Bonnethead Shark in the shallows

The bonnethead shark supplements its diet of invertebrates with seagrass, making it the first known omnivorous shark. Credit: Jay Fleming/Getty

Animal behaviour

The world’s first flexitarian shark grazes like a cow

A member of the hammerhead family munches on seagrass meadows as well as guzzling squid.

Sharks are notorious carnivores, but one small hammerhead shark also feels the need to eat its greens.

Bonnethead sharks (Sphyrna tiburo) live in seagrass meadows in the shallow waters off the coasts of the Americas. In addition to consuming invertebrates such as squid, they have been observed eating large amounts of seagrass. But whether the animals digest this dietary plant matter has been a mystery.

Samantha Leigh at the University of California, Irvine, and her colleagues fed five captive bonnetheads on seagrass grown in water containing carbon-13, an uncommon form of carbon. After several weeks on this regimen, the sharks’ blood contained high levels of carbon-13, which must have come from the seagrass in their diet. The team also found that the creatures’ guts host enzymes capable of breaking down cellulose and other carbohydrates found in grasses.

This makes the bonnethead the first omnivorous shark ever recorded. Because they are both predators and grazers, bonnetheads play an important part in seagrass ecosystems, the authors say.