Snailfish

Snailfishes make up the large family Liparidae, with species ranging from shallow tide pools to the very deepest parts of the ocean. The Mariana snailfish, Pseudoliparis swirei, described in 2017 from the Mariana Trench, holds the record for the deepest fish ever filmed, observed at depths beyond 8000 meters. Snailfishes are notable for a soft, gelatinous, scaleless body with no swim bladder, an adaptation that allows survival under the crushing pressure of hadal trench environments where few other vertebrates can persist. The family is globally distributed and includes hundreds of species occupying an unusually broad range of depths and habitats, from Arctic and Antarctic shallows to the abyss.

Identifying features of snailfishes:

• Soft, scaleless, gelatinous body, often tadpole-shaped
• Large, rounded head tapering to a slender tail
• Semi-translucent pinkish, whitish, or mottled coloration
• Small eyes and reduced pigmentation in deep-living species
• Continuous dorsal and anal fins merging with the tail fin
• No swim bladder

Deep-sea snailfish species can be distinguished from other soft-bodied deep fishes like blobfish by their more elongated, tapering tadpole shape and continuous fin margin running around the tail. Shallow-water snailfishes often have a ventral sucking disc formed from modified pelvic fins.

Snailfishes as a family occupy an extraordinarily wide habitat range, from rocky intertidal pools and shallow coastal waters to the hadal zone of ocean trenches deeper than 8000 meters. Deep-living species such as the Mariana snailfish are restricted to the near-freezing, lightless, extremely high-pressure environment of trench floors in the Pacific Ocean, where they appear to be the dominant fish predator due to the absence of larger competing species. Other snailfish species inhabit Arctic and Antarctic continental shelves, kelp forests, and tide pools in temperate coastlines worldwide. This breadth makes Liparidae one of the most depth-versatile fish families known.

Deep-trench snailfish species appear to move in small, loosely associated groups near the seafloor, feeding on tiny crustaceans and other invertebrates that are abundant in trench sediment. With few or no larger fish predators able to survive at such depths and pressures, they occupy something close to an apex position within the hadal food web. Shallower-water snailfish species are typically solitary and often use a ventral sucker to cling to rocks or kelp in areas with strong currents. Reproduction in deep-sea species is poorly studied, though related shallow-water snailfishes are known to lay egg masses on the seafloor. Their soft, flexible bodies and lack of a swim bladder are thought to reduce the physiological strain of extreme hydrostatic pressure.