Grass-Eating Sharks. Really.

SHARKS’ REPUTATION AS FIERCE, MAN-EATING APEX PREDATORS of the oceans takes a hit when you consider the bonnethead shark (Sphyrna tiburo), a small variety of hammerhead found mostly in warm waters along the Atlantic and Gulf coastlines of North and South America.

Along with their normal carnivorous diet, bonnetheads eat grass, as in seagrass. How much grass? It accounts for as much as 62 percent of its gut content in some juvenile populations.

The bonnetheads’ propensity for salad side dishes was first discovered some years ago, but researchers from the University of California, Irvine, and Florida International University decided to examine the issue of how well the sharks – with typical carnivorous-oriented shark digestive tracts – digest and assimilate nutrients from seagrasses.

They reported their results at the 2018 annual meeting of the Society for Integrative and Comparative Biology.

The researchers put several captive bonnetheads in a saltwater tank on a diet that was 90 percent eelgrass and 10 percent squid for three weeks, analyzing their blood weekly for assimilation of carbon from the grass. The results suggested that the sharks were digesting and using about 58 percent of the seagrass they were eating.

In fact, all the sharks gained weight, according to an online account of the project by Science Magazine. The Science article features a video and summary of the project.

While grass-eating sharks may jolt Sharkworld’s man-eating reputation, the carnivorous portion of bonnetheads’ diet in the wild isn’t stupendously impressive anyway. Only two to three feet long, they largely cruise shallow waters close to shore and dine on shrimps, crabs and small fishes they find in seagrass meadows.

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