The cutlassfishes are about 45 species of predatory fish in the family Trichiuridae of the order Scombriformes found in seas throughout the world. Fish of this family are long, slender, and generally steely blue or silver in colour, giving rise to their name. They have reduced or absent pelvic and caudal fins, giving them an eel-like appearance, and large fang-like teeth.

The largehead hairtail or beltfish is a member of the cutlassfish family, Trichiuridae. This common to abundant species is found in tropical and temperate oceans throughout the world. The taxonomy is not fully resolved, and the Atlantic, East Pacific and Northwest Pacific populations are also known as Atlantic cutlassfish, Pacific cutlassfish and Japanese cutlassfish, respectively. This predatory, elongated fish supports major fisheries.