First of its kind: An omnivorous shark

Ruining the reputation of sharks as bloodthirsty predators, California researchers have found a shark that enjoys a side of seagrass with its prey.

Sharks are not known for their taste for greenery. But bonnethead sharks not only eat grass while chomping fish and squid — they also digest the plant and gain nutrition from it, said scientists at the University of California, Irvine.

It turns out bonnetheads have high levels of enzymes that break down fiber and carbohydrates, compared with the low amount that carnivores typically have. That makes the bonnethead the first known omnivorous shark, researchers said.

The diminutive species, which reaches up to 3 feet in length, lives in the shallow sea grass meadows off both coasts of the Americas.

In 2007, researchers first reported that the digestive tracts of bonnethead sharks caught in the Gulf of Mexico were full of sea grass, up to 62 percent of the contents by weight. At the time, some reasoned that the grass might have been ingested incidentally. But Samantha Leigh, a graduate student who headed the four-year study, and her colleagues wondered whether there was more to it.

Leigh was also the lead author of the study published in Proceedings of the Royal Society B.

Dr. Sandy Trautwein of the Aquarium of the Pacific in Long Beach said she hopes the study "opens up the door for additional research" about seagrass communities and sharks in general.

For the study, the researchers caught bonnethead sharks, the smallest of the ten hammerhead species, just off the Florida Keys and transported them to an outdoor lab. There, the sharks received a meal every day consisting of a wad of seagrass wrapped in a piece of squid, resembling a large inside-out sushi roll. The sea grass, which made up 90 percent of the roll, had been loaded with a tracker isotope that could be detected later in their blood if the grass was truly being digested.

The sharks thrived on this diet, all of them gaining weight during the experiment. When the researchers checked their blood, they found very high levels of the tracker, indicating the grass was being digested and used for nutrients. Checking enzymes in their guts revealed that the sharks do possess enzymes needed for breaking down the carbohydrates in plants, much like other omnivores like humans; carnivores, on the other hand, have primarily enzymes for dealing with proteins. The researchers calculated that the sharks were digesting the grass with about 50 percent efficiency, much greater than carnivorous lizards, for instance, whose efficiency at digesting plant matter is about 30 percent.

Going forward, the researchers hope to understand why the shark has this ability.

Humans get our plant-digesting enzymes from our gut microbiome, and it seems likely the sharks do as well. Understanding that the bonnethead eats plants also changes its status in ecological models of the seagrass meadow ecosystem, an important tool for conservation.

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