Sharks evolved aircraft-like attributes to suit habitats

Sharks have evolved diverse physical attributes, such as those resembling the aerodynamics of high speed aircraft, to help them thrive in different ocean ecosystems, a study has found.

Researchers investigated the body composition of 32 shark species to understand their buoyancy control.

They found that these top predators mirror the aerodynamics of either zeppelins or fixed-wing high speed aircraft, depending on whether they evolved to cruise through the deep ocean or motor through shallow waters.

"Poorly known species such as Bramble Sharks and Birdbeak Dogfish that live in the dark, cold and nutrient poor oceans, have evolved enormous fatty livers that can make up more than a quarter of their bodies," said Adrian Gleiss, a biologist at Murdoch University in Australia.

"They are the zeppelins of the shark world, cruising near effortlessly at low speeds to save energy," said Gleiss, lead author of the study published in Proceedings of the Royal Society B: Biological Sciences.

"Conversely, shark species with smaller livers, like most whaler sharks, are negatively buoyant and so must swim at fast speeds so their wing-like fins can provide sufficient lift, overall expending more energy than their deep-dwelling cousins," said Gleiss.

The researchers wanted to understand why it is that those small-liver sharks managed to survive even while using such a costly lifestyle.

"This baffled the team, because for an animal to be wasteful with its energy expenditure should interfere with survival unless it would prove beneficial in some other manner," said Jeremy Goldbogen, biologist at Stanford University in the US.

"Looking into the hydrodynamics of swimming animals provided an answer to this question," said Goldbogen.

It turns out that as sharks evolved larger livers, they also became much bulkier and less hydrodynamic.

This, the team found, would slow sharks down when hunting for agile prey such as fish and squid or when escaping their own predators.

"The zeppelins of the shark world must live in cold places where slow swimming is the rule for both predator and prey," said Jean Potvin, physicist at Saint Louis University in the US.

"As with the sharks inhabiting shallow waters, trying to fly zeppelins at the speeds and accelerations common to fixed wing aircraft of same the weight class would require significantly more energy, unless that is, it is reshaped into a more javelin-like body," Potvin said.

"It would follow that slimmer, and thus more hydrodynamic sharks will require less energy for swimming at the high speeds necessary to catch agile prey, especially in environments where fast swimming is common," Potvin said.

The researchers found that the difference in the evolution of liver size is a result of this process.