Sharks have long been classified as strict carnivores, highly specialized marine predators adapted for a meat-based diet. Their powerful jaws, sharp teeth, and digestive systems are optimized for consuming fish, mammals, and other aquatic prey. The question of whether sharks eat plants, and gain nutrition from them, challenges this long-held biological assumption. While the vast majority of the over 500 known shark species are meat-eaters, a surprising scientific discovery has revealed a single exception that redefines our understanding of shark feeding behavior.
The Typical Shark Diet
Most shark species maintain a strictly carnivorous diet, preying on animals ranging from small crustaceans to large marine mammals. Their feeding habits generally fall into two main categories: active predation and filter-feeding. Apex predators like great white, tiger, and mako sharks consume seals, dolphins, and large fish. The tiger shark is also known for its opportunistic eating, consuming sea turtles, sea snakes, and seabirds.
Smaller, bottom-dwelling sharks frequently target invertebrates such as crabs, lobsters, and mollusks. The world’s largest sharks, like the whale shark and the basking shark, are planktivores. These colossal species filter enormous volumes of water to strain out tiny organisms such as krill and zooplankton. Across all these diverse groups, the digestive system is optimized for a high-protein diet, featuring a large, J-shaped stomach and a spiral intestine designed to maximize surface area for nutrient absorption.
The Omnivorous Exception
The only known exception to the purely carnivorous shark diet is the bonnethead shark (Sphyrna tiburo), a small member of the hammerhead family. Studies confirm that this coastal species consumes significant quantities of plant matter, specifically seagrass. Analysis of stomach contents in wild bonnetheads has shown that seagrass can constitute up to 62% of the total mass of food ingested.
Researchers initially dismissed this finding, believing the plant material was swallowed accidentally while the sharks hunted crabs and small fish within seagrass meadows. However, a definitive study demonstrated that seagrass ingestion is not incidental but a fully integrated part of the bonnethead’s diet. This research confirmed the shark actively seeks and consumes the plant material, making it the first known omnivorous shark species. The bonnethead’s ability to digest and assimilate nutrients from this plant matter challenges the classification of all sharks as carnivores.
Processing Plant Matter
The proof of the bonnethead’s omnivorous nature lies in its ability to break down and absorb nutrients from seagrass. Plant cell walls are made of cellulose, a complex carbohydrate difficult for most animals, especially carnivores, to digest. To test their digestive capacity, researchers fed captive bonnetheads a diet consisting of seagrass labeled with a traceable carbon isotope.
Analysis of the sharks’ blood and tissue showed they were assimilating carbon from the seagrass, proving the plant matter was being digested and utilized for energy. The bonnethead achieved a digestive efficiency for seagrass organic matter of approximately 50%, comparable to that of young green sea turtles. This efficiency is likely facilitated by the presence of beta-glucosidase, a cellulose-degrading enzyme detected in the shark’s hindgut. Since vertebrates do not produce this enzyme, its presence suggests the involvement of a symbiotic gut microbiome that helps the bonnethead break down the fibrous seagrass.