The bonnethead shark, a small member of the hammerhead family, was long thought to be a strict carnivore that would occasionally ingest greens purely by accident.
Not so, scientists said Wednesday. The bonnethead follows an omnivorous diet in which seagrass plays a key, nutritional role.
Although researchers have long known that Sphyrna tiburo eats copious quantities of sea grass, it was not believed to absorb any nutrients from it.
"Until now, most people thought that sea grass consumption was incidental when these sharks were hunting for crabs, etc. that live in the sea grass beds," study co-author Samantha Leigh, an expert in ecology and evolutionary biology at the University of California, Irvine, told AFP.
Leigh and a team found, however, that sea grass can form up to 62 percent of the bonnethead diet, alongside their preferred meal of crustaceans and molluscs.
"Bonnethead sharks are not only consuming copious amount of sea grass but they are actually capable of digesting and assimilating sea grass nutrients, making them clear omnivores," the researchers wrote in the journal Proceedings of the Royal Society B.
"This is the first species of shark ever to be shown to have an omnivorous digestive strategy."
Over the course of three weeks, the team conducted a series of lab-based trials in which they fed bonnethead sharks a diet of 90 percent sea grass and 10 percent squid.
They then analysed how much of the nutrients the animals digested, and how much they excreted.
Not all carnivores can digest plant material efficiently, but sharks fed the sea grass-heavy diet all gained weight, the team found.
The animals were found to be as good at digesting fibre and organic matter as young green sea turtles -- a species that transforms from eating an omnivorous diet in youth to committed vegetarianism in adulthood.