By Kathleen Berger
If fear of sharks is keeping you out of the ocean, a freshwater spot is considered safe. A Saint Louis University shark expert explains the science behind why the vast majority of the world’s hundreds of species of sharks stay out of streams and lakes where we swim.
One reason has to do with their body’s regulation of salt concentration. Sharks actively regulate their internal salinity to match the salinity of their outside environment through the process of osmoregulation.
Jean Potvin, PhD, is a physics professor at Saint Louis University who specializes in hydrodynamics. He explainshow osmoregulation pertains to the energetics of sharks. Potvin says the tissues of sharks have a slightly greater salt concentration than the oceans. When in freshwater, a greater amount of the water compared to saltwater will diffuse through their skin. So the body’s removal of freshwater would be costly, taking more energy.
For people who fear sharks, Potvin has more good news. Potvin teamed up with researchers at Murdoch University in Australia and the Hopkins Marine Station of Stanford University to perform calculations that now provide another reason why sharks don’t like freshwater.
Conclusively, sharks are negatively buoyant; that is, they sink. They calculated that sharks sink faster in freshwater than saltwater
“Saltwater is more dense and freshwater is less dense,” said Potvin. “So sharks are more negatively buoyant in freshwater than they are in saltwater. Because of that, sharks have to move faster to generate more lift in order to stay aloft at the given water depth. Therefore, having to move faster means that they have to spend more energy.”
Potvin says this is unlike fish that can remain at the given depth because they have a swim bladder. It’s an organ that fills with gas to change the buoyancy of thefish, allowingit to ascend and descend in the water.
What does this mean in freshwater? Sharks would have to move forward at an even higher speed in freshwater than saltwater to create more lift, much like an aircraft.
For the physical modeling, Potvin used hydrodynamics calculations that are borrowed from aircraft performance analysis.
“For sharks to remain at the given depth, they have to move forward. They have to fly through the water in the same way airplanes fly through the air,” Potvin explained. “They generate lift off of their body and fins, mostly pectoral fins, and fly forward like airplanes. Airplanes have propellers and jet motors. Sharks have their tail fin. That force compensates for their negative buoyancy.”
Potvin says sharks will crash on the bottom if they don’t move forward in the same way airplanes crash on the ground.
In this way, sharks are less like fish and more like aircraft. Potvin said sharks evolved from a different ancestor than fish, to have a partial buoyancy compensator of their own. They evolved having a low-density liver, which can be very large, helping them maintain necessary speeds.
“Without the liver, they would sink even faster than with the liver,” he said.
In some species of sharks, the liver is very large, up to 30% of its body volume. Slow moving sharks living deep in the oceans grew the larger fatty livers keeping them from sinking at slower speeds, allowing them to conserve energy. Faster moving sharks living in warmer, shallower waters have comparatively smaller livers than the others.
That makes swimming in the lower density freshwater all the more difficult, requiring more energy.
There are a handful of sharks in the world that evolved to live in freshwater or spend some of their time there.
While adapting to freshwater, those sharks still have the buoyancy issue, so Potvin says most of the freshwater sharks hang out at the bottom, as bottom dwellers.
Not bull sharks though! Bull sharks can live in the ocean and freshwater. Experts say they evolved to have a freshwater tolerance. They developed the ability to restrict removal of salt from their bloodstream by the rectal gland. The gills and kidneys also help adjust the amount of salt going in and out.
What about improved buoyancy? Potvin said “no”. Bull sharks have to swim forward to avoid sinking.
“They don’t spend all of their lives in freshwater,” he explained. “They come in, they eat, and then they come out. Bull sharks are also known to not procreate in freshwater, only in saltwater.”
There are claims of bull sharks seen in the Mississippi River north towards St. Louis, with one claim as far north as Alton, Illinois.
Bull sharks are aggressive and are believed to be responsible for shark attacks on people near shores, even near freshwater shores. They tend to be territorial and may attack anything that enters their zone.
Still, for anyone truly afraid of shark encounters, Potvin considers freshwater a safe option.