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.
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.
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.