AP Photo/Tony Dejak
Fish may actually be friends, not food, to bonnethead sharks.
- The bonnethead shark may be the world's first omnivorous shark, according to new research.
- While scientists have long known that this shark consumes seagrass, they thought it was incidental consumption when the sharks went after their bottom-dwelling prey.
- The researchers say this study may force a rethink about what it means to be a carnivore.
Despite what you may think, not all sharks are bloodthirsty killing machines. Some, like the bonnethead shark, prefer to munch on seagrass.
It's the first shark to receive the official designation of an omnivore, according to new research published in the journal Proceedings of the Royal Society B.
The bonnethead shark - which is a smaller cousin of the more famous hammerhead shark - resides in shallow, coastal areas in the Gulf of Mexico, and the US's Pacific and Atlantic coast. These areas have plenty of seagrass, along with the shrimp and crabs scientists thought was the shark's preferred sustenance.
Scientists have long known that seagrass ends up in the bonnethead shark's stomach, but were perplexed by the shark's gut, which, as with other sharks, seemed more well-suited to a carnivorous diet.
"It has been assumed by most that this consumption was incidental and that it provided no nutritional value," Samantha Leigh, one of the researchers on the team and an ecologist at the University of California in Irvine, 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."
In order to see if the sharks were actually seeking out the seagrass as food - rather than scooping it up accidentally when eating bottom-dwelling crabs or snails - the researchers fed captive sharks a mostly vegetarian diet coated with special isotopes to create a unique carbon signature.
After feeding the captive sharks, the scientists found that they were able to digest the seagrass. The researchers say the sharks may have special enzymes in their stomach acid that allows them to break down the cellulose in the plants.
The carbon signature from the plants also showed up in the sharks' blood and liver tissue, indicating that they absorbed the nutrients and used the plant-based diet for their metabolism.
"We have always thought of sharks as strict carnivores, but the bonnethead is throwing a wrench into that idea by digesting a fair amount of the seagrass that they consume," Leigh told Fox News.
While more research needs to be done, the scientists say that there may be more omnivorous shark species lurking in the ocean.
"Given that bonnetheads have a digestive system that resembles that of closely-related species that we know to be strict carnivores, we need to re-think what it means to have a 'carnivorous gut,'" Leigh told Fox.