Quirks & Quarks

A meat eater chooses a side of seagrass: This shark is an omnivore

Long thought to be exclusive meat-eaters, it turns out the bonnethead shark is an omnivore.

'This has never been seen before in a shark species,' says ecologist Samantha Leigh

Bonnethead shark (Sphyrna tiburo) at the Aquarium of the Americas, New Orleans (Kelly McCarthy)

Originally published on October 20, 2018.

We're used to thinking of sharks as meat eaters, but a new study has found that the bonnethead shark — a small shark found off both coasts of the Americas — is actually an omnivore. It's the first shark species shown to eat veggies as well as meat.

Bonnethead sharks are known to ingest a significant amount of seagrass along with their standard diet of fish and crustaceans — however, it was never clear how much of the seagrass was actually being digested. But a team led by Samantha Leigh, an ecologist at the University of California, Irvine, has found that they not only ingest the seagrass, they also digest around 50 per cent of it.

In other words, the sharks are extracting nutrients from the seagrass. 

"They were digesting about half of the organic matter from the seagrass — so, definitely acting like omnivores," Leigh told Quirks & Quarks host Bob McDonald. "This has never been seen before in a shark species."

Figuring out how much seagrass they digest

Leigh and her team studied the bonnethead sharks in an outdoor lab just off the Florida Keys. The sharks were fed a daily diet of seagrass wrapped in squid — rather like an inverse sushi roll.

The seagrass, which made up 90 per cent of the roll, was loaded with a tracker isotope to see where it ended up. Then, by measuring the amount of tracker in the sharks' blood, and also by studying the animals' feces, they could determine how much of the seagrass was being digested.

The finding raises questions about the shark's digestive system, which was believed to be similar to that of other, meat-eating sharks. The difference likely involves the microbiome of the bonnethead, Leigh said. "We're hypothesizing that these bonnethead sharks have specific bacteria that live inside their digestive system that help them break down the seagrass," she said.

Sharks' diet key to understanding their role in ecosystem

Understanding what sharks eat will give scientists a clearer picture of how they interact with their environment, which can in turn aid in developing strategies to protect delicate ecosystems like the seagrass meadows commonly found in North America's coastal waters. 

"What they're digesting, what they're getting out of their diet, and what they're excreting into the environment are very different as an omnivore than as a carnivore," Leigh said. "So their role in these ecosystems is vastly different from what we had expected."