A four-year study by scientists at the University of California, Irvine found that bonnethead sharks don’t just eat crabs and other small fish, but also live off a diet of sea grass.
Researchers at the School of Biological Sciences called the bonnethead shark, found along the U.S. coastline, “omnivorous.” Results of the study showed the sharks munched on — and digested — seagrass as well as animal protein.
“Experts already knew bonnetheads take in the grass while searching for crabs, squid and other similar fare,” the news release reads. “But they believed the sharks were consuming it unintentionally, without obtaining nutritional value.”
What’s a bonnethead?
Bonnethead sharks are the smallest of the ten hammerhead shark species, according to the Aquarium of the Pacific in Long Beach.
The head shape of these sharks is unique among the species.
“Unlike most species that have straight heads with notched edges, those of bonnetheads are smooth and rounded between the eyes,” according to the Aquarium’s website. “These sharks are highly migratory. Although they are common, coastal inshore sharks, only one unprovoked attack on a human has been recorded.”
They can be found on the western Atlantic from Rhode Island south to the Gulf of Mexico and Brazil, and throughout the Caribbean. Along the eastern Pacific, they are seen mostly from San Diego south to Ecuador, and like waters above 70 degrees.
The study was led by Samantha Leigh with the Department of Ecology & Evolutionary Biology at UCI’s School of Biological Sciences.
She and fellow researchers planted Florida Bay seagrass in a lab, and studied how five bonnethead sharks responded to a menu offering 10 percent squid and 90 percent grass.
The group found the animals digested and obtained nourishment from more than half of the seagrass’ organic matter, the news release reads.
The scientists also found the bonnetheads had high levels of enzymes that break down fiber and carbohydrates, compared to the low amount that carnivores typically have.
Diet and climate
The results highlight the urgency of fighting climate change, Leigh said.
“Seagrasses are important for the economy and the environment,” she said in the release. “They produce oxygen, screen toxins and provide species nursing habitats. However, they are suffering and declining to due to rising sea temperatures and acid levels in the ocean. The fact a highly abundant kind of shark feeds on the grasses is yet another indication of why we need to preserve this vegetation.”
Leigh says the study’s results can also help the public better understand shark behavior.
“Sharks have gotten such a bad reputation as really ravenous meat-eaters,” she said. “However, we’ve shown that there is at least one type that also consumes plant materials. There is so much more to discover about them.”