Snailfish

 

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. 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.
The habitats chosen by snailfish are as widely variable as their 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 having antifreeze proteins. It is the most species-rich family of fish in the Antarctic region, where 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. 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).