The tail or caudal fin is the engine that makes the fish move. The power behind the caudal fin rests with a group of mussels in the base of the tail or "caudal peduncle".

The shape of the caudal fin relates to how fast a fish moves. A caudal with a sharp cut in it, is characteristic of a fast, enthusiastic swimmer whereas a straight edged tail is indicative of a slow swimmer. Additionally, the less defined the caudal fin or the rounder in shape, the slower the fish to which it belongs.