What is the deepest fish in the sea?

There are plenty of fish in the sea – an estimated 3.5 trillion of them, it turns out – but only one can claim to be the deepest dwelling of them all. That crown belongs to an unknown snailfish species filmed swimming over 27,000 feet (or five miles) below sea level.

The remarkable footage was captured by a group of researchers from Western Australia and Tokyo. It is the first time a fish has been recovered from a depth greater than 8,000 meters (26,246 feet). The wreck of the Titanic, for context, is approximately 12,500 feet underwater.

It was discovered in the Izu-Ogasawara Trench in the western Pacific Ocean. The ocean trench stretches from the waters south of Japan to the northernmost section of the Mariana Trench, the deepest ocean in the world.

The unknown snailfish species, of the genus Pseudoliparis, was discovered during a two-month expedition that began in 2022, with the findings published earlier this year. The video was captured from an unmanned, autonomous deep ocean vessel known as a lander. Attached to the cameras was bait to lure deep-sea crustaceans, with snailfish in turn then attracted the crustaceans.

“When you picture what the deepest fish in the world should look like, the chances are it’s gnarly, black, with big teeth and small eyes,” Prof Alan Jamieson, chief scientist and founder of the Minderoo-UWA Deep Sea Research Centre, said to the Guardian. While snailfish are not unique to deep-sea environments like other species of fish, they have been found over 3,000 feet deeper than any other fish.

“One of the reasons [snailfish] are so successful is they don’t have swim bladders,” Jamieson explained. “Trying to maintain a gas cavity is very difficult at high pressure.” As well as no swim bladders, snailfish also do not have scales, instead being covered in a gelatinous layer described as a “physiologically inexpensive adaptation” by Jameison. Despite the mystery regarding the exact type of snailfish found, scientists know of over 400 different species with habitats ranging from shallow water to deep ocean trenches.

Young and small snailfish tend to live deeper than larger adults in order to avoid predators, Jamieson added.

“Because there’s nothing else beyond them, the shallow end of the range overlaps with a bunch of other deep-sea fish, so putting juveniles at that end probably means they’ll get eaten…When you get down to the mega deep depths, 8,000 plus [meters], a lot of them are very, very small.”

At this depth, the pressure is 800 times larger than at the ocean’s surface. Ichthyologist Prosanta Chakrabarty said: “Everything from gas exchange for breathing to nearly every physiological function seems impossible.”

The new record surpasses another of Jamieson’s snailfish discoveries. That came in 2017 in Mariana Trench.

“Each trench has its own snailfish in it,” Jamieson continued. “Once they’ve evolved to cope in a trench, they cannot decompress to get from one trench to another.”