''Pseudoliparis swirei'', the Mariana snailfish or Mariana hadal snailfish, is a species of

snailfish The Liparidae, commonly known as snailfish or sea snails, are a Family (biology), family of Saltwater fish, marine Scorpaeniformes, scorpaeniform fishes. Widely distributed from the Arctic Ocean, Arctic to Antarctic Oceans, including the ocea ...

found at

hadal The hadal zone, also known as the hadopelagic zone, is the deepest region of the ocean, lying within oceanic trenches. The hadal zone ranges from around below sea level, and exists in long, narrow, topographic V-shaped depressions. The cumula ...

depths in the

Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maximum know ...

in the western

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

. It is known from a depth range of , including a capture at the deepest point of Mariana Trench, which is possibly the record for a fish caught on the seafloor. Various

anatomical Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

and genetic adaptions help this species survive in such depths. This pale, tadpole-like fish reaches up to in

standard length Fish measurement is the measuring of individual fish and various parts of their anatomies. These data are used in many areas of ichthyology, including taxonomy and fisheries biology. Overall length * Standard length (SL) is the length of a fish m ...

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 Mackerel is a common name applied to a number of different species of pelagic fish, mostly from the family Scombridae. They are found in both temperate and tropical seas, mostly living along the coast or offshore in the oceanic environment. ...

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 A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

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 ''Abyssobrotula galatheae'' is a species of cusk eel in the family Ophidiidae. It is the deepest-living fish known; one specimen, trawled from a depth of in the Puerto Rico Trench in 1970, holds the record for the deepest fish ever captured. Alt ...

'' 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

, ''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 Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

of its bones (notably the skull) is incomplete, it appears to have little or no ability to see, there are mechanisms that allow

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

s in its body to still function, and differences in the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

s 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 The Challenger Deep is the deepest-known point of the seabed of Earth, with a depth of by direct measurement from deep-diving submersibles, remotely operated underwater vehicles and benthic landers, and (sometimes) slightly more by sonar bathym ...

, 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 A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

of ''P. swirei'' and at least some other

fish spend time in the open water at relatively shallow depths, less than .

References

{{Taxonbar, from=Q44096428 Liparidae Fish described in 2017