Whale sharks are the largest fish in the sea, though as filter feeders, they don’t have the same bloodthirsty reputation as their kin. Still, they are sharks, so it’s long been believed these gentle giants rely almost exclusively on animal protein.

That’s not what an intriguing new study published this month in the journal Ecological Monographs found, though. Careful investigation of blood and tissue samples from over a dozen whale sharks suggests that they actually have a pretty omnivorous diet that includes plants and algae.

The research team, led by University of Tokyo biologist Alex Wyatt, used a combination of samples from captive and wild sharks to demystify the feeding habits of these enigmatic ocean travellers. While previous studies had found seaweed in whale shark stomachs, this is the first study to suggest they might ingest such algae as a dietary staple.

“Whale sharks are a very charismatic creature that is globally threatened, but we still don’t know enough about their ecology for effective conservation,” Wyatt tells National Geographic. “I am very keen to contribute to an improved understanding of the species.”

Studying what an animal eats is “fundamental stuff,” says whale shark biologist and vice president of research and conservation at the Georgia Aquarium Alistair Dove, but “it’s also central to the sort of population models that are necessary when you are trying to develop enlightened conservation plans for an endangered species.”

Those making conservation decisions need concrete information about a species’ habits, including details like how individuals move around and what they need to eat, to determine the best ways to protect it. Tagging studies have elucidated the movement piece of the puzzle, but it’s basically impossible to follow these very mobile fish around all the time to see what they’re eating. And that’s why Wyatt and his team’s approach is so appealing—it uses samples of blood and other tissues that can be opportunistically collected to look back at what an animal has been eating.

“It's a great addition to knowledge of whale shark foraging and diet,” says Clare Prebble, a senior scientist with the Marine Megafauna Foundation.

Feast or famine lifestyle

To determine what the animals eat, the research team measured the different forms, or isotopes, of key atoms like nitrogen and carbon in the blood and tissue samples. The proportion of these isotopes differs in different food sources like algae, zooplankton, and fish, so looking at the ratios of these atoms in the sharks’ tissues can tell researchers a lot about what the animals are eating.

Plant-Eating Sharks Actually Exist? Like whale sharks, bonnethead sharks also aren't true carnivores. Watch them snack on an unusual bait: seagrass.

There’s usually a bit of a catch, though. While stable isotopes are frequently used to infer animal diets, the method often relies on making assumptions about how the animals process the nutrients from their foods. But this study was able to use five captive whale sharks with known diets from the Okinawa Churaumi Aquarium to ground-truth their data from eight wild animals. “It’s an epic amount of work that really nicely takes what we can learn from animals in aquariums and applies it to what we want to know about that same species in its natural setting,” says Dove.

Like the marine mammals they’re named after, whale sharks are huge. They can grow to be almost 40 feet long and weigh nearly 50,000 pounds. You’d think it takes a lot of food to support so much body mass—and it does. But the researchers found that the beasts can also go for four months or more without eating. That’s probably because they don’t eat while on the move, Wyatt says—perhaps because there isn’t anything worth stopping for along the way.

Prolonged fasting makes a lot of sense when you think about the animals’ environment, says Prebble. “Food availability in the open ocean is very patchy, so it would be a good strategy for them to gorge themselves when food is abundant, then use that energy to sustain them through periods of travelling or searching to find their next seafood buffet,” she says. (Learn about the chaotic world of whale shark tourism.)

But not all the animals showed signs of big gaps between meals. There was a lot of variation between individuals, something that Prebble and Dove both found intriguing. That may suggest that individual sharks specialize on feeding in certain areas or on certain foods. And in general, they just seem to eat a wider variety of things than anyone realized.

Eat food, a lot when you can… mostly plants?

According to Wyatt’s data, the animals are getting half or more of their nutrients from plants and algae. So why would whale sharks consume a large amount of algal matter? There are a few possible explanations.

“It is unclear if this is actually beneficial, or even a choice that whale sharks make,” says Wyatt. “But it is plausible that algal material could provide a food source when other prey are limited.”

“It makes sense that whale sharks might ingest a lot of plant matter given that their feeding method discriminates particles by size, not by species,” Dove says. That’s what a 2013 study concluded when researchers noted that marine algae has often been found in whale shark stomachs. But this new data shows the animals are using a lot of nutrients from the algae they consume, which could suggest a more vegetarian diet isn’t accidental.

To determine whether whale sharks are accidental or intentional omnivores, researchers will have to see whether the animals target areas with algae even in the absence of potential animal-based food. They’ll also need to study how effectively whale sharks use nutrients from algae. That’s what biologists did when bonnethead sharks kept turning up with seagrass in their digestive systems, and to their surprise, they found the animals do actually digest the grass thanks to special enzymes in their stomachs. (Learn more: Bonnethead sharks eat grass, and no one knows why.)