Sharks are carnivores. Almost every one of the more than 500 species survives on other animals. But at least two species break the rule by getting real nutrition from plants or seaweed, which makes them omnivores. No shark lives on plants alone, so none is a herbivore. The honest one-word answer is carnivore, with a short list of surprising exceptions.
What Most Sharks Actually Eat
The typical shark is smaller than the movie version and lives much closer to the seafloor. In the eastern North Pacific, fewer than half of the 73 shark species grow longer than about 2 meters (6.5 feet), and about 8 in 10 of those species live near the bottom on the continental shelf or slope.1PubMed. Diet Composition and Trophic Ecology of Northeast Pacific Ocean Sharks Small sharks such as houndsharks, and young sharks of larger kinds, mostly eat small fish and crustaceans like crabs and shrimp. Large sharks in open water do eat large fish and squid.2PubMed. Diet Composition and Trophic Ecology of Northeast Pacific Ocean Sharks
A crab or small fish is a far more typical shark meal than a seal. Films and news stories focus on great whites and tiger sharks, a small slice of all species. Still, all of these diets are made of animals, and that is what makes sharks carnivores.
Some sharks do sit at the very top of the food chain. Tiger sharks hold that spot in seagrass beds and shallow lagoons, above nearly every other shark there.3PubMed Central. The trophic role of a large marine predator, the tiger shark Galeocerdo cuvier Among open-ocean sharks in the northwest Atlantic, thresher sharks feed highest in the food chain.4Journal of the Marine Biological Association of the United Kingdom. Predicting trophic position in sharks of the north-west Atlantic Ocean using stable isotope analysis Even a top predator’s food shifts with its size, its habitat, and what prey is around.5PubMed Central. The trophic role of a large marine predator, the tiger shark Galeocerdo cuvier
Scientists learn all this without watching sharks eat. Stomach contents show the last meals a shark ate. Chemical markers in muscle and fin reveal what it ate over months, and they also show how high up the food chain it feeds. Using both gives a fuller picture than either alone.
The Biggest Sharks Eat the Smallest Food
The two largest sharks, the whale shark and the basking shark, eat zooplankton and small swimming animals rather than big prey.6PubMed. Diet Composition and Trophic Ecology of Northeast Pacific Ocean Sharks They are filter feeders, straining tiny food from the water with their huge mouths. Zooplankton means tiny animals such as small shrimp-like crustaceans and fish eggs, so this is still a meat diet.
The method is simple. The shark swims with its mouth open and water flows through. Structures in the gills trap food while the water leaves through the gill slits. These sharks have tiny teeth they barely use for eating.
Efficiency depends on finding dense plankton, which is why basking sharks pick the richest patches. In summer they feed where large plankton is thickest along temperature boundaries at the surface.7Nature. Selective foraging behaviour of basking sharks on zooplankton in a small-scale front Among the open-ocean sharks compared in one study, basking sharks feed lowest in the food chain.8Journal of the Marine Biological Association of the United Kingdom. Predicting trophic position in sharks of the north-west Atlantic Ocean using stable isotope analysis
The whale shark’s gigantic body is fueled by a very short food chain: tiny drifting animals go straight into the largest fish. The whale shark is also the one giant that takes in seaweed, as a later section explains.
The Bonnethead, a Shark That Digests Seagrass
An omnivore is an animal that eats and gets nutrition from both plants and animals. The bonnethead, a small relative of the hammerhead, regularly takes in seagrass along with its usual prey.9PubMed Central. Elasmobranch microbiomes: emerging patterns and implications for host health and ecology Bonnetheads digest about half of the seagrass they eat, and carbon from that seagrass ends up built into the shark’s own body.10PubMed Central. Seagrass digestion by a notorious ‘carnivore’ Swallowing something and digesting it are different things, and the bonnethead’s case counts because a share of the seagrass is absorbed. So the bonnethead qualifies as an omnivore, not a carnivore that eats plants by accident. Its main prey are crabs, shrimp, and small mollusks living among the grass in shallow, warm coastal seagrass beds.
Researchers showed this by feeding bonnetheads seagrass that carried a distinctive chemical signature, then checking whether it showed up in the sharks. It did, so the plant’s carbon had been absorbed into the shark’s tissue. That is stronger evidence than finding plants in a stomach, which can’t show whether anything was absorbed.
Sharks were assumed to be pure meat eaters, and plant material in a stomach was usually dismissed as accidental. Showing that the shark digests it changed the classification.
