Anguilliform

Anguilliform locomotion is a mode of axial undulatory propulsion used by elongated fishes in which most or all of the body participates in generating motion. The term derives from Anguilla, a genus of true eels, with the suffix -iform meaning “in the form of.” In anguilliform swimming, a wave of bending travels along the body from head to tail, pushing against the surrounding water to produce forward thrust.

The movement is produced by alternating contractions of the axial muscles on opposite sides of the body, creating a traveling wave that lifts and twists the body as it sweeps backward. The dorsal and anal fins often extend along much of the body and can aid in stability, while the caudal fin may be reduced or integrated into the body margin. The wavelength of the wave is long relative to body length, and the curvature amplitude is relatively high, especially around the midsection.

Organs and morphology influence performance: this mode favors flexibility and enables rapid navigation through cluttered or complex habitats, such as crevices and vegetation. It tends to be efficient at slow to moderate speeds but less suited to very fast swimming due to moving a large portion of the body.

Examples and scope: true eels (order Anguilliformes) and lampreys exhibit anguilliform swimming, including various larval stages of fishes before fin maturation. In comparative locomotion, anguilliform movement contrasts with carangiform and thunniform modes, which rely more on posterior body regions and tail thrust for propulsion.