow bigger, their tolerance towards low salinity reduces and they leave for the open ocean.
There are some species that permanently live in freshwater, however. These sharks are all part of the same genus, Glyphis, and have names that let you know where to find them. They are the Ganges shark in India, the northern river shark in New Guinea, and the Irrawaddy river shark in Myanmar.
However, one species has managed to not only colonize freshwater habitats, but also return to the sea whenever it wants: the bull shark.
A Load of Bull (Shark)
Bull sharks are infamous for their nasty temperament and their powerful bites, but perhaps their most fascinating feature is their tolerance of freshwater.
Bull sharks have been found in freshwater habitats all over the world, ranging from rivers and lakes in North and South America all the way to the Ganges river in India. One bull shark was even found 1,800 miles up the Mississippi River, in Illinois!
Bull sharks take the most advantage of having offspring in fresher water by using freshwater around the world. Lake Pontchartrain in Louisiana and Lake Nicaragua in Nicaragua and two freshwater lakes that are known to scientists to be breeding grounds for bull sharks pups.
The mother bull sharks give birth in the mouth of the rivers and the shark pups head inland to the lakes. In Lake Nicaragua, there’ve been stories of sharks up to four feet long jumping up waterfalls like salmon do! Once in the lakes, these sharks are safe from any and all large predators, and they live it easy until it’s time for them to return to the sea.
But how do bull sharks and the river sharks actually survive in freshwater while other sharks cannot? It all comes down to osmoregulation. Osmoregulation is, “the ability of an organism to maintain a constant concentration of water in its body even when its outside environment would normally cause it to lose or gain water.” All fish, both freshwater and saltwater, osmoregulate. Saltwater fish need to get rid of the extra salt, while freshwater fish want to retain salt.
However, bull sharks differ from other fishes because they can adapt their osmoregulation to their external environment. All sharks regulate the salt levels in their body via their kidneys. As they drink the salt water their kidneys filter out all the salt and expel it from their body as waste material, in this case urea.
Bull sharks can gradually alter the amount of salt they excrete from their bodies as they move from salt to fresh water. As they enter the fresh water they begin drinking more and more, reducing the salt concentration in their blood to that of the surrounding water. Since this is a gradual process it does not kill the shark. This does, however, have a side effect: bull sharks living in fresh water pee about 20 times more frequently than those in salt water.
“The Encyclopedia of Sharks” by Steve Parker
“Sharks: The Animal Answer Guide” By Gene Helfman and George H. Burgess.
#sharks #freshwater #bullsharks #TSF