Trichiurus lepturus is a marine fish species in the family Trichiuridae, commonly known as largehead hairtail, beltfish, Pacific cutlassfish. It is widely distributed in tropical and temperate oceans worldwide, including the Atlantic, Indian, and Pacific Oceans, where it inhabits coastal and offshore waters. The genus name Trichiurus derives from Greek thrix (“hair”) and oura (“tail”), referring to its long, ribbon-like body, while the species epithet lepturus comes from Greek leptos (“slender”) and oura (“tail”), further emphasizing its elongated form. In East Asia, it is commonly known as tachiuo (太刀魚, “sword fish” in reference to its blade-like shape) in Japan and is an important commercial species.

Trichiurus lepturus has a distinctive morphology characterized by a long, laterally compressed, silvery body that tapers to a pointed tail, with reduced or absent caudal fin structures. It is a predatory fish, feeding on smaller fish, squid, and crustaceans, often inhabiting midwater to near-bottom zones. Its metallic sheen and elongated shape make it easily recognizable among marine fishes. The species is highly adaptable and supports significant fisheries in many regions, particularly in East and Southeast Asia.

As a food source, Trichiurus lepturus is valued for its soft, white flesh and rich flavor. It is commonly prepared by grilling, pan-frying, braising, or steaming, and is frequently salted or lightly dried before cooking to enhance texture and taste. In Japanese cuisine it is often grilled with salt (shioyaki), while in Chinese and Korean cuisines it is used in braised dishes, stews, or fried preparations. The flesh is delicate and contains a relatively high fat content compared with many other white fish, contributing to its desirable mouthfeel.

Nutritionally, Trichiurus lepturus provides high-quality protein and is a source of omega-3 fatty acids, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). It also contains vitamins such as B12 and minerals including selenium and phosphorus. Bioactive compounds include taurine and small peptides that may have antioxidant and metabolic functions in the human body. As with many predatory marine fish, there may be concerns about bioaccumulation of trace contaminants such as mercury depending on habitat and size, though levels vary by region.