By Kathleen Berger

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 was to evolve the sleek and streamlined torpedo, or javelin, shaped bodies in order to manage drag and conserve energy.

Understanding the evolution of sharks may one day help conservation efforts.

“In the context of climate change, for example, knowing how animals live and how they react or how they live in waters of given temperatures, you can start guessing what may happen if the waters are getting warmer,” Potvin said. “That is, which species would be more successful negotiating climate change.”