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Definition of THRESHER SHARK
: a large nearly cosmopolitan shark (Alopias vulpinus) that has a greatly elongated curved upper lobe of the tail which is often used to thresh the water to round up the schooling fish on which it feeds — see shark illustration
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.