Caudal fin
The tail fin, which drives most fish forwards with side-to-side strokes.
Also: tail fin, tail
The caudal fin is the tail. Its shape tells a lot about how a fish lives. A deeply forked, crescent-shaped tail, as in tuna, marlin and mackerel, is built for long, fast cruising. A broad rounded or square tail, as in groupers and many cichlids, gives quick bursts and tight turns among rocks and reefs. In most bony fish the two lobes look alike (homocercal). Sharks and sturgeons have a heterocercal tail, with the backbone running up into a longer top lobe. Swimming speed is often estimated from the tail's height and stiffness.