Some ocean-dwelling fish, including tuna, mackerel, and sharks, have a form of swimming called thunniform. In thunniform swimming, most of the lateral movement occurs in the tail and adjacent area of the body with very little bending of the fish’s body. The tail or caudal fin is usually large and crescent shaped to increase the power of each sweeping motion. This form of swimming is ideal for species that cover long distances and swim fast because it conserves energy.
“Yellowfin tuna (Thunnus albacares) swimming kinematics was studied in a large water tunnel at controlled swimming velocities (U). Quantified kinematic variables included the tail-beat frequency, stride length (l), caudal amplitude, yaw, the propulsive wavelength, the speed of the propulsive wave (C) and the sweepback angle of the pectoral fins. In general, all variables, except the propulsive wavelength and consequently C, are comparable to values determined for other teleosts. The propulsive wavelength for the tunas (1.23–1.29L, where L is fork length) is 30–60 % longer than in other cruise-adapted teleosts such as salmonids. The resulting thunniform swimming mode and the morphological and anatomical adaptations associated with the long propulsive wavelength (e.g. fusiform body shape, rigid vertebral column) act to minimize anterior resistance and maximize caudal thrust. The long propulsive wavelength also increases the maximum l which, in concert with the elevated muscle temperatures of tunas, increases their maximum swimming velocity.” (Dewar and Graham 1994:45)