Fish
Mariana Snailfish
Pseudoliparis swirei · also called mariana snailfish, hadal snailfish
The Mariana snailfish is the deepest fish known to exist. It lives on the floor of the Mariana Trench, more than eight kilometres down, in water pressure around 800 times that at the surface — and it is not some armoured monster but a small, soft, translucent-pink animal that would slump into a shapeless blob if you brought it up.
Overview
The Mariana snailfish lives in the Mariana Trench in the western Pacific, mostly between about 6,000 and 8,000 m. It is 20–29 cm long, tadpole-shaped, with no scales, small eyes, loose translucent skin, and watery flesh. It is the top predator of that zone, eating the amphipods that swarm over anything edible on the trench floor.
The deepest fish
It has been caught and filmed at roughly 7,000–8,000 m, and related snailfish a little deeper still. Below about 8,400 m, no fish of any kind have ever been found, even though the trenches go much deeper. The likely cause is TMAO, a molecule fish use to protect their proteins from pressure: past that depth a fish would need more of it than its salt balance can tolerate.
Built to be squeezed
Its skull is only partly hardened, its bones are thin, it has no swim bladder, and its body is largely water and gel — all of which let it withstand the pressure without being crushed.
Status
Described in 2017 and named for an HMS Challenger officer, the Mariana snailfish has not been assessed. It appears locally common on the trench floor, beyond the reach of any fishery.
Mariana Snailfish: common questions
How deep does the Mariana snailfish live, and is anything deeper?
It has been reliably caught and filmed between roughly 6,000 and 8,000 metres in the Mariana Trench. Snailfish in the same genus have been recorded a little deeper in other trenches, with the current record for any fish sitting at around 8,300 metres. Below roughly 8,400 metres, despite many baited camera drops, no fish have ever been seen, even though the trenches go down past 10,000 metres. Amphipods and other invertebrates live all the way to the bottom, but fish appear to stop, so the Mariana snailfish and its close relatives are at or very near the absolute depth limit for fish.
Why can fish not live below about 8,400 metres?
The leading explanation involves a chemical called trimethylamine N-oxide, or TMAO. Deep-sea fish accumulate TMAO in their tissues because it helps their proteins keep working under crushing pressure, and the deeper a fish lives, the more it needs. But TMAO also makes the fish's body fluids saltier, and there is a point, reached at roughly 8,000 to 8,400 metres, at which a fish would need so much TMAO to survive the pressure that its internal salt balance would tip past what its body can manage. Invertebrates use different biochemistry and are not bound by the same limit.
What would happen to a Mariana snailfish brought to the surface?
It does not survive well, and its body changes markedly. Adapted to a world of extreme, constant pressure, the fish has soft, water-laden flesh, weak bones, and no gas-filled organs, and when the pressure is removed on the way up, the tissues lose their structure. Specimens recovered by researchers arrive slack and fragile, and the fish cannot live in normal aquarium conditions. Almost everything known about it comes from baited traps and cameras lowered to the trench floor and from careful study of the few preserved bodies.
How does anything find food that deep?
Trenches act as funnels. Dead animals, faecal matter, and other organic material sinking from the sunlit ocean above tend to be channelled down the steep sides and collect on the trench floor, so there is more food at the very bottom than at the mid depths just above it. Amphipods swarm in huge numbers onto any carcass or bait that lands there, and the Mariana snailfish makes its living by eating those amphipods. Baited cameras exploit exactly this, dropping a lump of mackerel that draws first a cloud of amphipods and then the snailfish that hunt them.