Convergence of undulatory swimming kinematics across a diversity of fishes

Fishes exhibit an astounding diversity of locomotor behaviors, from
classic swimming with their body and fins to jumping, flying, walking, and
burrowing.  Fishes that use their body and caudal fin (BCF)
during undulatory swimming have been traditionally divided into modes
based on the length of the propulsive body wave and the ratio of head:tail
oscillation amplitude: anguilliform, sub-carangiform, carangiform and
thunniform. This classification was first proposed based on key
morphological traits, such as body stiffness and elongation, to group
fishes based on their expected swimming mechanics. Here, we present a
comparative study of 44 diverse species quantifying kinematics and
morphology of BCF-swimming fishes. Our results reveal that most species we
studied share similar oscillation amplitude during steady locomotion that
can be modeled using a second-degree order polynomial. The length of the
propulsive body wave was shorter for species classified as anguilliform
and longer for those classified as thunniform, although substantial
variability existed both within and among species. Moreover, there was no
decrease in head:tail amplitude from anguilliform to thunniform mode of
locomotion as we expected from the traditional classification. While the
expected swimming modes correlated with morphological traits, they did not
accurately represent the kinematics of BCF locomotion. These results
indicate that even fish species differing as substantially in morphology
as tuna and eel exhibit statistically similar two-dimensional midline
kinematics and point toward unifying locomotor hydrodynamic mechanisms
that can serve as the basis for understanding aquatic locomotion and
controlling biomimetic aquatic robots.

The data and code available here allow to reproduce the results
presented in the article.