Sharks evolved aircraft-like attributes to suit habitats

November 8, 2017

A bull shark (Credit: Associate Professor David Morgan)

Spitfire speed: Bull sharks are among the species to have evolved smaller livers resulting in negative buoyancy (Image: Associate Professor David Morgan)

Shark species have evolved diverse physical attributes to help them thrive in different ocean ecosystems.

Murdoch University researcher Dr Adrian Gleiss from the Centre for Fish and Fisheries Research led an international team to better understand the buoyancy control of sharks.

Researchers from Stanford and St Louis Universities in the United States helped Dr Gleiss to investigate the body composition of 32 shark species.

Results showed the movement of these top predators through the water had evolved to mirror the aerodynamics of aircraft. Sharks which cruise through the deep ocean mirror zeppelins and species which motor through shallow waters were similar to fighter aircraft like spitfires.

“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,” Dr Gleiss said.

“They are the zeppelins of the shark world, cruising near effortlessly at low speeds to save energy.

“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.”

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.

Professor Jean Potvin, a physicist from St Louis University specialising in the hydrodynamics of swimming animals, provided an answer to this question.

It turns out that as sharks evolved larger livers, they also became much bulkier and less hydrodynamic. This, the team suggests, would slow sharks down when hunting for their agile prey such as fish and squid or when escaping their own predators.

“Whereas deep sea sharks can hide in perpetual darkness, ambushing unaware prey, those living in shallow waters will have to out run their prey or predator, providing very different challenges for survival. We found that the difference in the evolution of liver size is a result of this process,” Dr Gleiss said.

Understanding the evolution of marine fish from largely bottom-dwelling organisms to those capable of swimming at a range of depths represents one of the key evolutionary transitions, Dr Gleiss added. The study has shed important light on how the environment has shaped this process.

“It is incredible to think that many hundreds of million years ago, the early ancestors of fish only lived near the sea-bed, sporting heavy armour that prevented them from swimming in mid-water,” Dr Gleiss said.

“Sharks represent a relic of this time and were probably among the first fish to exploit most depths of the ocean. Our study contributes to our understanding of the evolutionary processes that led to them being such a successful group.”

The study has been published in the Proceedings of the Royal Society B journal.

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