Fish Are Friends, Not (Always) Food: Meet the World’s First Omnivorous Shark Species

Bonnethead sharks enjoy a diet of up to 60 percent seagrass, as well as crab, shrimp, snails and bonyfish

Until now, it’s been unclear whether the bonnethead's seagrass consumption was intentional or the result of indiscriminate feeding Wikimedia Commons

The beloved Pixar film Finding Nemo popularized the image of friendly vegetarian sharks with a catchy refrain coined by Bruce, a great white shark and leader of the Fish-Friendly Sharks support group: “Fish are friends, not food.” Unfortunately for Marlin and Dory, the tale’s fishy protagonists, the shark’s pledge fails to override his natural instincts, and soon after offering these overtures of friendship, Bruce pounces on his unsuspecting pals.

For one shark subspecies, this fictional scenario isn’t actually too far from the truth, according to a new study published in Proceedings of the Royal Society B. Although sharks are often portrayed as the quintessential bloodthirsty marine predator, the bonnethead—a close relative of the hammerhead—adheres to an omnivorous diet, chomping down on a mixture of smaller creatures and seagrass.

Bonnethead sharks are smaller than their famed hammerhead kin. The National Aquarium states that the sharks typically grow to a length of 30 to 48 inches and weight of up to 24 pounds. Members of the species have narrower and more rounded heads than hammerheads. Roughly 4.9 million bonnetheads reside in the coastal outskirts of North America, making them one of the most populous species in the region, Hannah Osborne writes for Newsweek.

Gizmodo’s George Dvorsky reports that the bonnethead is the first shark species to receive the official designation of omnivore rather than carnivore. The shark’s meat-based diet features crab, shrimp, snails and bony fish, while its plant-based diet mainly consists of seagrass.

Scientists have known about the bonnethead’s unusual inclination toward seagrass since 2007, when an ecologist named Dana Bethea first recorded the phenomenon, but until now, it’s been unclear whether consumption was intentional or the result of indiscriminate feeding.

Lead author Samantha Leigh, a marine biologist at the University of California, Irvine, tells The Guardian’s Ian Sample that previous research tended to point toward the latter option.

“It has been assumed by most that this consumption was incidental and that it provided no nutritional value,” Leigh says. “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 test the bonnethead’s response to seagrass, Leigh and her colleagues fed five sharks a diet of 90 percent seagrass—grown in water sprinkled with sodium bicarbonate powder to create a unique carbon isotope signature—and 10 percent squid. At the end of the three-week testing period, the team analyzed the sharks’ fecal matter and digestive systems, ultimately concluding that the bonnetheads did, in fact, digest and absorb nutrients offered by the seagrass.

According to Dvorsky, the bonnethead shark possesses special digestive enzymes that enable it to break down seagrass. The animals tested digested more than half of the organic material found in the seagrass, Sample adds, and even used these nutrients to build up and maintain their overall health, as evidenced by traces of the seagrass carbon isotope found in the sharks’ blood and liver tissue. Overall, the researchers suggest that seagrass constitutes up to 60 percent of the shark species’ diet.

The scientists’ findings point to the need for further study of marine ecosystems. As the authors note in their paper, “Understanding how the consumption and digestion habits of bonnethead sharks impacts seagrass ecosystems is important, as these omnivores may stabilize food web dynamics and even play a role in nutrient redistribution and transport.”

Still, the study has its critics: Gavin Naylor, director of the Florida Program for Shark Research at the University of Florida, tells Newsweek’s Osborne that the scale of the experiment was “insufficient to be compelling. … A three-week study probably only served to demonstrate a stress response as evidenced by the variable responses among the individuals.”

Naylor further argued that apex predators like sharks influence vegetation patterns via indirect effects beyond direct ingestion.

“Predators control the populations of the herbivores upon which they prey, which, in turn, control the vegetation upon which they feed,” he continued. “The notion that bonnethead sharks might have a major impact on seagrass beds through the seagrass they consume directly as juveniles is—in my opinion—unsubstantiated speculation.”

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