Bonnethead: first omnivorous shark species discovered by scientists

Cristina Cross
September 7, 2018

That makes the bonnethead the first known omnivorous shark, researchers said.

"Until now, most people thought that seagrass consumption was incidental when these sharks were hunting for crabs, etc. that live in the seagrass beds", Samantha Leigh, co-author of the work, told reporters.

Unswayed by this popular opinion, the Leigh and her team chose to run an experiment to see if bonnethead sharks were, in fact, capable of digesting and acquiring nutrients from seagrass. Indeed, up to 60% of the bonnethead shark's diet is seagrass found in coastal waters around the Americas, with the rest made up of bony fish, crabs, snails, and shrimp, reports the Guardian.

For three weeks, Leigh and her team conducted several lab trials, feeding the bonnethead sharks 90 percent seagrass and only 10 percent squid. These showed that the fish successfully digested the seagrass with enzymes that broke down components of the plants, such as starch and cellulose. Surprisingly, the experiment saw the initially thought carnivorous sharks gained weight from the seagrass-heavy diet.

"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 said.

"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".

Sharks don't just eat meat they eat plants as well, a new study finds.

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The bonnethead shark eats underwater plants as well as other sea creatures (PA).

- About 4.9 million alone in the Atlantic and Gulf of Mexico coastal waters of the U.S. -combined with their eating habits suggest that we need to re-evaluate the role that bonnetheads play in seagrass meadows.

Although sharks lack a secondary jaw that many herbivores use to chew plants, they have highly acidic stomachs which may aid digestion, the researchers said.

Researchers claim the small relative of the hammerhead is the first known omnivorous shark.

They found, however, that seagrass can form up to 62 per cent of the bonnethead diet, alongside their preferred meal of crustaceans and molluscs. 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.

For one shark subspecies, this fictional scenario isn't actually too far from the truth, according to a new study published inProceedings of the Royal Society B.

In fact, seagrass meadows are the most widespread coastal ecosystems on Earth, providing a home for thousands of fish and invertebrates, while at the same time filtering water and absorbing excess carbon dioxide from the atmosphere.

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