The RATTAIL aka Grenadier Fish

Rattail fish are deep sea fish who tend to travel alone. This species of fish are known to account for 15% of the deep sea fish community.

These Grenadiers or rattails (less commonly whiptails) are generally large, brown to black gadiform marine fish of the family Macrouridae. Found at great depths from the Arctic to Antarctic, members of this family are among the most abundant of the deep-sea fishes.
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The Macrouridae are a large and diverse family with some 34 genera and 383 species recognized (well over half of which are contained in just three genera, Caelorinchus, Coryphaenoides and Nezumia). They range in length from approximately 10 centimetres (3.9 in) in the graceful grenadier (Hymenocephalus gracilis) to 1.5 metres (4.9 ft) in the giant grenadier, Albatrossia pectoralis. An important commercial fishery exists for the larger species, such as the giant grenadier and roundnose grenadier, Coryphaenoides rupestris. The family as a whole may represent up to 15 percent of the deep-sea fish population.
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Typified by large heads with large mouths and eyes, grenadiers have slender bodies that taper greatly to a very thin caudal peduncle or tail (excluding one species, there is no tail fin): this rat-like tail explains the common name rattail and the family name Macrouridae, from the Greek makros meaning "great" and oura meaning "tail". The first dorsal fin is small, high and pointed (and may be spinous); the second dorsal fin runs along the rest of the back and merges with the tail and extensive anal fin. The scales are small.
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Grenadiers or rattails at a debth of 2660m webres

The giant grenadier, Albatrossia pectoralis , is a very large rattail, the only member of the genus Albatrossia, found in the north Pacific from northern Japan to the Okhotsk and Bering seas, east to the Gulf of Alaska, and south to northern Baja California in Mexico, at depths of between 140 and 3,500 m. Its length is up to 2.1 m.
The giant grenadier has the usual greatly elongated pointed tail of the rattails.
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As with many deep-living fish, the lateral line system in grenadiers is well-developed; it is further aided by numerous chemoreceptors located on the head and lips, and chemosensory barbels underneath the chin. Benthic species have gas bladders with unique muscles attached to them. The animals are thought to use these muscles to "strum" their gas bladders and produce sound, possibly playing a role in courtship and mate location. Light-producing organs called photophores are present in some species; they are located in the middle of the abdomen, just before the anus and underneath the skin.

Living at depths from 200 to 6,000 metres (660 to 20,000 ft), rattails are the most common benthic fish of the deep (however, two genera are known to prefer the midwater). Rattails may be solitary or they may form large schools, as with the roundnose grenadiers. The benthic species are attracted to structural oases, such as hydrothermal vents, cold seeps, and shipwrecks. Rattails are thought to be generalists, feeding on smaller fish, pelagic crustaceans such as shrimp and amphipods, cumaceans and less often cephalopods and lanternfish. As well as being important apex predators in the benthic habitat, some species are also notable as scavengers.
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As few rattail larvae have been recovered, little is known of their life history. They are known to produce a large number (over 100,000) of tiny (1–2 millimetres/0.039–0.079 inches in diameter) eggs made buoyant by lipid droplets. The eggs are presumed to float up to the thermocline (the interface between warmer surface waters and cold, deeper waters) where they develop. The juveniles remain in shallower waters, gradually migrating to greater depths with age.
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Spawning may or may not be tied to the seasons, depending on the species. At least one species, Coryphaenoides armatus, is thought to be semelparous; that is, the adults die after spawning. Non-semelparous species may live to 56 years or more. The Macrouridae in general are thought to have low resilience; commercially exploited species may be overfished and this could soon lead to a collapse of rattail fisheries.

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Source: Wikipedia