Scientists recently exploring some of the world’s deepest oceanic trenches in the North Pacific have redefined the known limits that fish can survive.

On 15 August 2022, a juvenile Pseudoliparis snailfish (species yet to be determined) was captured on film investigating a baited camera 8,336 m (27,349 ft) below the surface in the Izu-Ogasawara Trench off Japan. It claims the undisputed record as the world’s deepest fish.

The unprecedented depth – more than double the vertical extent of Mount Fuji – is approaching what is thought to be the biological bottom line for fish.

Just days later, during the same expedition, two P. belyaevi snailfish were successfully retrieved from a depth of 8,022 m (26,319 ft) in the nearby Japan Trench. A watershed moment, this is the first time that any fish have been caught categorically from below 8,000 m (26,247 ft).

The snailfish were documented by marine biologists from the University of Western Australia (UWA) and the Tokyo University of Marine Science and Technology (TUMSAT), during a two-month survey on the research vessel DSV Pressure Drop (since renamed Dagon). The expedition was supported by Caladan Oceanic and Inkfish, with filmmakers from the Japanese broadcaster NHK also on board.

The chief scientist conducting this field study was UWA Professor Alan Jamieson, founding director of the Minderoo-UWA Deep-Sea Research Centre. A consultant for GWR on deep-sea marine life, Professor Jamieson is arguably the world’s leading authority on hadal organisms – i.e., those that live deeper than 6,000 m (19,685 ft) – and has made numerous record-setting discoveries over his career.

These include the deepest squid (6,212 m; 20,381 ft), the deepest octopus (6,957 m; 22,825 ft), the deepest decapod (7,703 m; 25,272 ft) and the deepest hydrozoan (10,063 m; 33,015 ft), to name just a few.

Given his fathomless expertise in this area, was he surprised to find a snailfish so far down, when he himself has proposed that the absolute maximum for fish lies not much beyond this point? “The limit of 8,200 m [26,903 ft] came with the caveat of it being in the region of 8,200–8,400 m [26,903–27,559 ft] as temperature likely plays a part here,” Professor Jamieson told GWR.

“Temperature and pressure have similar perturbing effects on cells, therefore warmer waters should allow fish to go deeper.” He continued: “Two years ago, we published a paper on all ultra-deep-sea fish and concluded that the deepest is likely off Japan as the trenches there are both deep enough and slightly warmer than the previous record in the Mariana Trench. And hey presto, there it was!”

“The 8,200–8,400 m limit is pretty solid. Fish use an osmolyte in their cells that balances pressure, essentially stopping the cells imploding at depth! It is called Trimethylamine-n oxide (TMAO for short). Concentrations of TMAO increase in cells with depth and the region where it reaches isosmosis – i.e., saturation point, when they can’t put any more in – is 8,200–8,400 m” – Professor Alan Jamieson

As Professor Jamieson alludes to, this isn’t the first time that snailfish have hit the headlines for plunging to record lows in the ocean.

Prior to this, the deepest fish on record was a Mariana snailfish (P. swirei) observed at 8,178 m (26,831 ft) in the Mariana Trench in the western Pacific on 18 May 2017. This was reported by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and NHK in August 2017.

This sighting came in the wake of another yet-identified snailfish species at 8,145 m (26,716 ft) – for now dubbed the “ethereal snailfish” – also in the Mariana Trench, filmed by Professor Jamieson in 2014.

Situated around 320 km (200 mi) south-west of Guam, the Mariana Trench is home to the oceans’ deepest point: the 10,935 m (35,872 ft) Challenger Deep.

It begs the question could another snailfish – or indeed a different fish species – be found even deeper still? Either in the 9,780 m (32,087 ft) Izu-Ogasawara Trench, the Mariana Trench or in another ocean trench entirely?

Professor Jamieson believes we are close to hitting a baseline. “Since we first started finding these snailfish around 2007–10, and the TMAO hypothesis was established (in 2014), we have done an exponentially increasing number of visits with baited cameras.

“We have now done 500 deployments of deep-sea cameras all over the world, and so our understanding of where these fish are has progressed a lot.

“Pretty much every film up to 8,336 m has fish in it, and then they simply disappear after that. We have honed into this limit in such a way that if a deeper fish is found it is likely no more than a few tens of metres, and probably then, falling within the error bars of the depth sensors. I think this might be the last significant increase in their maximum depth.”

