Pseudoliparis swirei
Pseudoliparis swirei, the Mariana snailfish or Mariana hadal snailfish, is a species of snailfish found at hadal depths in the Mariana Trench in the western Pacific Ocean. It is known from a depth range of, including a capture at, which is possibly the record for a fish caught on the seafloor. Various anatomical, physiological, molecular and genetic adaptions help this species survive in such depths.
This pale, tadpole-like fish reaches up to in standard length and in weight. It is apparently the top predator along certain stretches of the Mariana Trench, feeding on tiny crustaceans in a deep-water habitat with few larger predators. Pseudoliparis swirei are abundant in their deep-sea habitat and lay relatively large eggs that are almost in diameter.
Discovery
The first specimens of this species were caught in 2014 during an expedition of the research vessel Falkor. To catch the fish, deep-water traps designed to minimize the damage to the caught fish during the ascent were used. Mackerel was used as bait. Findings of hadal fish are limited, as the trawl catch method used for catching causes them to end up in very low quantities and poor condition. The first representative of this species was caught on November 15, 2014. In the next 10 days another 35 were caught. Another was caught on January 29, 2017, during the expedition of the research ship Shinyo-maru. The holotype of the species is the immature specimen USNM 438975 / HADES 200060, caught on November 21, 2014, at a depth of.Depth record and adaptations
Abyssobrotula galatheae has generally been recognized as the record-holder based on one caught at the seafloor at, but it might have been caught with a non-closing net and therefore was perhaps shallower. Another contender is Echiodon neotes, captured at, but this was likely from open water. Only one other species of fish has been recorded from depths in excess of, the so-called ethereal snailfish, but it has only been seen on film and remain undescribed.Due to the extreme pressure, appears to be around the theoretical maximum depth possible for fish.
Compared to shallow-water snailfish, Pseudoliparis swirei has several unusual adaptions for its dark and high pressure habitat, including transparent skin that lacks pigment, certain organs and eggs that are enlarged, the muscles are thinner, the ossification of its bones is incomplete, it appears to have little or no ability to see, there are mechanisms that allow proteins in its body to still function, and differences in the cell membranes for maintaining their flexibility.