Ribbon fish (more often written ribbonfishes) are deepwater marine fishes in the family Trachipteridae, also commonly called dealfishes.
According to Fishes of Australia (Museums Victoria), they have long, ribbon-like, highly compressed bodies, large eyes, and greatly protrusible mouths; adults lack ribs and pelvic fins, and they swim by undulating a very long dorsal fin. The family occurs worldwide in meso- to bathypelagic waters, reaches about 2.2 m in length, and undergoes dramatic shape changes between larval and adult stages.
Source: Fishes of Australia — Family Trachipteridae (Ribbonfishes, Dealfishes)