High Quality Content by WIKIPEDIA articles! The common thresher (Alopias vulpinus) is the largest species of thresher shark, family Alopiidae, attaining a maximum known length of 6 m (20 ft). Almost half of that length consists of the elongated upper lobe of its caudal fin. This structure, the source for many a fanciful tale about this shark through history, is employed by the thresher in a whip-like fashion to deliver incapacitating blows to its prey. The common thresher resembles (and has often been confused with) the pelagic thresher (A. pelagicus), which also has a streamlined body, short pointed snout, and modestly sized eyes. It can be distinguished from the latter species by the white of its belly extending in a band over the bases of its pectoral fins.