The whale shark is not really a whale but a massive fish — the largest known fish species and one of the gentlest creatures of the oceans. They typically eat krill, a small crustacean, but scientists have now discovered a new dimension to their dining habits: plants. This makes whale sharks (Rhincodon typus) the largest omnivores on Earth.

Preferring warm waters, whale sharks can be found on tropical seas and are known to migrate in the spring to Australia in search of krill. They reach lengths of 40 feet (12 meters) or more and are listed as a vulnerable species, as they are hunted in parts of Asia, such as the Philippines.

Despite their size, whale sharks are docile and friendly creatures, and they sometimes even allow swimmers to hitch a ride. Just like the basking shark, the world’s second-largest fish, are filter feeders. This means that in order to eat, they open their massive jaws and passively filter everything in their path. But as well as catching krill, they also scoop up algae and other photosynthesizing organisms.

While this can’t be avoided, researchers were intrigued if this vegetation was just a garnish for the carnivore or if it was more relevant to their diet. Now, a study examining skin samples of several of these massive ocean hoovers found they are actually making proper use of algae in their organisms, rather than just accidentally ingesting it. In other words, plants are an important part of their diet.

“This causes us to rethink everything we thought we knew about what whale sharks eat. And, in fact, what they’re doing out in the open ocean,” fish biologist and lead author Mark Meekan in an Australian Institute of Marine Science statement.

Looking at whale sharks

To get a better idea of the diet of whale sharks, the researchers collected samples of possible food sources at Western Australia’s Ningaloo Reef — from plankton to seaweed. They then compared the fatty acids and the amino acids in plankton and plants to those in the whale sharks, doing a dermal tissue analysis of 15 sharks.

The analysis showed a fatty acid profile more consistent with omnivores than carnivores. The tissue samples were rich in N-6 polyunsaturated fatty acids (PUFA), especially arachidonic acid (ARA). These are compounds that are found in high proportions in Sargassum – a brown seaweed that breaks off the reef and floats at the water surface.

Whale sharks aren’t the only sharks that are omnivores. Bonnethead sharks (Sphyrna tiburo) also eat a lot of seagrasses. These animals, called shovelheads, shallow the plant materials as they hunt small prey like crabs and mollusks. So their need to cope with plants passing through their bodies is probably what led to their ability to digest them.

The same could have happened with whale sharks, the researchers suspect. As they evolved in the past, they may have originally been eating algae to digest the animals that live on it. But now they can also digest the algae and make use of it too. This means they don’t go to the Australian reefs just to eat krill but algae too, Meekan concludes.

“On land, all the biggest animals have always been herbivores. In the sea, we always thought the animals that have gotten really big, like whales and whale sharks, were feeding one step up the food chain on shrimp-like animals and small fishes,” Meekan said. “Turns out that maybe the system of evolution on land and in the water isn’t that different after all.”

The study was published in the journal Ecology.