Sharks are known globally as efficient apex predators. Their classification has long centered on a diet composed exclusively of meat, a reputation based on their specialized anatomy and hunting strategies. However, recent scientific discoveries have revealed a surprising exception to this long-held rule, prompting a re-evaluation of the dietary flexibility possible within the shark family.
The Default Shark Diet
The vast majority of the over 500 known shark species are classified as obligate carnivores, meaning they cannot sustain themselves without animal flesh. Their primary prey varies widely, ranging from large marine mammals like seals, to smaller bony fish, squid, rays, and crustaceans, depending on the species and habitat.
Most sharks are adapted to this meat-based lifestyle. Their teeth are designed for grasping, cutting, or crushing prey, and their digestive systems are highly acidic to efficiently break down bone and protein. If plant matter is ingested, it is generally considered incidental consumption, often swallowed while attacking prey in dense areas. For most species, this plant material passes through the digestive tract largely undigested, providing negligible nutritional benefit.
The Unexpected Omnivore
The bonnethead shark, Sphyrna tiburo, the smallest hammerhead species, challenged the carnivorous narrative when research showed its diet included a substantial amount of seagrass. Earlier investigations noted plant matter in their stomachs, but initially assumed it was consumed accidentally while hunting preferred prey like crabs, shrimp, and snails.
Further comprehensive studies demonstrated that seagrass could account for up to 62% of the bonnethead shark’s gut content mass in the wild. This high proportion of plant consumption is too significant to be merely incidental, leading to the species being reclassified as the first known omnivorous shark. The species inhabits shallow seagrass meadows along the coasts of the Americas, using its broad head to sweep the seabed for crustaceans and surrounding vegetation.
The Science Behind Plant Digestion
Scientific validation of the bonnethead’s omnivory required confirming their ability to extract nutrition from plant matter. Scientists conducted feeding trials using seagrass chemically labeled with a specific carbon isotope (C-13) to track its fate within the sharks’ bodies. The results showed that the sharks were able to digest and assimilate the seagrass with a moderate efficiency of about 50% of the organic material.
This absorption was confirmed by finding the C-13 carbon signature in the digestive tract, blood, and liver tissue. The ability to process plant cellulose, highly unusual for a shark, is attributed to the detection of specific digestive enzymes, such as beta-glucosidase, in their hindgut. These enzymes break down the fibrous components of the seagrass, distinguishing the bonnethead from other sharks that simply pass plant material as waste.