A skin appendage covered with scales on either side of the tail of sharks, mackerels, jacks and swordfish. They help reduce turbulence when swimming, increase the stiffness of the tail and act as a horizontal rudder.

Fast swimming open water sharks, such as mackerel sharks, have crescent-shaped tails with the upper and lower lobes of the tail almost equal in size. The high aspect ratio of the tail is used to increase swimming power and efficiency. These species also usually have lateral keels on the tail shaft. The whale shark and giant shark also have this type of tail, although they are generally more sedentary than other examples.