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In a hostile realm of the ocean, where the pressure is over 830 times greaterthan on Earth's surface, scientists spotted a fish casually swimming around. No big deal.

It's a curious-looking snailfish, and at 27,349 feet (8,336 meters) down, it's the deepest fish ever observed. Researchers spotted the critter on a deep sea expedition in the Izu-Ogasawara Trench, located south of Japan, after lowering a camera with bait down into the ocean's "hadal zone." This cryptic region is named for the Greek god of the underworld, and is home to the deepest of the seas. The record-breaking observation, announced in early April 2023, was made by scientists at the University of Western Australia and Tokyo University of Marine Science and Technology.

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Even at such remote hadal depths, researchers noted that snailfish generally spotted in the region were a "large and somewhat lively population of fish." Yet how might these animals survive such crushing pressure and isolated conditions? The answer is that snailfish, like this deepest-observed fish of the genus Pseudoliparis, are fantastically strange, with clever adaptations.

"Overall, very deep sea fishes (those living in the abyssal and hadal zones) tend to be small, flabby or jelly-like, eel-shaped fishes living slow lives, [that] like to binge eat and hunt using the blue light of their prey," Jessica Arbour, a biologist at Middle Tennessee State University, told Mashable over email.

I asked Alan Jamieson, the chief scientist of this hadal expedition and founder of the Minderoo-UWA Deep Sea Research Centre, how he and his team reacted when they saw the record-breaking fish in the video. "In total admiration for how deep these little goofy fish can go," Jamieson said.

You can see the record-breaking fish in the clip below.

SEE ALSO:Scientists discover ancient shark swimming in a really strange place

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Resisting unfathomable pressure

At tens of thousands of feet under the surface, the microscopic proteins in animal cells are weakened and become unstable. This is problematic, because proteins are critical to the functioning of animals' organs and tissues. But snailfish churn out a chemical, called trimethylamine N-oxide, or TMAO, that stops proteins from changing under such extreme physical stress.

"This name may be unfamiliar, but TMAO is what gives fishes their characteristic 'fishy' smell," Mackenzie Gerringer, a biologist at the State University of New York at Geneseo, told Mashable.

TMAO is quite effective. For reference, the pressure in the deepest part of the ocean, the Mariana Trench, is like an elephant standing on your fingernail, Geneseo explained. This deepest-observed snailfish wasn't observed at quite this depth, but it's a decent approximation.

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Snailfish have evolved their bodies in other major ways to withstand the relentless pressure. Fish in shallow waters use a gas bladder to float (aka "maintain neutral buoyancy"). But these bladders compress in the deep sea. So snailfish don't even have this organ. Instead, they remain buoyant with fewer or smaller bones, more cartilage, and smaller structures like fins, which makes them somewhat eel-like, Arbour explained. And snailfish skin is jelly-like — as opposed to scaly — to help stay buoyant.

Snailfish captured on camera in the hadal zone of the Izu-Ogasawara Trench.Credit: Minderoo-UWA Deep Sea Research Centre / University of Western Australia

Expert eaters in the deepest seas

Food in the ocean's remote trenches generally isn't nearly as plentiful as near the surface, where hungry creatures will find bountiful algae and other creatures traversing rich reefs.

In the deeps, life depends on nutrients sinking down from above. Creatures subsist on these particles, then bigger animals eat those critters. And snailfish are well equipped to capitalize on this prey.

"One hadal snailfish can have more than a hundred of these amphipods in its stomach at a time!"

"They have very large mouths and stomachs, so they can cash in on a large food item when available," Arbour explained.

A CT scan of both snailfish jaws.Credit: Adam Summers / University of WashingtonTwo deep sea snailfish collected on the expedition into the Izu-Ogasawara Trench.Credit: Caladan Oceanic

Their food often takes the form of small crustaceans, called amphipods. In the video above, the snailfish are actually chowing down on amphipods (which were attracted to the fish). Snailfish use "suction feeding" to catch these prey, Gerringer explained, by rapidly expanding their mouth and creating a "suction force that pulls in the amphipod." But that's not all: Hadal snailfish have a second set of jawsin the back of their throat that crush the hapless prey. It's an effective means of eating.

"One hadal snailfish can have more than a hundred of these amphipods in its stomach at a time!" Gerringer emphasized.

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Seeing in the dark

The deep, deep sea is either a lightless or largely lightless realm.

Many fish are adapted to see the type of blue-green light that can penetrate through deep waters, though in hadal zones there's little sunlight to be found. But snailfish capitalize on the bioluminescence, or light created by organisms, in the darkness. Creatures luminesce for hunting or attracting mates, among other reasons.

"Snailfish seem particularly well adapted to seeing the dim blue light specifically produced by bioluminescence of their prey," Arbour explained.

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Snailfish are indeed marvelously adapted to thrive in one of the harshest realms on Earth. Soak in this deep sea footage, which scientists capture not just to learn about the mysterious deep sea ecosystem, but to help us understand how to protect it.

"This is great footage of an incredible group of animals. It's exciting to see the deep oceans in the news, these are beautiful and important habits that are worth understanding and protecting," Gerringer said. "We often think of the deep oceans as being remote and otherworldly, but we are already seeing impacts from human activities in the deep ocean, including from climate change and pollution."

This story was originally published on April 8 and has been updated with more information and images from the expedition.

TopicsAnimals