Fish use the caudal fin to generate swimming power and aid in maneuverability. The shape of the caudal fin, which is also known as the tail fin, determines a fish's swimming speed and movement.
A fish with a thin, crescent-shaped caudal fin, such as a swordfish, can swim over long distances while maintaining high speeds. These fish sacrifice their maneuverability and ability to survive at low depths to focus on speed. Broad caudal fins provide a balance between speed and maneuverability. Sea bass and grouper are two common species of fish that share this adaptation. The disadvantage of this type of tail is that increased drag fatigues the fish more quickly than other species.
The single fin running continuously along an eel's body is also a type of caudal fin. This caudal fin provides the eel with great maneuverability, allowing it to move quickly into crevices. Continuous caudal fins are beneficial to reef-dwelling fish for the same reason: the increased maneuverability provided by the fin allows for easier movement in a coral reef.
Because they breathe air, marine mammals such as dolphins and whales have unique tail fins. Their fins move up and down rather than side to side, providing lift rather than forward thrust. This lift is necessary for them to reach the surface and breathe.