1. Range and habitat
Largehead hairtails are found worldwide in tropical and temperate oceans. In the East Atlantic it ranges from southern United Kingdom to South Africa, including the Mediterranean Sea. In the West Atlantic it ranges from Virginia (occasionally Cape Cod) to northern Argentina, including the Caribbean Sea and Gulf of Mexico.
In the East Pacific it ranges from southern California to Peru. It is widespread in the Indo-Pacific region, ranging from the Red Sea to South Africa, Japan, the entire coast of Australia (except Tasmania and Victoria) and Fiji, but is absent from the central Pacific Ocean, including Hawaii. Some populations are migratory.
A study of largehead hairtails in southern Japan's Bungo Channel indicated that the optimum water temperature is 20–24 °C (68–75 °F). Based on fishing catches in the Jeju Strait of South Korea, the species mainly resides in water warmer than 14 °C (57 °F), while catches are poor in colder water. Off southern Brazil it mainly occurs in waters warmer than 16 °C (61 °F).
It is absent from waters below 10 °C (50 °F). The largehead hairtail prefers relatively shallow coastal regions over muddy bottoms, but it sometimes enters estuaries and has been recorded at depths of 0 to 589 m (0–1,932 ft). In European waters most records are from 100 to 350 m (330–1,150 ft), off southern Brazil it is most abundant between 40 and 120 m (130–390 ft), it has been recorded between 55 and 385 m (180–1,263 ft) in the East Pacific, and in southern Japan's Bungo Channel it is primarily known from 60 to 280 m (200–920 ft) but most common between 70 and 160 m (230–520 ft). It is mainly benthopelagic, but may occur at the surface during the night.
Although often considered a single highly widespread species, it has been argued that it is a species complex that includes several species with the main groups being in the Atlantic (Atlantic cutlassfish), East Pacific (Pacific cutlassfish), Northwest Pacific (Japanese cutlassfish) and Indo-Pacific. If split, the Atlantic would retain the scientific name T. lepturus, as the type locality is off South Carolina.
The Northwest Pacific (Sea of Japan and East China Sea) differs in morphometrics, meristics and genetics, and is sometimes recognized as T. japonicus. Morphometric and meristic differences have also been shown in the population of the East Pacific (California to Peru), leading some to recognize it as T. nitens. Neither T. japonicus nor T. nitens are recognized as separate species by FishBase where considered synonyms of T. lepturus, but they are recognized as separate species by the Catalog of Fishes.
The IUCN recognizes the East Atlantic population as a distinct, currently undescribed species. This is based on genetic evidence showing a divergence between West and East Atlantic populations. However, this would require that T. japonicus, T. nitens and the Indo-Pacific populations also are recognized as separate species, effectively limiting T. lepturus to the West Atlantic (contrary to IUCN), as they all show a greater divergence.
Additional studies are required on the possible separation and nomenclature of the Indo-Pacific populations. Based on studies of mtDNA, which however lacked any samples from the southern parts of the Pacific and Indian Oceans, there are three species in the Indo-Pacific: T. japonicus (marginal in the region, see range above), T. lepturus (West Pacific and Eastern Indian Ocean; the species also found in the Atlantic) and the final preliminarily referred to as Trichiurus sp. 2 (Indian Ocean, and East and South China Seas).
It is likely that Trichiurus sp. 2 equals T. nanhaiensis. The names T. coxii and T. haumela have been used for the populations off Australia and in the Indo-Pacific, respectively, but firm evidence supporting their validity as species is lacking.