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 during the 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 representative of this species 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. This species is known from a depth range of , including a capture at . This is possibly the depth record for a fish caught on the seafloor. "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 (a net that is open on the way up and down into the deep) and therefore was perhaps shallower. Another contender is "Echiodon neotes", captured at , but this was likely from open water (not at the seafloor). Only one other species of fish has been recorded from depths in excess of , the so-called ethereal snailfish (living in the same region as "Pseudoliparis swirei" but somewhat deeper), 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 (notably the skull) 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. Etymology. "Pseudoliparis swirei" is named for Herbert Swire, the First Navigating Sub -Lieutenant of and author of "The Voyage of the "Challenger", a personal narrative of the historic circumnavigation of the globe in the years 1872-1876". In 1876 the expedition found the Challenger Deep, habitat of the Mariana snailfish. Reproduction. A study of mature females showed that mature eggs of this fish species are unusually large, up to in diameter. In total, there were up to 23 mature eggs for each female (each larger than in diameter), which were alternated with immature, small eggs, the number of which was about 850. Eggs of intermediate size were rarely seen. Even in the  largest eggs, no structures were found that indicate the development of the embryo within the eggs. There are indications that the larvae of "P. swirei" and at least some other hadal fish spend time in the open water at relatively shallow depths, less than .