Sharks eat lots of things. Great whites famously hunt large mammals, basking sharks scoop up tiny plankton and some sharks even eat each other! But the thing they tend to have in common is that they're carnivores. Their teeth, their organs, their senses – all are beautifully adapted for catching and eating tasty animal prey.

And then there's the bonnethead shark. The shark that eats grass.

Bonnethead sharks – also called shovelheads or bonnetnose – hunt fish and crustaceans in seabeds of the warm coastal waters of the Americas. Image: Samantha Leigh

Bonnetheads – close relatives of hammerheads – are pretty weird as sharks go. Inside their uniquely shovel-shaped heads are two different types of teeth: the blade-like teeth in the front are standard slicers, but farther back, they have unusual flat, molar-type teeth for crushing up hard-shelled food like crabs and shrimp (some other crustacean-chomping sharks have this feature as well). Even their stomachs are a strange shape, straight instead of bent like most sharks.

But they also eat grass!

"[T]he bonnethead shark is the only known shark species to consume copious amounts of seagrass (up to 60% of their diet)," said Samantha Leigh of the University of California, Irvine. Leigh's shark research, which is funded through National Geographic, focuses on digestion, so you can understand her interest in a shark with a belly full of plants.

"Sharks, to our knowledge, are largely carnivorous," she explained, "and presumably have guts optimised for a high-protein/high-lipid diet."

In the Florida Keys, Leigh escorts a female bonnethead toward the research boat. As you can see, these animals aren't nearly as big as some of their hammerhead cousins, typically growing around 1-1.5 metres (3-5 feet) long. Image: Samantha Leigh

But are the bonnetheads grabbing grass on purpose, or are they getting an accidental "seaweed wrap" around their sushi? Leigh presumes the latter: "I suspect that the sharks are incidentally consuming the seagrass while foraging on other prey items that live in the seagrass beds (crabs, shrimp, small fish, etc.)."

If this is true, then it's even more surprising to learn that unlike typical carnivores, the bonnetheads are actually capable of partially digesting the plants!

Examination of the sharks' intestinal juices has revealed enzymes for digesting fats and proteins (found in meat) and enzymes for digesting chitin (found in crustacean shells), but also enzymes for breaking down cellulose, the famously hard-to-digest component of plants. That means bonnethead guts are extremely effective at digesting crabs, fish ... and grass!

By comparing the nutritional value of grass before and after it's made its way through the sharks, research has found these animals are digesting about 56% of the seagrass they eat. "To put that in perspective," Leigh said, "this is comparable to juvenile sea turtles, which are strict herbivores, feeding on the same seagrass and living in the same habitat as the bonnetheads."

The reality of grass-eating sharks raises so many questions, and Leigh is looking to answer two of the big ones. How exactly are these sharks able to digest the grass so well? And are their bodies actually capable of taking in nutrition from the grass?

Leigh passing a bonnethead up to her colleague during another shark-fishing research expedition. Image: Yannis Papastamatiou

Regarding that first question, she thinks microbes might have something to do with it. Lots of plant-eating animals, from cows to termites, digest cellulose with help from gut bacteria. Part of Leigh's plan going forward is to get a close look at the microscopic community inside shark intestines. Beyond that, she'll be sampling belly juices, cutting into gut tissues and generally feeding grass to a lot of sharks, all to put together a newly detailed look at the inner chemistry of sharks.  

"Answers to these questions could lead one to re-evaluate the role that these sharks are playing in their environment," Leigh suggested.

And if it turns out these sharks really are gaining nutritional value from the grass, then maybe their herbivorous habits aren't an accident after all. But finding out whether or not the sharks gobble grass on purpose will take careful observations in the field, Leigh says, "which is always tricky considering you are dealing with wild animals living in a vast ocean."



Top header image: John Turnbull/Flickr