The World's Largest Shark Wasn't Actually A Carnivore, Scientists Find

The largest shark in our oceans already has a reputation for being gentle giants, and it seems there are more than we ever realized. whale shark (Typhoid rhino) is a filter feeder, considered carefully combing the water for small animals such as krill.

Among the litany of small swimmers they take are leafy greens made up of algae and other photosynthetic organisms.

It’s unavoidable, but researchers wonder if this vegetation is just a garnish for carnivores, or if it provides the side salad needed to keep them swimming.

Researchers examining dirt and skin samples identified what these 10-meter (32-foot) long sea hoovers were actually taking advantage of from the giant pool of water they breathed through their system.

“The droppings suggest that they ate krill,” said University of Tasmania marine biologist Patti Virtue. “But they don’t metabolize much.”

In contrast, whale sharks, which are true sharks with cartilage instead of bones, seem to extract nutrients from a lot of algae.

“This caused us to rethink everything we thought we knew about what whale sharks eat,” said Mark Meekan, fish biologist from the Australian Institute of Marine Sciences. “And, in fact, what they do on the high seas.”

Meekan and colleagues’ network analysis also revealed a fatty acid profile that was more consistent with omnivores than carnivores. They found a shell rich in arachidonic acid (ARA), which is only present in amounts large enough to explain the levels found in whale sharks, in the floating macroalgae Sargassum.

In 2019, another study using tissue samples also found evidence that whale sharks actually eat at least some organisms that are low on the food chain, such as plants and algae. What’s more, they’re not the only sharks that are omnivores: the Bonnethead Shark (Sphyrna tiburo) significant seagrass feeding.

These animals, also called shovelheads for obvious reasons, often ingest plant material as a result of hunting small prey such as crabs, mollusks, and fish in dense seagrass habitats. So their need to cope with plant material passing through their bodies is most likely what ultimately happens to their ability to digest it.

The same may be true of whale sharks, the researchers suspect. In their evolutionary past, they may have initially ingested algae to digest the animals living on them (epibion), but now they can also digest and utilize the algae themselves as well.

“So the vision we have of whale sharks coming to Ningaloo just to feast on these little krill is only half the story,” explains Meekan. “They’re actually out there eating quite a lot of algae too.”

Unfortunately, to find enough of this floating organic material, whale sharks have to follow ocean features such as surface currents that gather these floating food sources together. This same feature also collects pollutants in the ocean like plastic – so whale sharks end up accidentally eating them too.

Meekan has observed some of this plastic passing through whale shark droppings. But it would likely reduce their intestinal capacity, slow their digestion or cause them to regurgitate their food, the team noted in their paper. This could endanger the endangered animal, which has seen a 62 percent population decline over the past 75 years.

“On land, all the largest animals have always been herbivores,” says Meekan. “In the ocean, we’ve always thought of animals that got really big, like whales and whale sharks, feeding one step of the food chain on animals like shrimp and small fish.

“It turns out that perhaps the evolutionary systems on land and in water are not at all different.”

This research was published in Ecology.

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