60% of the bonnethead shark’s diet is made up of seagrass, which they happily graze upon as well as eating fish, crabs, snails and shrimp. 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.
Scientists at the University of California in Irvine and Florida International University in Miami decided to investigate the sharks’ dietary habits 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,” said Samantha Leigh, a researcher on the team. “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.”
To see whether the sharks are truly flexitarian, the scientists retrieved seagrass 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 of shark,” said Leigh. The findings, published in Proceedings of the Royal Society B, overturn the idea that all sharks are exclusive meat-eaters, but that is not all. “This has implications for fragile and crucial seagrass meadow habitat management,” said Leigh. From now on, researchers who look after such habitats will have to factor in that bonnetheads not only eat the plants but can make seagrass as much as 60% of their diet.
The revelation has raised questions about how well scientists understand the feeding habits of other species, on land as well as at sea. “We should be taking a closer look at what animals are consuming, digesting, and excreting in their environments around the world because it impacts habitats that we depend on as well,” Leigh added. 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 re-evaluate their role as top predators,” said Leigh.
Credit: Ian Sample for The Guardian, 5 September 2018.