Watch: World's Deepest Fish Lurks 5 Miles Down in Mariana Trench

Things get weird for fish that live on the edge. A potentially new species of deep-sea snailfish with a bulbous head and partly transparent body has smashed the record for deepest fish in the world by over 1,600 feet (500 meters).

Filmed December 6 over five miles deep (eight kilometers) in the Pacific's Mariana Trench—which extends nearly seven miles (11 kilometers) down—the video shows a fish at the deepest point any living fish has ever been found. (See "How the Mariana Trench Became Earth's Deepest Point.")

It's also unlike any other snailfish ever seen, says Alan Jamieson, a deep-sea biologist at Aberdeen University in the United Kingdom, who co-led the expedition.

Its head looks like a cartoonish version of a dog's head, he says. But the body is incredibly delicate. "You can see its liver through the side of the fish," Jamieson says. "It's like tissue paper being dragged through the water." (Watch video of another weird deep-sea fish—this one has a transparent head.)

And the way it moves, "it's like there's no structure to it," says the deep-sea biologist. "It just glides."

Video Cliffhanger

The roughly six-inch-long (15 centimeters) snailfish sports broad, winglike fins that likely help it search for food, says expedition co-leader Jeff Drazen, a deep-sea ecologist at the University of Hawaii, Manoa. Those fins could be covered in sensors, something like taste buds, that would help the fish detect small animals in the mud.

In fact, "we actually saw it strike at something [in the mud] and try to eat it," Drazen says.

The fish's presence just goes to show that these deep trench communities are much more active and interesting than previously thought, he explains.

Most expeditions go charging to the Challenger Deep, the deepest point in the Mariana Trench, Drazen says. All you see there is a flat, barren landscape. But this time researchers looked along the sides of the trench and found a much more interesting view. "That's where the story is," he says.

Unfortunately, it's a story with a cliffhanger. The new snailfish is the ultimate "one that got away": The researchers drew fish to a deep-sea camera using bait, but had no way to catch it. Without a specimen, they can't give the fish a formal description or scientific name.

That will have to wait until scientists can get back to this remote part of the world with a remotely operated vehicle or submersible that has sampling equipment. "Unfortunately," says Jamieson, "it's going to remain nameless until someone can catch it."

Under Pressure

Snailfish are no strangers to the crushing depths of the world's trenches. In fact, the Kermadec Trench near New Zealand and the Japan Trench farther north each have their own species of snailfish, says Drazen. The deep-sea fish can usually be found between 21,300 and 24,600 feet (6,500 and 7,500 meters) down. (Learn more about deep-sea trenches around the world.)

However, Jamieson and colleagues believe there is a limit to how deep a fish can live, and this new record-holder is right at the edge. Below about 26,900 feet (8,200 meters), a fish's body hypothetically can't produce any more of a substance called an osmolyte, which helps their cells withstand the incredible pressures of the deep sea.

"The deeper you go, the higher the concentration of this substance," Jamieson says. "When you get to 8,200 meters, there's no way a theoretical fish can produce more." If it were to go any deeper, the pressure would likely crush its cells, he says.

"If a fish can go deeper, it's different from any other fish on the planet."

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