The experiment used seagrass grown with a carbon-13 label so its atoms could be tracked. Bonnetheads fed a diet of about ninety percent seagrass and ten percent squid gained mass, digested more than half the organic matter, and the labelled carbon turned up in their blood and liver tissue. Their guts also carry enzymes capable of breaking down carbohydrate.
Seagrass had long been reported in bonnethead stomachs, sometimes at more than half the contents by volume, but was assumed to be incidental material swallowed alongside crustaceans and passed undigested. The trial showed it is a nutritional contribution.
The finding has limits worth keeping. It concerns one small hammerhead species, and nobody has shown any other shark doing this; the sharks were still gaining most of their protein from animal prey. The bonnethead is now listed as Endangered, largely because of coastal fishing pressure and estuarine habitat loss.