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 6,198–8,076 m (20,335–26,496 ft), including a capture at 7,966 m (26,135 ft), 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 28.8 cm (11.3 in) in standard length and 160 g (0.35 lb) 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 1 cm (0.4 in) 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 7,949 m (26,079 ft).