World’s First Plant-Eating Shark Has Just Been Discovered

Living on a diet that's made up of more than 60 percent seagrass, bonnethead sharks are the first omnivorous sharks to ever be identified.

Bonnethead shark (Sphyrna tiburo).
Vladimir Wrangel / Shutterstock

Living on a diet that's made up of more than 60 percent seagrass, bonnethead sharks are the first omnivorous sharks to ever be identified.

Hold on to your bonnets for this one, as the world’s first plant-eating shark has just been discovered and its name is the bonnethead shark.

Smaller cousins of the hammerhead shark, bonnetheads had been spotted munching on seagrass in the past, in addition to crustaceans and other shellfish, but marine biologists had always assumed the veggie mouthfuls were purely accidental, reports the New Scientist.

However, a recent study into the animals’ diet revealed that bonnetheads are actually the first omnivorous shark species to ever be identified. In fact, it turns out that up to 62 percent of their diet is made up of seagrass, from which they absorb a great deal of nutrients, notes the Smithsonian website.

The discovery was made by a team of scientists led by ecologist and evolutionary biologist Samantha Leigh of the University of California, Irvine, who tracked down five bonnethead sharks and investigated their digestion. The researchers monitored the bonnetheads for a period of three weeks, during which time they fed the sharks a diet of 90 percent seagrass and 10 percent squid.

For the purpose of this study, the seagrass was grown in special conditions that involved water sprinkled with sodium bicarbonate powder. This created a unique carbon isotope signature, which the team traced through the sharks’ digestive system to see if the vegetation had been assimilated at all and had any nutritional value for these animals, previously considered to be strictly carnivorous.

The research uncovered that bonnetheads, scientifically known as Sphyrna tiburo, have a real stomach for greens, as Phys.org puts it, and possess special digestive enzymes that allow them to break down seagrass and digest more than half of its organic material.

“Until now, most people thought that seagrass consumption was incidental when these sharks were hunting for crabs, etc. that live in the seagrass beds,” Leigh said in a statement.

Yet, it would appear that “bonnethead sharks are not only consuming copious amount of seagrass, but they are actually capable of digesting and assimilating seagrass nutrients, making them clear omnivores,” the researchers concluded.

Their experiment showed that, unlike other known carnivores which can’t digest plants efficiently, these sharks have highly acidic stomachs. This makes them as proficient at digesting plant matter as young green sea turtles, a reptile species that starts out as omnivorous and switches to a fully vegetarian diet in adulthood.

According to Leigh, this discovery has come as a big surprise.

“Bonnetheads have a digestive system that is very similar to other closely-related species that are definitely strictly carnivorous, so the fact that they are acting like omnivores is truly remarkable!”

The team argues that these sharks are thriving on seagrass. The five bonnethead sharks studied by the researchers actually gained weight while on a predominantly vegetarian diet and even built up their overall health, as shown by the seagrass carbon isotope found in their blood and liver. Their findings are published in the journal Proceedings of the Royal Society B.

One more mystery remains to be solved, namely finding out when the bonnetheads first took up the habit of chomping down on plants. This detail is particularly intriguing considering that their shark ancestors were seemingly 100 percent carnivores, notes Phys.org.