SAN FRANCISCO, CALIFORNIA—Sharks are infamous as meat eaters—and human eaters. But now, scientists have discovered the first species to feast on plants. The bonnethead shark (Sphyrna tiburo), which sticks close to shore and hunts in shallow waters frequented by humans, usually dines on shrimp, small crabs, and small fish in seagrass meadows. But in 2007, researchers found something strange in the guts of juvenile bonnethead sharks: the seagrass itself. So one adventurous graduate student put several sharks in an outdoor salt-water tank on a diet of 90% eelgrass and 10% squid three times a day for 3 weeks. Despite this mostly vegetarian fare, all the sharks gained weight, she reported here this week at the Society of Integrative and Comparative Biology annual meeting. What’s more, carbon tracers in the seagrass—along with an analysis of the sharks’ poop—suggested that they were digesting and using at least half the seagrass they were eating. But the bonnethead has the same short intestines as its hammerhead relatives, not the longer guts that plant eaters use to digest fibrous material. That makes researchers suspect that microbes in the bonnethead shark’s guts are doing the digesting for it. Just don’t expect hammerheads to go green anytime soon.