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 strike and stun the small schooling fish on which it feeds; the tail also is flailed from side to side to sweep prey in front of the shark. The thresher also slaps the water with its tail to frighten its prey. 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.