Deepest-ever fish filmed at a depth of 8,336 metres

A fish found off the coast of Japan could be the deepest ever recorded.

Japanese and Australian researchers filmed a Pseudoliparis snailfish at 8,336 metres down as part of an expedition to some of the world's deepest ocean trenches. This is 158 metres deeper than a previous recording made by the team in 2017, and close to the depth limit for all fish.

Professor Alan Jamieson, the chief scientist of the expedition, says, 'We have spent over 15 years researching these deep snailfish; there is so much more to them than simply the depth, but the maximum depth they can survive is truly astonishing.'

'In other trenches such as the Mariana Trench, we were finding them at increasingly deeper depths just creeping over that 8,000-metre mark in fewer and fewer numbers, but around Japan they are really quite abundant.'

Though another fish has been reported from greater depths, the reliability of this record has been questioned. The recording makes a strong case for Pseudoliparis being the deepest-ever fish.

What are snailfish?

Snailfishes are a family of fish that have been successful in exploiting inhospitable habitats such as cold and deep water, allowing them to rapidly evolve into a multitude of species.

Dr Rupert Collins, Senior Curator in Charge of Fishes at the Museum, says, 'There are over 400 species of snailfishes, and there are likely to be more which haven't yet been described. They are one of the fastest radiating groups of fishes, and can be found in marine habitats all the way from rockpools down to some of the deepest parts of the ocean.'

The deepest living snailfishes are found in the genus Pseudoliparis, and were among the first deep sea fishes to have their genome sequenced. This has revealed how these animals adapted to life in a place where the pressure is 1,000 times greater than at sea level.

Its stomach, for instance, takes up much more space than in other snailfishes, which is thought to allow it to eat a lot when it comes across scarce prey on the ocean floor. Its skeleton, meanwhile, is soft and flexible to allow it to tolerate the extreme pressure, while it is covered in a gelatinous layer rather than scales.

'Snailfishes produce a gelatinous mucus externally and internally, which makes them very slippery and gives them their name,' Rupert says. 'It's an energetically cheap way for them to grow, as energy efficiency is crucial in the deep sea where there's limited food, and it reduces friction as they swim.'

'The mucus is also thought to help them adapt for life in the deep as it allows them to stay neutrally buoyant without a gas-filled swim bladder, which couldn't be tolerated at that depth.'

At a molecular level, snailfish have specialised fats which keep its cells flexible, while it produces compounds such as trimethylamine N-oxide (TMAO) that help to stabilise its proteins. These adaptations are common in many species of deep-sea fish.

Unlike these species, however, the deepest living snailfish tend to be the youngest. They hatch from enlarged eggs, which may allow them to hatch as larger juveniles that can better tolerate the pressure than larvae can.

This meant the researchers weren't overly surprised when they saw an extremely small juvenile swimming towards their bated trap over 8,300 metres beneath the waves in the Izu-Ogasawara Trench.

While its exact species isn't known, it has been identified as a Pseudoliparis snailfish.

What are the deepest living fish?

Since scientists first became interested in studying the deep sea during the 1800s, plenty of research has been directed into understanding the species that live there. However, as these depths are extremely difficult to work in, much of the life found there remains unknown to science.

Historically, the main way of catching fishes from the deepest parts of the ocean, known as the hadal zone, has been trawling. One of the earliest fish to be caught in this zone was the cusk-eel Abyssobrotula galatheae in 1901, followed by another specimen found at 7,160 metres in the 1950s.

As the pace of ocean exploration picked up in second half of the twentieth century, even deeper fishes may have been found. In the first manned dive to the Mariana Trench, the deepest part of the ocean, the crew reported seeing what they interpreted as flatfish at more than 10,000 metres deep.

As this is much deeper than any flatfish has ever been found, this claim has been heavily disputed. It's thought the crew may have mistaken a deep-living species of sea cucumber for a fish.

In 1970, a decade after this dive, another A. galatheae cusk-eel was trawled from 8,370 metres deep. It is often credited as being the deepest fish ever found, but as it was found in an open trawl it may have been caught at a shallower depth as the net was lowered and raised.

Today, much of the exploration of the deep sea is carried out with remotely operated vehicles (ROV) and landers. This allows the depth of any observed fish to be more accurately reported, and even recorded on camera.

In recent years, this has led to reliable records of fish from over 8,000 metres. In 2017, the Mariana snailfish was found at 8,178 metres while another undescribed species was discovered at 8,145 metres.

These fish are pushing themselves to the limit of depths they are expected to tolerate.

'Between 8,200 and 8,400 metres, it's estimated that fish can no longer accumulate sufficient TMAO, and so their proteins would stop working normally,' Rupert says. 'At the moment, this hypothesis appears to put a limit on how deep fish can live.'

While fish may be able to tolerate greater depths for a short period, they would soon have to return upwards. Unless our understanding of life at depth changes, it's unlikely that many fish, if any, will be found deeper.