|Snailfish (probably Elassodiscus tremebundus) caught in the eastern Bering Sea|
T. N. Gill, 1861
Widely distributed from the Arctic to Antarctic Oceans, including the oceans in between, the snailfish family contains more than 30 genera and about 410 described species, but there are also many undescribed species. Snailfish species can be found in depths ranging from shallow surface waters to greater than 8,000 meters, and species of the Liparid family have been found in seven ocean trenches. They are closely related to the sculpins (family Cottidae) and lumpfish (family Cyclopteridae). In the past, snailfish were sometimes included within the latter family.
The snailfish family is poorly studied and few specifics are known. Their elongated, tadpole-like bodies are similar in profile to the rattails. Their heads are large (compared to their size) with small eyes; their bodies are slender to deep, tapering to very small tails. The extensive dorsal and anal fins may merge or nearly merge with the tail fin. Snailfish are scaleless with a thin, loose gelatinous skin; some species, such as Acantholiparis opercularis have prickly spines, as well. Their teeth are small and simple with blunt cusps. The deep-sea species have prominent, well-developed sensory pores on the head, part of the animals' lateral line system.
The pectoral fins are large and provide the snailfish with its primary means of locomotion although they are fragile. They are benthic fish with pelvic fins modified to form an adhesive disc; this nearly circular disc is absent in Paraliparis and Nectoliparis species. Snailfish range in size from Paraliparis australis at 5 cm (2.0 in) to Polypera simushirae at some 77 cm (30 in) in length. The latter species may reach a weight of 11 kg (24 lb), but most species are smaller. Snailfish are of no interest to commercial fisheries.
It was difficult to initially study snailfish species that dwell at deeper levels because they would implode upon being brought to the surface, but researchers did manage to study the bones of the animal.
Occurrence and habitat
The habitats occupied by snailfish are as widely variable in terms of size. They are found in oceans worldwide, ranging from shallow intertidal zones to depths of slightly more than 8,000 m (26,000 ft). This is a wider depth range than any other family of fish. They are strictly found in cold waters, meaning that species of tropical and subtropical regions strictly are deepwater. They are common in most cold marine waters and are highly resilient, with some species, such as Liparis atlanticus and Liparus gibbus, having type-1 antifreeze proteins. It is the most species-rich family of fish in the Antarctic region, generally found in relatively deep waters (shallower Antarctic waters are dominated by Antarctic icefish).
The diminutive inquiline snailfish (Liparis inquilinus) of the northwestern Atlantic is known to live out its life inside the mantle cavity of the scallop Placopecten magellanicus. Liparis tunicatus lives amongst the kelp forests of the Bering Strait and the Gulf of St. Lawrence. The single species in genus Rhodichthys is endemic to the Norwegian Sea. Other species are found on muddy or silty bottoms of continental slopes.
In October 2008, a UK-Japan team discovered a shoal of Pseudoliparis amblystomopsis snailfish at a depth of approximately 7,700 m (25,300 ft) in the Japan Trench. These were, at the time, the deepest living fish ever recorded on film. The record was surpassed by a snailfish that was filmed at a depth of 8,145 m (26,722 ft) in December 2014 in the Mariana Trench, and extended in May 2017 when another was filmed at a depth of 8,178 m (26,831 ft) in the Mariana Trench. The species in these deepest records remain undescribed, but it has been referred to as the "ethereal snailfish". The deepest-living described species is Pseudoliparis swirei, also of the Mariana Trench, which has been recorded to 8,076 m (26,496 ft). In general, snailfish (notably genera Notoliparis and Pseudoliparis) are the most common and dominant fish family in the hadal zone. Through genomic analysis it was found that Pseudoliparis swirei possesses multiple molecular adaptions to survive the intense pressures of a deep sea environment, including pressure-tolerant cartilage, pressure-stable proteins, increased transport protein activity, higher cell membrane fluidity, and loss of eyesight and other visual characteristics such as color. There are indications that the larvae of at least some hadal snailfish species spend time in open water at relatively shallow depths, less than 1,000 m (3,300 ft).
Reproduction and life span
Reproductive strategies vary extensively among snailfish species. As far as known, all species lay eggs that are relatively large in size (diameter up to 9.4 mm or 0.37 in). The number of eggs varies extensively depending on species. Some deposit their egg mass among cold-water corals, kelp, stones, or xenophyophores. It is possible that the male guards the egg mass. At least one species, Careproctus ovigerus of the North Pacific, is known to practice mouth brooding; that is, the male snailfish carries the developing eggs around in his mouth. Some other species of the genus Careproctus are parasitic, laying their eggs in the gill cavities of king crabs. One species, Careproctus rhodomelas, was found instead to be a batch spawner, laying multiple batches of large eggs multiple times throughout its lifetime. After the eggs hatch, some species rapidly reach the adult size and only live for about one year, but others have life spans of more than a decade.
Upon a 2007 study of fish in the hadal zone, it was revealed that snailfish usually feed on amphipods, which were also attracted to the chum that the researchers left out, Larval snailfish feed on a mix of plankton, small and large copepods, and amphipods. The diet of larval snailfish contains 28 food categories, mainly copepods and amphipods. 
Snailfish prey can be grouped in six main categories: gammarid, krill, natantian decapods, other crustaceans, fish, and others. Size also affects snailfish diets. Species smaller than 50 mm primarily eat gammarids, while species larger than 100 mm primarily eat natantian decapods. Species larger than 150 mm have the highest proportion of fish in their diet. Larger snailfish species tend to be piscivorous. 
This family contains these genera as of 2020:
- Acantholiparis Gilbert & Burke, 1912
- Aetheliparis Stein, 2012
- Allocareproctus Pitruk & Fedorov, 1993
- Careproctus Krøyer, 1862
- Crystallichthys Jordan & Gilbert, 1898
- Eknomoliparis Stein, Meléndez C. & Kong U., 1991
- Elassodiscus Gilbert & Burke, 1912
- Eutelichthys Tortonese, 1959
- Genioliparis Andriashev & Neyelov, 1976
- Gyrinichthys Gilbert, 1896
- Liparis Scopoli, 1777
- Lipariscus Gilbert, 1915
- Lopholiparis Orr, 2004
- Menziesichthys Nalbant & Mayer, 1971
- Nectoliparis Gilbert & Burke, 1912
- Notoliparis Andriashev, 1975
- Osteodiscus Stein, 1978
- Palmoliparis Balushkin, 1996
- Paraliparis Collett, 1879
- Polypera Burke, 1912
- Praematoliparis Andriashev, 2003
- Prognatholiparis Orr & Busby, 2001
- Psednos Barnard, 1927
- Pseudoliparis Andriashev, 1955
- Pseudonotoliparis Pitruk, 1991
- Rhinoliparis Gilbert, 1896
- Rhodichthys Collett, 1879
- Squaloliparis Pitruk & Fedorov, 1993
- Temnocora Burke, 1930
- Volodichthys Balushkin, 2012
Careproctus ovigerus (juvenile)
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- Media related to Liparidae at Wikimedia Commons