In addition to forward undulatory swimming, eels (and some other elongated swimmers) can swim backwards in a similar way. We compared the kinematics (wave speed, cycle frequency, amplitude, local bending and estimated muscle strain) of forward and backward swimming in the European eel Anguilla anguilla. Both swimming modes are characterised by a wave of undulation that travels over the body in the direction opposite to that of swimming. We observe two major kinematic differences. First, the slope of wave frequency against swimming speed is significantly higher for backward than for forward swimming. Second, the amplitude profile along the body of the propulsive wave differs greatly. During forward swimming, the yaw at the head is minimal and the amplitude of the propulsive wave increases to approximately 15 % (left-to-right) of total body length towards the tail tip. During backward swimming, the amplitude profile is rather uniform along the body (with values similar to the tail-tip amplitude during forward swimming), resulting in considerable lateral head oscillation. Strikingly, the head remains approximately parallel to the swimming direction, which presumably enhances visual and acoustico-lateral perception. Furthermore, muscle strain is much higher in the rostral part of the body during backward swimming than during forward swimming. Values for stride length and propeller efficiency suggest that backward undulatory swimming is mechanically less efficient than forward swimming. We suggest that the typical anguilliform body shape is an important feature that allows these animals to swim backwards using an undulatory mechanism that resembles the forward undulatory swimming mechanism. Most other fishes, if able to swim backwards at all, do so using fin oscillations or undulations.

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