Deepest fish

The deepest fish in the ocean to have been observed in situ are several species of Pseudoliparis snailfish belonging to the Liparidae family. The current record holder is a specimen of Pseudoliparis (species unknown) observed at a depth of 8,336 metres (27,349 feet) in the western Pacific’s Izu-Ogasawara Trench on 15 August 2022. On 20 August 2022, two specimens of P. belyaevi snailfish were collected at 8,022 metres (26,319 feet) in the neighbouring Japan Trench; this represents the deepest fish ever captured. These discoveries were made on a joint research expedition by the landers Flere and Closp respectively, deployed from the support vessel DSSV Pressure Drop by scientists from the University of Western Australia (Australia) and the Tokyo University of Marine Science and Technology (Japan). The operation was overseen by deep-sea marine biologist Professor Alan Jamieson (UK). This expedition was filmed for a documentary by the Japanese broadcaster NHK.

Hadal snailfish (i.e., those living in the hadal zone, more than 6,000 m (19,685 ft) beneath the surface, have evolved several adaptations to withstand the extreme conditions at this depth, including a largely cartilaginous skeleton, no swim bladder and gaps in their skulls. It’s been estimated that snailfish can cope with pressure akin to the weight of 1,600 elephants.

Prior to this, the greatest depth at which a snailfish had been observed was a Mariana snailfish (P. swirei) at 8,178 m (26,831 ft) in the Mariana Trench in the western Pacific on 18 May 2017 that was reported by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and NHK in August 2017. This followed in the wake of the Chinese Academy of Sciences announcing in April 2017 that it had logged a snailfish at 8,152 m (26,745 ft), also in the Mariana Trench. There have also been observations of another yet-formally-described snailfish species at 8,143 m (26,716 ft) – for now referred to as the “ethereal snailfish”.

The deepest live snailfish to have been caught before the 8,022 m specimen in the Japan Trench was a Mariana snailfish collected at 7,966 m (26,135 ft) in the Mariana Trench by Alan Jamieson and Thomas Linley on 23 November 2014.

There are two other deepest fish depths records that are somewhat contentious. The first is the "Trieste flatfish" that was reported to have been seen by Jacques Piccard when he and Lt Don Walsh became the first people to dive to the Challenger Deep in 1960 on board the Trieste bathyscaphe. This record (never published scientifically) was widely questioned at the time as being a misidentification, and more likely to have been a holothurian (sea cucumber).

In the scientific literature, it was long thought that the deepest fish was a cusk eel named Abyssobrotula galatheae (originally Bassogigas profundissimus, Staiger, 1972, but reclassified by Nielson, 1977). It was recorded as being from 8,370 m (27,460 ft) in the Puerto Rico Trench (the deepest point in the Atlantic Ocean) and captured by trawl (so was not seen in situ) from the RV John Elliott Pillsbury in 1970. However this species is reasonably well known, and known to be more common at abyssal depth (3,000–6,000 m; 9,843–19,685 ft) and also known to swim thousands of metres above the seafloor. With no other sightings of this species at such depths since, it is possible that this was an accidental capture of a mid-water fish during the lowering or hauling of the trawl.

The scientific team consists of University of Western Australia, Tokyo University of Marine Science and Technology, in collaboration with Nagoya University, JAMSTEC, Niigata University, the University of Tokyo, South Denmark University, Nippon Marine Enterprises, Caladan Oceanic Co., Ltd., Inkfish Co., and NHK.