Scientists have captured footage of a fish swimming more than 8km underwater, setting a new record for the deepest fish ever recorded.
The animal, an unknown snailfish species belonging to the genus Pseudoliparis, was filmed at a depth of 8,336 metres in the Izu-Ogasawara trench, south-east of Japan.
The footage was captured from an autonomous deep ocean vessel as part of a two-month expedition that began last year.
A few days after the snailfish was filmed, scientists caught two other snailfish, of the species Pseudoliparis belyaevi, in the Japan trench from a depth of 8,022 metres.
The team said these are the first fish to have ever been collected from a depth greater than 8,000 metres.
Scientists from the Minderoo-University of Western Australia Deep Sea Research Centre and the Tokyo University of Marine Science and Technology had set out to explore the Japan, Izu-Ogasawara and Ryukyu trenches – which are 8,000, 9,300 and 7,300 metres deep respectively – as part of a decade-long study into the deepest fish populations in the world.
Using unmanned submersibles known as landers, researchers deployed baited cameras in the deepest part of these trenches.
There are more than 400 known species of snailfish, which live in a wide variety of habitats ranging from shallow waters to the darkness of the deep ocean.
The expedition’s chief scientist and founder of the Minderoo-UWA Deep Sea Research Centre, Prof Alan Jamieson, said specific adaptations enabled some snailfish species to live about 1,000 metres deeper than the next deep-sea fish.
At 8,000 metres underwater, the pressure is 800 times greater than at the ocean surface.
“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,” Jamieson said. “Chances are it’s got nothing to do with deep sea – that has to do with being dark.”
Deep-sea adaptations tended to be less visibly obvious, he said. “One of the reason [snailfish] are so successful is they don’t have swim bladders. Trying to maintain a gas cavity is very difficult at high pressure.”
Snailfish also do not have scales, but instead have a gelatinous layer that Jamieson describes as a “physiologically inexpensive adaptation”.
The deepest individual caught on film was a juvenile fish. Unlike other deep-sea fish species, young snailfish are generally found at greater depths than adults.
“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,” Jamieson said. “When you get down to the mega deep depths, 8,000 plus [metres], a lot of them are very, very small.”
The depth of the Izu-Ogasawara fish beats the previous record of 8,178m, set in 2017 by a Mariana snailfish in the Mariana trench, by 158 metres.
Jamieson, who also made the 2017 discovery, said the Japan expedition confirmed a longstanding theory that the Mariana snailfish would not be the deepest fish in the world.
“Temperature and pressure are both very much interlinked,” Jamieson said. “When we go to colder places, everything goes shallower, and when we go to warmer places, things go deeper.”
The water in Izu-Ogasawara, which lies further north of the Mariana trench in the Pacific Ocean, is a fraction of a degree warmer, which Jamieson said made a significant difference at depth.
Nearly a decade ago, Jamieson and his colleagues had hypothesised that it may be biologically impossible for fish to survive at depths greater than 8,200 to 8,400 metres.
“Fish all have osmolyte, a fluid in their cells that they use to counteract pressure – it’s the thing that makes that fishy smell,” Jamieson said. “One of the only things, when you look at fish from a biochemical point of view, that is linear with depth is the concentration of that fluid.
“When you get to about 8,200 to 8,400 metres – the variation is probably temperature-dependent … it reaches what’s called isosmosis, which means you can’t increase the concentration of that fluid in the cells anymore.
“After all these years of hammering away at this [theory], it seems to be pretty solid. We’ve done close to 250 deployments … the window is narrowed to the point where on this Japanese expedition, we were seeing snailfish every single deployment down to this last one [of 8,336 metres].”
