Scientists Discover World’s Biggest Shark Isn’t Really a Carnivore

The biggest shark in our oceans already has a reputation for being a gentle giant, and it seems there’s more to it than we ever imagined. Whale sharks (type of rhinoceros) are filter feeders, thought to carefully comb the waters for tiny animals like krill.

Among the litany of tiny swimmers they pick up are greens made up of algae and other photosynthetic organisms.

This can’t be avoided, but the researchers wondered if this vegetation was just garnish for the carnivore, or if it provided the side salad needed to keep it swimming.

Researchers examining poop and skin samples have pinpointed what these 10-meter-long (32-foot) ocean vacuums actually use from the giant pools of water they suck through their systems.

“The poo showed they were eating krill,” says University of Tasmania biological oceanographer Patti Virtue. “But they don’t metabolize a lot of it.”

Instead, whale sharks, which are real sharks with cartilage instead of bone, seem to extract nutrients from a bunch of algae.

“It’s causing us to rethink everything we thought we knew about what whale sharks eat,” says Mark Meekan, a fish biologist at the Australian Institute of Marine Science. “And, in fact, what they do in the open sea.”

Meekan and colleagues’ tissue analysis also revealed a fatty acid profile that was more consistent with the omnivore than the carnivore. They found skin rich in arachidonic acid (ARA), which is only present in sufficient amounts 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 feed on at least some organisms lower in the food chain, such as plants and algae. Moreover, they are not the only omnivorous sharks: Bonnethead sharks (Sphyrna tiburo) eat a lot of seagrass.

These animals, also called shovelheads for obvious reasons, frequently swallow plant matter due to hunting small prey like crabs, molluscs, and fish in dense seagrass habitats. So their need to deal with this plant material passing through their body is likely what resulted in their ability to digest it.

The same may have happened to whale sharks, the researchers suspect. In their evolutionary past, they may have originally swallowed algae to digest the animals that live there (epibionts), but now they can also digest and use the algae themselves.

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

Unfortunately, to find enough of this floating organic matter, whale sharks must follow ocean features such as surface currents that bring these floating food sources together. These same characteristics also aggregate pollutants in the ocean like plastic – so whale sharks end up accidentally feasting on them too.

Meekan observed some of this plastic seep into the whale shark’s poop. But doing so risks reducing their intestinal capacity, slowing their digestion, or causing them to regurgitate their food, the team notes in their paper. This could harm these endangered animals which have seen a population decline of 62% over the past 75 years.

“On land, all of the larger animals have always been herbivores,” Meekan explains. “In the sea, we’ve always thought that animals that got really big, like whales and whale sharks, fed on a rung of the food chain with shrimp-like animals and small fish.

“It turns out that maybe the system of evolution on land and in water isn’t so different after all.”

This research was published in Ecology.

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