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 angui
lla,
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 sw
imming speed is significantly higher for backward than for forward swimming
, Second, the amplitude profile along the body of the propulsive wave diffe
rs greatly. During forward swimming, the yaw at the head is minimal and the
amplitude of the propulsive wave increases to approximately 15% (left-to-r
ight) 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 considera
ble lateral head oscillation. Strikingly, the head remains approximately pa
rallel to the swimming direction, which presumably enhances visual and acou
stico-lateral perception. Furthermore, muscle strain is much higher in the
rostral part of the body during backward swimming than during forward swimm
ing. Values for stride length and propeller efficiency suggest that backwar
d 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 undulat
ions.