Bonnethead's eating habits reveal it's the first omnivorous shark

September 7, 2018, 12:35 p.m.
Bonnethead sharks enjoy a tasty sea salad when meat isn't readily available.
Photo: Vladimir Wrangel/Shutterstock

Sharks have a reputation as voracious carnivores. It turns out that one shark, however, is content with a tasty salad of seagrass instead of fish or crab.

The bonnethead shark (Sphyrna tiburo) only grows to about five feet long, making it small by shark standards. The shark has been known to consume seagrass since 2007, with the plant sometimes accounting for 60 percent of the shark's overall diet. What we didn't know until now was whether or not the shark was digesting the seagrass and using it for nutrients as well.

"It has been assumed by most that this consumption was incidental and that it provided no nutritional value," Samantha Leigh, a graduate student at the University of California, Irvine, and a researcher on the study, told The Guardian. "I wanted to see how much of this seagrass diet the sharks could digest, because what an animal consumes is not necessarily the same as what it digests and retains nutrients from."

Leigh and her team's findings were published in Proceedings of the Royal Society B with one conclusion:

"The bonnethead shark is the first known omnivorous species of shark," Leigh said.

To determine if the bonnethead shark was getting any actual benefit from the seagrass, they collected bonnethead sharks from the Florida Keys, took them to a lab and fed them a diet of 90 percent seagrass. Researchers added a specially-made sodium bicarbonate powder containing a specific carbon isotope that provided the powder with a distinct chemical signature.

After three weeks of this, the researchers ran tests on the sharks. These tests showed that the sharks were indeed breaking down the seagrass, to get nutrients, like starch and cellulose. In fact, more than half of the organic matter was being absorbed by the sharks. As for the special powder, researchers used that to see if it was appearing in the sharks' blood and liver tissue. Since it was, it helped scientists to conclude that the seagrass was indeed being used to keep the animals healthy.

Now that we know that this shark at least relies on plants to survive — perhaps out of a desire to avoid competing with larger sharks in the area — we may need to reevaluate what we know about not only sharks, but all species. This will help us better maintain habitats for the animals to thrive.

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