By Kathleen Berger, Executive Producer for Science & Technology

When looking at the different types of sharks, you may notice that some are sleek and streamlined, like an airplane, while others are shaped like a blimp. But why is that? Jean Potvin, PhD, professor of physics at Saint Louis University, has joined researchers from around the globe logging hours by boat and in the lab in the study of hydrodynamics, his specialty. 

On one of his latest projects, Potvin and his colleagues at Murdoch University in Australia and the Hopkins Marine Station of Stanford University were focused on the energetics of shark swimming and its relationship to buoyancy, along with the drag force the varieties of sharks generate when they swim.

Slow moving sharks are shaped more like a zeppelin, or blimp, while the faster sharks are shaped like a high-speed javelin or torpedo.

Potvin and his colleagues discovered two different evolutionary paths for sharks.

“The difference may be due to the drag that each kind generates,” said Potvin. 

In order to investigate, Potvin and the research team studied the body composition of 32 species of sharks. For the physical modeling, he used hydronamics calculations that are borrowed from aircraft performance analysis.  

They discovered sharks evolved attributes of aircrafts to suit their habitats.

“The blimpy sharks tend to live in cold water, versus the torpedo sharks that tend to live in the tropics, near the surface,” he said. “The cold water sharks tend to prey on slower fish, which require a slower speed in order to catch them. The torpedo-like, tropical sharks have to move fast and accelerate swiftly in order to catch their prey.”

Potvin explained how the hydrodynamics of drag in each case is difference. The slow-moving sharks can get away with being shaped like a zeppelin, or blimp, because they don’t generate a lot of drag.  For blimp-shaped sharks, moving faster, resulting in more drag, requires more energy. Sharks that live in warmer and shallower waters have to accelerate fast which generates more drag. The only way for sharks in the faster-moving environments to thrive were to evolve the sleek and streamlined torpedo-shaped or javelin-shaped bodies in order to manage drag and conserve energy.

The research also proves sharks are negatively buoyant, meaning they sink. Through evolution, Potvin explained how sharks developed larger, low-density livers. He said their livers have become partial buoyancy compensators, 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. 

“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 said the research offers a thorough explanation as to why the vast majority of the world’s hundreds of species of sharks stay out of streams and lakes.

First, sharks actively regulate their internal salinity to match the salinity of their outside environment through the process of osmoregulation, which 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. 

And his research discovered sharks would spend even more energy swimming in freshwater than saltwater because they are negatively buoyant, calculating how 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.” 

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, evolving to have a freshwater tolerance. 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.”