Sorry Twitter, this Greenland shark is not actually full of pee

Twitter can be a wasteland of heart-wrenching breaking news, trolls, and poorly placed promoted content — but then there are the metal cats and the random factoids, like this viral Twitter thread about Greenland sharks that threw the Verge newsroom into a frenzy today.

The Twitter thread went on to say that the Greenland sharks — which live in Arctic waters — can live 500 years, have glowing parasites attached to their eyes, and are eaten in a “fermented pee shark dish” in Iceland. So I called some scientists to fact-check this thread. And I even found one that had eaten that fermented dish.

First things first: there’s no such thing as a pee shark. Sorry. Greenland sharks have high concentrations of urea in their bodies, that’s true. But “urea does not equal pee,” says Peter Bushnell, a professor of physiology at Indiana University South Bend, who studies Greenland sharks. Urea is just a by-product of protein breakdown that we humans filter out in pee. Sharks — all ocean sharks, and rays, and skates — retain urea so that their bodies are at the same salt concentration as the salt water outside. This is called osmotic balance, and without it, sharks would lose or gain water to the ocean and die.

But urea is toxic — it breaks down proteins in tissue. (That’s why we expel it in pee, and why if you have kidney failure, you’ll die.) So to balance the urea and protect its tissue, sharks also have high concentrations of a compound called trimethylamine oxide, or TMAO. It’s not exactly clear why, but Greenland sharks have higher concentrations of urea and TMAO than other sharks, skates, and rays. Because your body turns TMAO into a poisonous compound during digestion, you can get sick from eating Greenland shark meat that hasn’t been dried or soaked.

The untreated meat will literally make you “shark drunk” because of the toxins. You’ll feel dizzy and fall over, says Holly Shiels, an associate professor in the Faculty of Biology, Medicine and Health at Manchester University, who has studied Greenland sharks. Bushnell ate some Greenland shark meat on a research expedition, after soaking it in milk and frying it up with a lot of soy sauce. “Half of us ate it and the other half waited, so there were witnesses,” he says. No one got sick and the shark tasted “nothing special.” But remember that Icelandic “fermented pee shark dish” mentioned on Twitter? It’s called Kæstur hákarl and Bushnell tried it. “It really does taste foul,” he says. “It’s an acquired taste, let’s put it that way.” Then he adds, “Not for me, thank you very much.”

Greenland sharks are also the longest-living vertebrates known on Earth. In fact, these sharks can be 400 years old. Think about that: there are sharks on this planet that have been swimming in the Arctic sea since well before the French Revolution. Why do they live so long? “That’s the thousand dollar question,” Bushnell says. Researchers all over the world are trying to figure that out — after all, if these sharks don’t get cancer, they could hold the secret to cure the nasty disease in humans. Bushnell says it may have to do with their very slow metabolic rate — the sharks grow super slowly, about half an inch a year, and aren’t sexually mature until they’re about 150 years old. So it could just take longer for those DNA glitches that cause cancer to occur.

Although they grow slowly, they can become massive: up to 24 feet long and over 2,500 pounds heavy. They feed on fish, crustaceans, seals — likely gobbled down as the seals sleep in the water column — and polar bears. Now, Greenland sharks have never been observed actually attacking and eating a polar bear, Shiels says. But the bear contents have been found in shark stomachs. So it could be that the polar bears were dead and had fallen into the water where they were scavenged by the Greenland sharks.

But here’s another one for you: most Greenland sharks have small crustacean parasites dangling from their eyes. In the long run, these copepods can make the sharks blind. Whether the crustaceans are actually glowing, “eating its eyes in exchange for attracting more prey with their light,” as our Twitter friend suggested, it’s up for debate. A paper reported that the copepods are fluorescent, Bushnell says, but he and his team “took a number of them and put them in a dark closet” and the crustaceans wouldn’t glow, he says.

Still, these sharks are pretty metal. Because they live in super cold and deep waters — over 600 miles below the surface — it’s not clear how many Greenland sharks populate our planet. At the beginning of the 1900s, they were overfished for their livers, which were used to make oil. Today, they’re mostly caught by mistake when people are fishing other prey. But because they take so long to grow and reproduce, any shark that’s killed — either to make Kæstur hákarl or as a mistake — takes a toll.

I don’t know about you, but I think these blind, toxic Sequoia-like sharks should be protected. Even if they’re not really pee sharks after all.

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