First Shark Species Identified As Omnivorous

First Shark Species Identified As Omnivorous

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 sharks’ dietary habits was investigated after reading reports of the fish chomping on seagrass, the flowering marine plant that forms subsea meadows in some coastal waters.

“It has been assumed by most that this consumption was incidental and that it provided no nutritional value,”

To see whether the sharks are truly flexitarian, the scientists retrieved sea grass from Florida Bay and hauled it back to the lab where they re-planted it. As the seagrass took root, the researchers added sodium bicarbonate powder made with a specific carbon isotope to the water. This was taken up by the seagrass, giving it a distinctive chemical signature.

The researchers next caught five bonnethead sharks and brought them back to the lab. Once the fish had settled in, they were fed on a three week diet of the seagrass and squid. All of the fish put on weight over the course of the study.

The scientists then 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.