See the Baby Glow-in-the-Dark Shark Hatched at the Tennessee Aquarium

In coastal waters, swell sharks appear a glowing green color to other members of their species due to a special eye adaptation

A green-glowing baby swell shark next to a blue ruler showing that it's about four inches long
The young swell shark glows under a blacklight. The swell shark's eyes have adapted to filter out other wavelengths of light so they can better see other sharks. Tennessee Aquarium

A baby shark has hatched at the Tennessee Aquarium—and it glows.

The speckled newborn is a swell shark, also known by the scientific name Cephaloscyllium ventriosum. The young fish is only about four inches long now, but it could grow up to three feet as an adult, according to a statement from the aquarium.

“We’ve had these swell sharks living off-exhibit for a couple years now, but this is the first time we’ve had a successful hatch of an egg,” Kyle McPheeters, a senior aquarist at the Tennessee Aquarium, says in a video with the statement.

Swell sharks don’t generate their own light, but they glow through a technique called biofluorescence. This means they absorb sunlight and transform it within their bodies, emitting it as a different color.

Glow In The Dark Swell Shark Pups

To some other animals, including humans, the sharks don’t appear to glow when seen with the naked eye. Instead, they look brown and mottled, and they blend in well with their surroundings. “To a predator, like a seal or sea lion, they don’t really see them,” McPheeters tells Local 3 News’ Abigail Martin. “They’re kind of camouflaged.”

But glowing could allow swell sharks to see each other in dark waters. The sharks’ eyes have adapted a yellow lens that makes their companions appear green and glowing. A 2019 study found that molecules in the sharks’ scales absorb blue light, the only wavelength of visible light that reaches deep underwater, and emit green light, JoAnna Klein wrote for the New York Times when the findings were published. Glowing might also play a role in helping the sharks find mates, since males and females fluoresce in different patterns.

McPheeters can see the sharks glow at the aquarium by shining blue light on them and wearing yellow glasses.

Chain catsharks, a deep-sea species, also have this glowing property. And they’re not alone. In all, 57 known shark species glow, which is more than 10 percent of all sharks, Jérôme Mallefet, a marine biologist at the Catholic University of Louvain in Belgium, told Mongabay’s Elizabeth Claire Alberts in 2021.

Swell sharks live in coastal waters between central California and southern Mexico, as well as along the coast of Chile, according to the Monterey Bay Aquarium. The fish earned their name because they can chug large amounts of sea water when threatened, swelling to twice their normal size. This makes it difficult for predators to bite them or pull them out of a hiding place.

The sharks are most commonly found in waters 16 to 121 feet deep, but they have been spotted as far as 1,500 feet below the ocean’s surface, per the California Department of Fish and Wildlife. They live between 20 and 35 years.

Another novel characteristic of swell sharks is their egg cases, which are long and thin, brownish-yellow pouches. Spindly, spiraling tendrils like curly hair grow from the ends of the case. The mother can wrap these tendrils around the base of a plant or rock to anchor it, McPheeters explains in the video.

A long, thin egg case with light shining through, illuminating the unborn swell shark inside
A light shines through a swell shark egg case, illuminating an unborn pup inside. The sharks are only a few inches long when they're born, but they grow to up to be up to three feet long. Tennessee Aquarium

“It’s not like any egg I’ve ever seen,” he says in the video. “They’re very unique. Until you understand what they are, it looks, like, other-worldly.” Due to their fantastical appearance, the egg cases are called “mermaid purses.”

At the Tennessee Aquarium, McPheeters watched the eggs for five or six months before this swell shark pup hatched. Eight more eggs are still developing at the aquarium, according to Local 3 News. For now, the newborn swell shark, its mother and the other eggs are off-exhibit.

“When we have eggs coming from the adults, that’s exciting in itself, because it means we have a healthy environment where they feel safe reproducing,” McPheeters says in the statement. “But then to come in and see that come full circle… with a baby shark hatching from its egg is really exciting.”

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