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Scientists Discover the World's Largest Omnivore, and It's a Fish

And we thought we knew what whale sharks liked to eat.

Wide-mouthed, dappled whale shark swims in a deep blue sea.
Not a whale, but whale sharks are the largest fish in the sea.
Andre Rerekura, Australian Institute of Marine Science

Despite the name, whale sharks aren't whales, they're a type of fish that qualify as absolute units of the deep. They typically chow down on krill, a type of small crustacean, but scientists have discovered a previously unknown dimension to their diet: plants. That makes whale sharks the largest omnivores in the world.

Omnivores are animals (like humans and most bears) that eat and draw nutrients from both meat and plants. Whale sharks can grow to nearly 60 feet (18 meters) long. That's the length of a bowling lane.

How big are whale sharks? This big.

Andre Rerekura/Australian Institute of Marine Science

A team of researchers published its findings on the whale shark diet in the journal Ecology last week. 

"This causes us to rethink everything we thought we knew about what whale sharks eat," said fish biologist and lead author Mark Meekan in an Australian Institute of Marine Science statement on Monday.

It took some clever work to figure out that algae is an important part of the whale shark diet and not just something they accidentally ingest. The researchers compared compounds found in plant life -- including seaweed and plankton -- to compounds found in tissue samples from the fish. They found a match for sargassum, a brown seaweed, indicating that the animals are using the seaweed for energy and growth.

Sargassum grows along the Ningaloo Reef in Western Australia, a spot where whale sharks are known to hoover up krill. "So, the vision we have of whale sharks coming to Ningaloo just to feast on these little krill is only half the story. They're actually out there eating a fair amount of algae too," said Meekan.

Whale sharks have now ridden their heft to the top of the omnivore size chart, easily unseating other contenders, like Kodiak bears. These jumbo fish might be behemoths, but there's good news for wary humans: Whale sharks don't eat people.