There are some pretty sizable sharks out there, from the great white to the tiger. The biggest, though, is the whale shark, which can grow as long as 65 feet. Turns out they’re not just the biggest sharks, though. A new study shows they’re also the largest omnivores.
Researchers studying whale sharks in the Ningaloo Reef of Western Australia discovered something interesting about the species’ diet. Though they’re known for being filter feeders that take in large amounts of krill, tissue samples from whales in the reef showed they’re also eating plants. This development came as a surprise to the team.
Australian Institute of Marine Science fish biologist Dr. Mark Meekan, co-author on the study, says, “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.”
The researchers studied the sharks’ diets by collecting samples of possible food sources at the reef and comparing the amino and fatty acids in these materials to those in whale shark tissues. This analysis, published in the journal Ecology, showed that the shark samples contained compounds from Sargassum, a brown seaweed common at the reef.
With the evidence that the species was eating some plant life, that earned the ocean giants the coveted title of world’s largest omnivore. It also shows they have a little something in common with large land mammals.
Dr. Meekan explains, “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. Turns out that maybe the system of evolution on land and in the water isn’t that different after all.”
The dietary discovery wasn’t the only thing that surprised researchers, either. As they examined how the sharks’ bodies generate energy and grow, as well as their droppings, there were more findings they hadn’t expected.
Dr. Patti Virtue, co-author and associate professor at the University of Tasmania’s Institute for Marine and Antarctic Studies, explains, “It’s very strange, because in their tissue they don’t have a fatty acid or stable isotope signature of a krill-feeding animal. The poo did show that they were eating krill, but they’re not metabolizing much of it.”
These enigmatic omnivores aren’t in any particular rush to eat, as they swim at the rate of about 3 miles per hour, but their fins can take them thousands of miles in search in feeding grounds. Such grounds are located throughout the world’s tropical and warm, temperate seas.