thresher shark, long-tailed, warm-water shark, genus Alopias. The upper fork of its tail is slender and sickle-shaped and is about equal in length to the rest of the body. This shark uses its tail to herd the small schooling fish on which it feeds; the tail is flailed from side to side, sweeping the prey in front of the shark. It also slaps the water with its tail to frighten the fish. Threshers are found chiefly in offshore, tropical waters, but are also known in temperate regions. The common thresher, A. vulpinus, is widely distributed throughout the Atlantic and the E Pacific; it is common off the New England coast in summer and is fished commercially on the S California coast. It may reach a length of 20 ft (6.2 m) and weigh 1,000 lb (450 kg). A second species, A. pelagicus, is found in the W Pacific. The big-eyed thresher, A. superciliosus, is a deep-sea fish of the tropical and subtropical Atlantic. It is smaller than the common thresher, but its eyes may measure 4 in. (10 cm) in diameter. Thresher sharks are classified in the phylum Chordata, subphylum Vertebrata, class Chondrichthyes, order Selachii, family Alopiidae.

Any of five species (family Alopiidae) of sharks with a long, scythelike tail that may constitute almost half their total length. They are found in tropical and temperate seas worldwide. They eat squid and schooling fishes, attacking after circling and herding their prey into small groups. They sometimes use their tail to stun their prey or, by thrashing the water, to frighten them. They are not considered dangerous to humans. The long-tailed thresher, or fox shark (Alopias vulpinus), is a big, dark fish that grows about 20 ft (6 m) long.

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