The world’s largest fish is very mysterious and endangered

Don’t let the name fool you. Whale sharks are not whales. They are sharks. 

And they’re the largest fish species on the planet. They can grow as long as a city bus. What’s more, scientists recently concluded that whale sharks — which have unique polka dots on their bodies akin to fingerprints — are the biggest living omnivores on Earth, officially dethroning Alaska’s Kodiak bear. (Whale sharks chomp on shrimp, plants, and beyond.)

This large species, which is unfortunately listed as endangered by the International Union for Conservation of Nature, is mysterious and continues to amaze biologists.

“The thing that I love the most about whale sharks is that they challenge everything we think we know about sharks.”

“The thing that I love the most about whale sharks is that they challenge everything we think we know about sharks,” Alistair Dove, a marine biologist and the vice president of the Atlanta-based Georgia Aquarium, told Mashable. 

Mysterious creatures

One of the most perplexing things about whale sharks is how they give birth. There’s a hypothesis that female whale sharks make their eggs, the eggs hatch inside them, and they eventually release live babies into the water. But so far, researchers haven’t witnessed a whale shark mate or give birth. However, fishermen harpooned, killed, and then cut open a pregnant female shark in a fish market in Taiwan in 1995. Inside were some 300 dead baby whale sharks, which is an unusually high number for a shark.

When fishermen dissected the animal, they realized they had never seen a whale shark with hundreds of pups, and called local marine scientists, explained Jennifer Schmidt, a molecular biologist and the director of the Shark Research Institute in New Jersey. The pups were then safely collected and frozen.

“We can never track one animal for its whole life.”

Years later, Schmidt and her team decided to perform a genetic analysis on 10 percent of the pups to determine their paternity. They found that all the pups had the same father, which suggested that whale sharks could mate once, store the sperm, and eventually fertilize their own eggs.

It’s difficult to know what the lives of whale sharks are like. That’s because they are long-lived animals, likely surviving for over a century. This makes it challenging to track a single animal, and how it migrates, over its lifetime.

“We can never track one animal for its whole life,” Schmidt said. 

(Usually with fish, the number of rings on their bones indicates how old they are. But whale sharks don’t have hard bones like other fish, and calculating their age becomes tricky, Georgia Aquarium’s Dove told Mashable.)

Whale sharks spend considerable time on the surface of the water and easily get hit by ships. Scientists say it’s one of the reasons for their population decline.
Credit: AvigatorPhotographer / Getty Images

Endangered creatures

Knowledge about how whale sharks behave and migrate is crucial to conserving this endangered species. Over the last 75 years, we have lost an estimated 50 percent of whale sharks, said Gonzalo Araujo, a whale shark researcher, and also a member of the International Union of Conservation of Nature’s Shark Specialist Group. 

The number one threat to whale sharks is fishing — including accidental catch, or bycatch — despite attempts to protect them in most countries, Dove told Mashable. “People in small villages in developing nations happen to luckily catch a whale shark. That’s enough to feed their village for some time,” he explained. 

The next big threat, Dove said, is ships colliding with whale sharks. Whale sharks that spend considerable time on the sea surface eating plankton are easily hit by commercial fishing ships. Unlike whales, which tend to float after they’ve died, whale sharks often sink after they’ve been killed. “Commercial shipping can be a silent killer,” he said.

“Commercial shipping can be a silent killer.”

Climate change might also threaten the dwindling whale shark population. One of their primary food sources, plankton, is sensitive to shifting conditions in the oceans like temperature and acidity, and whale sharks will redistribute according to how their prey shifts, Araujo told Mashable. They might not succeed in new oceanic regions. “But that’s going to be over longer time scales,” he said.

There’s another looming, potential peril to their survival, and that’s plastic pollution. Whale sharks are filter feeders, which means they make a living by ingesting bounties of water and filtering out small animals. They can’t differentiate between plankton and little pieces of plastic, Dove said. We don’t yet know what impact ingesting this plastic has on their health, he noted.

Despite years of whale shark research, we still know relatively little about their biology and behavior. To better protect these animals, we need to know where they’re born, where they breed, and where their babies go, Dove emphasized. Since the Tyrannosaurus rex stomped around in the Cretaceous Period millions of years ago, whale sharks have been doing their thing in the ocean “I think we have a responsibility to work out how to live alongside them a little bit better,” Dove said.