“[After snailfish] the next deepest fish are the grenadiers (Macrouridae) and cusk eels (Ophididae), which occupy most of planet Earth, but despite such huge horizontal distributions, they pretty much drop off at a depth of around 7,200 m (23,622 ft), and usually shallower. After then, it appears to be snailfish all the way” – Professor Alan Jamieson

Certain snailfish have been able to push beyond the depth barrier of all their other kin owing to a number of special adaptations that enable them to withstand the extreme conditions. These evolutionary attributes include a largely cartilaginous skeleton and also gaps in their skulls to allow for the immense pressure, which beyond 8,000 m can exceed more than 800 times that at the surface.

However, these traits are not shared across the whole family, as Professor Jamieson explains: “There are 300 species of snailfish and most of them are shallow, even estuarine. They have percolated down into the depths during their radiating evolution and now we have a multitude of ultra-deep snailfish that live 1,000 m [3,280 ft] deeper than any quintessentially ‘deep-sea’ fish.

“It is perhaps their lack of swim-bladder, gelatinous body and efficiency in consuming small crustaceans that happen to be in the trenches, that have made them so successful.”

While the 8,336 m snailfish is garnering much of the media spotlight in terms of it fundamentally being the deepest recorded fish in history, Professor Jamieson is eager to emphasize the significance of the two P. belyaevi specimens that were collected five days later, slightly higher at 8,022 m in the Japan Trench.

So how do you go about fishing at depths akin to 90% the height of Mount Everest? “We use standard fish traps from the local fishing shop, pop a bit of bait in it and tie to a $200,000 deep-sea camera,” Professor Jamieson revealed. “A lovely mix of hi-tech meets low-tech!”

He continued: “We film a significantly larger number of snailfish than we ever catch, and I am happy with that. I don’t really want to catch them; I’d rather learn as much from videoing them than having to bring them up.

“But, to know what it is, how genetically related they are to other populations, how they survive, what they eat and things like the TMAO-driven limit, we must bring up what we call ‘voucher specimens’. Normally one or two fish is enough for this type of work.”

As you might suspect given their native pitch-black habitat several kilometres below the waves, which until relatively recently has been out of reach to humans, the record for the deepest fish has had a complex and at times controversial history. A few past contenders have been thrown into doubt as more light is cast on the hadal world thanks to advances in technology and the dedicated research of explorer-scientists like Professor Jamieson.

One such of these was the so-called “Trieste flatfish”. It was reported on the ocean floor in the Mariana Trench by the Swiss oceanographer Jacques Piccard, who along with pilot Don Walsh (USA) performed the first crewed dive to the Challenger Deep in the Trieste bathyscaphe on 23 January 1960.

This account of a “flatfish” (never published scientifically) was widely questioned at the time as being a misidentification, and it’s now generally believed that the creature Piccard observed was more likely to have been a holothurian (sea cucumber).

Another more recent contender has proven more debated. In the scientific literature, it was long thought that the deepest fish was a cusk eel of the species Abyssobrotula galatheae. A specimen was reportedly collected from 8,370 m (27,460 ft) in the Puerto Rico Trench – the deepest point in the Atlantic Ocean – by the RV John Elliott Pillsbury in 1970. This is, of course, even deeper than the newly documented snailfish at 8,336 m.

However, the key bone of contention here is that the cusk eel was trawled and so not observed or filmed in situ at that depth. Therefore, there’s every chance it was ensnared when the net was being lowered or lifted at some point between the sea floor and the surface.

This explanation is bolstered by the fact that there have been no other sightings of these cusk eels at such extreme depths in the 50-plus years since. Furthermore, this species is reasonably well documented, and known to be more common in the ocean’s abyssal zone – i.e., 3,000–6,000 m (9,843–19,685 ft) – and typically seen swimming thousands of metres above the seafloor.

By comparison, the deepest cusk eel to have been recorded in situ are examples of Bassozetus recorded at 7,176 m (24,543 ft) in the Indian Ocean’s Java Trench. This discovery too was made by Professor Jamieson, as reported in Deep Sea Research Part I in Dec 2021.

All images courtesy of: Alan Jamieson / Minderoo-UWA Deep-Sea Research Centre / Caladan Oceanic / Inkfish