After being narrowly beaten by a close relative, the so-called "Mariana snailfish" has reclaimed its title as the world's deepest-diving fish. This pale piscine creature was clocked at 8,178 metres (over five miles!) beneath the surface.

The sighting was made by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Japan Broadcasting Corporation (NHK), who recently set sail for the Mariana Trench in the western Pacific Ocean during a collaborative cruise to film the region's inhabitants in 4K. Using mackerel for bait, the team dropped a lander – armed with a camera capable of operating below 10,000 metres – into the deepest part of the world's oceans.

"Deep-sea organisms and their ecosystems have attracted great scientific interest," says the team. "However, extremely high pressure in deep-sea trenches has prevented sampling as well as video recordings."

Small crustaceans and other invertebrates were the first to show up to the free feast fathoms below. Then, after 17 hours, the snailfish (family Liparidae) made its remarkable appearance at the lander's deepest site. For a bit of perspective: this creature is lallygagging about in waters four times deeper than diving specialists like sperm whales can reach (and it also surpasses the human SCUBA record 2,600 times over). 

The previous depth-defying record-holder was encountered back in 2014, when the "ethereal snailfish" (which has yet to be officially described) was seen at 8,145m. Earlier this year, the Chinese Academy of Sciences reportedly encountered a fish at 8,152m – but the details surrounding that observation are still unclear. 

Fish sightings below 6,000 metres are few and far between, and most of what we know about the animals that cope in such extreme habitats comes from trawlers. University of Aberdeen Oceanlab researcher Dr Alan Jamieson, who has done work on deep-water snailfish, notes that an interesting trend is emerging as we continue to explore the ocean's underwater canyons. 

"Each trench has its own snailfish species," he told BBC News after the ethereal snailfish made its chart-topping debut. 

Just last month, the NOAA Office of Ocean Exploration and Research released footage of what they suspect to be yet another new snailfish species, encountered near the Johnston Atoll Unit of the Pacific Remote Islands Marine National Monument. A depth of 1.5 miles won't earn that animal any diving titles, but the fish's black-spotted body is utterly unique: 

Meanwhile, during various stock-assessment surveys in the Aleutian Islands over the years, Alaska Fisheries Science Center biologist Jay Orr has discovered nearly a dozen new snailfishes – including the mischievous, comic and tomato snailfish (naming points, all round!).

We don't know for sure, but it's thought that geographically isolated populations of deep-sea snailfishes may be the result of their unique reproductive habits. Unlike the many fish species that have planktonic (or drifting) larvae, snailfish lay their eggs on the seafloor and the young hatch only when they're big enough to fend for themselves. That means juveniles are more likely to stay put, in contrast with fish that begin life at the mercy of midwater currents. In fact, some species of snailfish possess a suction-cup-like disc in lieu of pelvic fins. 

Jamieson aptly describes snailfish as having the consistency of "wet tissue paper", and there's a good reason for that flabby physique. Many deep-dwelling species are similarly gelatinous (paging the bony-eared assfish) because malleable skeletons and muscles are less prone to pressure damage. 

Snailfish also lack scales, which makes attempting to identify a dead one all the more challenging. "They get damaged easily," Orr explained in a NOAA Fisheries blog post. "In the past, biologists might just call all snailfish 'snailfish', instead of attempting to identify them."

For this reason, high-definition footage like that captured by NHK and JAMSTEC is an invaluable resource. 

The "Mariana" snailfish has yet to be officially described, and it's possible that the recently sighted specimens are a different species altogether. But going on morphology alone, the teams suspect this fish has been seen before. 


Top header image: JAMSTEC/NHK