Scientists Identify the First-Known Omnivorous Shark
The bonnethead shark's diet is about 60% seagrass according to new research.
Image: D Ross Robertson
Sharks are such feared carnivores that there is an entire movie genre dedicated to dreaming up ludicrous ways for them to tear apart unsuspecting human populations. This reputation is well-earned—sharks are apex predators with astonishing hunting skills—but new research reveals that at least one species likes a nice salad, too.
According to a study published Wednesday in Proceedings of the Royal Society B, the bonnethead shark pairs its meaty meals with copious amounts of seagrass. This makes the bonnethead the first shark species to be officially recognized as an omnivore.
Researchers have observed bonnetheads, which are close relatives of hammerhead sharks, ingesting seagrass for over a decade. But it was not clear if the animals were actually digesting nutrients from the plants, or if it was an incidental byproduct of their feeding strategy: bonnetheads gulp up whole piles of grassy sediment in order to trap crabs, shrimps, mollusks, and other seafloor dwellers.
To shed light on this question, a team led by Samantha Leigh, a marine biologist at the University of California, Irvine, planted an underwater seagrass garden at Florida International University in Miami. They dosed the aquatic laboratory with carbon-13, a traceable isotope, that became integrated into the seagrass.
The researchers captured five wild bonnethead sharks from the Florida Keys area and placed them on a controlled diet of 90% seagrass and 10% squid for three weeks. When they analyzed the carbon-13 content in the sharks’ feces, blood, and digestive systems, they found the isotope, and they also identified plant-digesting enzymes in the animals’ guts.
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“We show that a coastal shark is digesting seagrass with at least moderate efficiency, which has ecological implications due to the stabilizing role of omnivory and nutrient transport within fragile seagrass ecosystems,” the team concluded. Leigh and her coauthors estimated that seagrass comprises up to 60% of the bonnethead’s diet.
More research is needed to understand how the bonnethead evolved to digest plants, or if any other sharks share its omnivorous leanings.
The discovery probably disqualifies the bonnethead from a starring role in future sharkspoitation films, but it showcases the incredible diversity of forms and abilities within the shark family.
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