They have a fearsome reputation as bloodthirsty monsters of the deep, but some sharks are actually quite happy to munch on greens, US scientists have discovered.

The coastal bonnethead shark, a smaller relative of the hammerhead, is thought to be the first known omnivorous shark.

Originally believed to be carnivorous, the Sphyrna tiburo is actually a flexitarian, meaning it can survive on either plant or meat-based diets.

It had been known for some time that bonnethead sharks typically consume a lot of seagrass (with it constituting up to 62 percent of their stomach contents), but it had been assumed this was just something the fish were assimilating during their hunt for squid and crabs.  It was unknown whether they could actually digest or obtain any nutrients from it.

To test this theory, the findings of which are published in the Proceedings of Royal Society B, captive bonnethead sharks were fed a majority seagrass diet, with the results proving the fish were able to successfully digest it using stomach acids.

The authors wrote: “Remarkably, the bonnethead’s digestibility of organic matter is comparable to juvenile green sea turtles.

“As green sea turtles mature, they become almost entirely herbivorous, and their digestibility of seagrass increases to 65 percent.

“Bonnetheads are capable of digesting components of seagrass, with similar effectiveness to omnivores, making them the only shark species know to have the ability to digest plant material.”

Samantha Leigh, a researcher on the team, told the Guardian: “The bonnethead shark is the first known omnivorous species of shark.

“This has implications for fragile and crucial seagrass meadow habitat management.”

The bonnethead shark is found in shallow estuaries an bays on the Atlantic and Pacific Ocean coasts of the Americas, as well as the Gulf of Mexico.