Whale sharks are the biggest fish and shark in the world. Adults can weigh 41,000 pounds and average about 40 feet long, but they may grow as large as 65 feet. According to World Wildlife Fund (WWF), these gentle giants roam all the planet’s tropical oceans, generally alone. Like with human fingerprints, each whale shark has a unique pattern of white spots that allow individual sharks to be identified.

Since whale sharks spend most of their lives in the deep ocean, they remain mysterious to us. Adult whale sharks are often found feeding at the surface, but they can dive 3,000 feet—or more. The maximum-recorded dive depth is 4,462 feet. Where pupping occurs and where the youngest animals situate is still unknown, as they are rarely found.

Now, however, researchers have made a surprising discovery about the dining habits of whale sharks. Not only do they gulp down enormous mouthfuls of krill, but they also swallow huge helpings of seaweed. This bestows the largest fish in the sea with another world title: the world’s biggest omnivore, officially taking that honor from 1,500-pound Kodiak bears.

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Kodiak bears have just lost their title as the world’s largest omnivores (using a variety of foods). They spend most of their time eating berries, grass and other plants. Fish, too, are an important part of their diets, but few Kodiak bears expend the time or effort necessary to chase and kill mammals.

The sargassum shocker

While it’s long been known that the biggest fish in the sea gorges on krill, new research, published in the journal Ecology in July 2022, shows that they can benefit from eating plant material, too.

Algae has turned up before in the stomachs of beached whale sharks. But because of how whale sharks feed—by swimming with their mouths open through swarms of zooplankton—it was thought it was accidental ingestion. Carnivores typically can’t digest plant life, so some scientists suspected that algae just passed through whale sharks’ guts.

To find out if that assumption was correct, researchers from the Australian Institute of Marine Science followed whale sharks as they gathered near Ningaloo Reef off the coast of western Australia in 2017. Since the gigantic fish are well camouflaged in the water, the team located 17 individuals that had surfaced to feed from an airplane. Then, they intercepted the sharks by boat and jumped into the water to snap pictures, scrape off parasites and collect tissue samples. Most of the whale sharks didn’t react when they were poked by the spear, which is roughly the width of a pinky finger.

Every whale shark has a unique pattern of white spots that allow individuals to be identified. The spot patterns on whale sharks bear a remarkable resemblance to the star patterns that NASA studies. So, with a few minor adjustments, that same algorithm that identified stars is now identifying whale sharks. ©Erratic Eric, flickr

The scientists also collected the sharks’ possible food sources—ranging from tiny crustaceans and plankton to large clumps of seaweed—and then chemically analyzed the samples to reveal their amino and fatty acids. After cross-referencing these acids with those found in skin samples taken from the whale sharks, it was discovered that the whale sharks were rich in arachidonic acid, an organic molecule found in brown-algae seaweed, called sargassum. Since the sharks can’t make this molecule themselves, they probably got it by digesting algae.

This finding rules out the possibility that seaweed was getting scooped up accidentally but not actually digested. The presence of the acid demonstrates that whale sharks are using plant matter as a means of growing. Surprisingly, though, the whale sharks didn’t have a fatty acid or stable isotope signature of a krill-feeding animal; and krill was less well metabolized than expected. It appears that when the whale sharks make their annual migration to western Australia, they’re dining on a buffet of not just krill but algae, too.

These results contradict the common assumption that large land creatures are typically herbivores, but those that live in the sea occupy a different niche on the food chain, feeding on fish and small shrimp. It turns out that the system of evolution on land and in the water isn’t that different, after all.

Whale sharks can stretch their mouths to four feet wide. They feed by swimming through swarms of zooplankton with their jaws open. Teeth don’t play a major role in feeding; food moves through filtering pads that cover the entrance of their throats. The filtering pads are full of millimeter-wide pores that act like a sieve, allowing water to pass through while capturing food particles. ©Brian Lauer, flickr

The plants/plastics paradox

While the researchers admit that they can’t say for certain that whale sharks specifically seek out sargassum, the amount they eat isn’t incidental, either. Their analysis clearly shows that plant material makes up a very large part of their diets. And regardless of whether whale sharks actively seek out plant snacks, the animals do have the ability to digest them.

One theory to explain this new finding is that the large size of whale sharks has prompted an evolutionary response to digest accidentally swallowed sargassum. Since whale sharks expend a lot of energy pushing their mouths open like a huge net through the water, throwing up a gut full of food with a lot of algae in it would be very costly compared to the amount of energy spent collecting it. Some scientists think whale sharks have simply gotten around this in an evolutionary sense by effectively turning bycatch—such as the sargassum common at Ningaloo—into food.

Having a broader range of food sources might sound like good news regarding whale sharks, as it could help them withstand potential upheavals to their marine ecosystems brought about by climate change. But the scientists say there’s a negative side to that:  it’s possible that the sharks’ propensity for swallowing most of what is swept into their mouths could make them far more likely to swallow copious amounts of ocean-borne plastics.

Whale sharks are very big animals; and when you’re a huge being, you need lots of food. That prompted an evolutionary response, according to scientists. ©Marcel Ekkel, flickr

While whale sharks can pass some plastics through their guts, the researchers write that ingesting larger plastic pieces could cause the sharks to vomit up their meals and could reduce their gut capacity, interfering with their digestion. 

The whale shark wonderment

Climate change and plastics aren’t the only threats that whale sharks face today. They are highly valued on international markets for their fins, meat and oil. They’re also victims of bycatch. Although whale sharks are now protected from fishing in many countries, they are decreasing in some areas and are considered endangered by the International Union for Conservation of Nature (IUCN).

Although loners most of the time, large numbers of whale sharks sometimes gather in areas with abundant plankton food—making them popular with divers and snorkelers and attractive to tourists. While whale shark tourism does present a threat to the species as it can interrupt their feeding and sharks can be injured by boat propellers, organizations such as World Wildlife Fund are working to ensure that this type of tourism keeps the animals safe.

Swimming with whale sharks is very popular in Mexico. WWF developed a code of conduct for swimming with these “gentle giants” and works with tour boat operators to educate them about the animals’ movements. ©Bruce & Amara Thayer

For example, in Mexico where whale shark tourism is very popular, WWF educates tourists on codes of conduct for swimming with whale sharks and raises awareness with tour boat operators about the movements of the animals, which has resulted in fewer collisions with them.

Whales sharks are not only wonders of the waters, emblems of enigma and emissaries of enormity, they indicate the presence of plankton and the overall health of our oceans.

And that may be their most enchanting quality of all.

Here’s to finding your true places and natural habitats,