Sharks don’t make the enzymes needed to break down tough plant fiber on their own. Bonnethead guts, though, show spikes in fiber-breaking enzymes near the end of the gut that most likely come from microbes.11PubMed. Digestive enzyme activities in the guts of bonnethead sharks (Sphyrna tiburo) provide insight into their digestive strategy and evidence for microbial digestion in their hindguts They also show signs of the same microbe-driven breakdown that helps cows digest grass.12Marine Biology. Gut microbial diversity and digestive function of an omnivorous shark It is still unclear whether bonnetheads graze on purpose or swallow plants while catching crabs.13PubMed Central. Elasmobranch microbiomes: emerging patterns and implications for host health and ecology
Whale Sharks and Seaweed
Chemical traces in whale shark tissue and droppings show that floating Sargassum seaweed and the small creatures on it are a significant food source.14PubMed. The world’s largest omnivore is a fish That makes the whale shark, the world’s largest fish, the world’s largest omnivore too.15PubMed. The world’s largest omnivore is a fish Unlike a bite swallowed by chance, the seaweed’s nutrients show up in the shark’s own body.16PubMed. The world’s largest omnivore is a fish
Sargassum is a brown seaweed that floats in large mats in warm oceans. Small crabs, shrimp, and fish larvae live in it, so a whale shark takes in the seaweed and its residents together. That mix makes its diet partly plant and partly animal. Plankton remains a major part of its diet, so the seaweed adds to it rather than replacing it.
Plant matter in a stomach is not the same as a plant diet. Nurse sharks sometimes have algae and even coral in their stomachs.17Discover Fishes. Nurse Shark Even so, they are not considered omnivores, because the algae is most likely swallowed by accident while they grab prey from the seafloor.18San Diego Zoo Wildlife Alliance. Nurse Shark The test is whether the animal gets nutrition from plants, not just whether plants are found inside it.
Picky Eaters and Eat-Anything Sharks
Tiger sharks are considered the least selective feeders of any shark.19PubMed Central. Diet and trophic ecology of the tiger shark (Galeocerdo cuvier) from South African waters Their stomachs have held land animals and even human trash they cannot digest, so they scavenge as well as hunt, eating whatever they come across.20PubMed Central. Diet and trophic ecology of the tiger shark (Galeocerdo cuvier) from South African waters
A hunter catches live prey. A scavenger eats animals already dead, like a whale carcass. Many sharks do both depending on what’s available, and either way the food is animal, so the shark is still a carnivore.
Deep-water sharks in the far north fall into groups that mainly eat fish, mainly eat small deep-sea fish, or mainly eat crustaceans.21Marine Ecology. Deepwater Sharks at Their Northern Limits—Distribution, Diet and Trophic Relations The Mediterranean blackmouth catshark eats crustaceans, squid, and fish and shifts its menu with season and place.22Sustainability. Diet of the Deep-Sea Shark Galeus melastomus Rafinesque, 1810, in the Mediterranean Sea: What We Know and What We Should Know Greenland sharks in Arctic Norway have had minke whale DNA in their stomachs.23Polar Research. Minke whale Balaenoptera acutorostrata DNA in the stomachs of Greenland sharks Somniosus microcephalus sampled in Svalbard, Norway
How a Shark’s Diet Changes as It Grows
Young great white sharks eat mostly fish. Adults longer than about 3 meters (10 feet) increasingly eat seals and other marine mammals.24PubMed Central. Form, Function and Feeding: Changes in Tooth Size and Shape Associated With Ontogenetic Changes in Prey Consumption by Australian White Sharks (Carcharodon carcharias) Their teeth change to match, from narrow, pointed grasping teeth to broad, serrated slicing teeth.25PubMed Central. Form, Function and Feeding: Changes in Tooth Size and Shape Associated With Ontogenetic Changes in Prey Consumption by Australian White Sharks (Carcharodon carcharias) The change is gradual, so a mid-sized great white may eat both. Seals are larger and richer in fat, which gives a growing body more energy per meal. Deep-sea black dogfish and great lanternsharks make a similar shift, from crustaceans to fish as they get bigger.26Marine Ecology. Deepwater Sharks at Their Northern Limits—Distribution, Diet and Trophic Relations A small shark can’t catch or swallow large prey, and young sharks often live in shallow nursery areas full of small fish and crabs.
Teeth record these choices. Pointed teeth grip slippery fish. Serrated triangular teeth cut flesh and bone. Flat crushing teeth break the shells of crabs and clams. A fossil tooth can hint at what its owner ate, though teeth alone don’t settle a diet.
Jaws and Teeth Built for Different Meals
Shark jaws trade off between a strong but slow bite and a fast but weaker one, matched to how they catch prey.27PubMed Central. Shark mandible evolution reveals patterns of trophic and habitat-mediated diversification A fast-closing jaw snaps up quick fish, while a strong slow one cracks shells.
Some deep-sea sharks have a single row of blade-like teeth for gouging chunks from larger animals.28PubMed Central. Shark mandible evolution reveals patterns of trophic and habitat-mediated diversification Houndsharks and horn sharks have broad, rasp-like teeth for grabbing and handling prey.29PubMed Central. Shark mandible evolution reveals patterns of trophic and habitat-mediated diversification Many bottom and deep-sea sharks crush hard-shelled prey, and some use suction to pull food in.30eLife. Integration of multi-level dental diversity links macro-evolutionary patterns to ecological strategies across sharks
One body feature gives no clue. Many sharks have a spiral intestine that increases surface area for absorbing food. You might expect a different diet to produce a different shape, but the spiral shape does not match diet type.31PubMed Central. Shark spiral intestines may operate as Tesla valves