Bonnethead shark (Sphyrna tiburo) Seaquarium du Grau du Roi, France © Getty Images

First ever omnivorous sharks identified

Bonnethead sharks get the nutrition they require through a diet of vegetation and meat.

We’re going to need a bigger salad cart. Researchers at the University of California have found that bonnethead sharks, smaller cousins of the more famous hammerheads, supplement their diet of crabs, shrimp and squid with mouthfuls of seagrass. It is the first time sharks have ever been confirmed to be omnivorous, they say.

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Traces of seagrass have often been found in the guts of bonnethead sharks but it was assumed that they were consuming it unintentionally rather than seeking it out. To test this the team grew seagrass in water laced with powdered sodium bicarbonate. As the seagrass grew it absorbed a distinctive form of carbon into its structure that the team were able to detect the presence of in the sharks livers, plasma and red blood cells.

They then fed five captive sharks a diet consisting of 90 per cent of the modified seagrass and ten per cent squid for three weeks. All of the sharks gained weight during this time. The resulting analysis of the sharks revealed that not only were the sharks deliberately consuming the seagrass but that they were also able to break it down and absorb nutrients from it. It turns out that bonnetheads have special enzymes in their stomach acids that allow them to breakdown the cellulose found in the plant material. They were able to digest 45 per cent of the organic matter found in the seagrass putting them on a par with juvenile green sea turtles.

Why have hammerhead sharks evolved their distinctive heads? © iStock

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