It is one of the most radical rebrandings in history: contrary to their bloodthirsty image, some sharks are not irrepressible meat eaters, but are happy to munch on vegetation too. According to US researchers, one of the most common sharks in the world, a relative of the hammerhead which patrols the shores of the Americas, is the first variety of shark to be outed as a bona fide omnivore.
The bonnethead shark is abundant in the shallow waters of the eastern Pacific, the Western Atlantic, and the Gulf of Mexico, where they feed on crab, shrimp, snails and bony fish. Though small by shark standards, adult females – the larger of the sexes – can still reach an impressive five feet long.
The scientists ran a series of tests on the sharks. These showed that the fish successfully digested the seagrass with enzymes that broke down components of the plants, such as starch and cellulose. Lacking the kind of teeth best suited for mastication, the fish may rely on strong stomach acids to weaken the plants’ cells so the enzymes can have their digestive effects. In all, more than half of the organic material locked up in the seagrass was digested by the sharks, putting them on a par with young green sea turtles.
Further tests found high levels of the seagrass carbon isotope in the sharks’ blood and liver tissue, demonstrating that the digested food was being used to build and maintain the animals. The results led the scientists to their inevitable conclusion: “The bonnethead shark is the first known omnivorous species of shark,” said Leigh.At the moment, there are few suspicions that other shark species have broadened their diets to consume plants as well as meat. That would call for an even more drastic rewriting of the textbooks. “If other species are taking omnivorous digestive strategies as well, then we’d need to reevaluate their role as top predators,” said Leigh.