Undulatory Locomotion in Elongate Aquatic Vertebrates: Anguilliform Swimming since Sir James Gray

Anguilliform locomotion is the mode of undulatory swimming used by eels and other elongate vertebrates in which waves (of shorter length than the body of the animal) are propagated posteriorly along the length of the animal, propelling it forward. Although a broad array of vertebrate taxa convergently use this locomotor mode, much of what we know regarding the kinematics of anguilliform swimming derives from the classic work of Sir James Gray in the 1930s on the European eel, Anguilla anguilla. To better understand the variability within this phylogenetically widespread mode of locomotion, I have reviewed recent work on the swimming kinematics of elongate fishes, salamanders and snakes. The amplitude and relative speed of the undulatory waves propagated along the body during swimming differs among elongate vertebrate tax, and can also vary with swimming speed. In addition, the lateral velocity and orientation of tail segments can reach their maxima at different times within a tailbeat cycle (in contrast to Gray's suggestion that these two events occurred simultaneously). Finally, the angle of attack of tail segments is negative during a consistent portion of each tailbeat cycle in the elongate salamander Siren. While this has yet to be examined in other anguilliform taxa, it is possible that they will also show this pattern. Hence, although Gray's descriptions of anguilliform swimming are generally accurate, gaining a broader understanding of this mode of locomotion requires the characterization of the diversity and complexity found among animals using this undulatory mode.