Vegetarian Sharks? World's First Omnivorous Sea Beasts Discovered off U.S. Coast

The world's first omnivorous sharks have been confirmed by scientists, who say the bonnethead shark is the first and apparently only of its kind to get nutrients from vegetation.

Earlier this year, researchers from the University of California Irvine announced they had discovered a species of shark that eats and digests seagrass. The team has now published their findings in Royal Society B: Biological Sciences, showing how this "notorious carnivore" is, in fact, a "clear omnivore."

The bonnethead shark is a small relative of the more famous hammerhead. There are an estimated 4.9 million living along the Atlantic and Gulf of Mexico coasts of the U.S., making it one of the most abundant shark species in these coastal waters. They live in seagrass meadows and it was generally thought their diet mainly consisted of crustaceans and mollusks.

In 2007, scientists announced bonnetheads ingest huge quantities of seagrass. At the time, it was not clear if they were eating the seagrass because it formed part of their diet. It is accepted that sharks are uniformly carnivorous, so it was assumed the sharks could be ingesting seagrass by accident.

However, the sheer number of bonnetheads ingesting seagrass raised the possibility they were eating vegetation for the nutritional value and that it formed a major part of their diet. After analyzing the sharks and their diet, scientists have now confirmed bonnetheads are, indeed, omnivores.

Bonnetheads have been confirmed as the world's first omnivorous sharks. Mills Baker/CC

Study author Samantha Leigh, from the university's Department of Ecology and Evolutionary Biology, told Newsweek the discovery was a huge surprise. "We've always known sharks to be strict carnivores," she said. "This is the first species we know of that is taking this omnivorous digestive strategy."

The team fed captive sharks a diet made up of 90% seagrass and 10% squid. They then analyzed how well they digested and assimilated seagrass material. Findings showed the sharks were able to digest the seagrass with "at least moderate efficiency," revealing it could retain nutrients and potentially survive on vegetation alone.

"We fed the shark a 90% seagrass diet for three weeks and they all gained weight and remained healthy," Leigh said. "However, right now, we don't know how this diet would affect them long term."

Because of the vast numbers of bonnetheads living along the U.S. coast, the findings raise major questions about the ecosystem the sharks live in and the role they play in maintaining it. "They could be playing a very important role in food web stability and the transportation of nutrients throughout the ecosystem," Leigh said.

Not everyone is convinced by the study, however.

Gavin Naylor, Director of the Florida Program for Shark Research at the University of Florida, told Newsweek that while the paper is interesting and has raised the issue that sharks could be getting energy from non-meat sources, it is premature to say bonnetheads are playing a major role in maintaining the ecosystem of their habitat by eating seagrass.

Furthermore, Naylor takes issue with the way the study was conducted: "The experiment was done on five animals over a three week period. This is insufficient to be to be compelling. They would need to run the experiment with many many more individuals for at least six months. A three week study probably only served to demonstrate a stress response as evidenced by the variable responses among the individuals."

He added: "Sharks generally sit close to the apex of trophic hierarchies. Apex predators have far more influence on vegetation patterns via indirect effects than they do via direct ingestion. Predators control the populations of the herbivores upon which they prey, which, in turn, control the vegetation upon which they feed. The notion that bonnethead sharks might have a major impact on seagrass beds through the sea grass they consume directly as juveniles is—in my opinion—unsubstantiated speculation."

This story has been updated to include quotes from Gavin Naylor.