Long-axis twisting during locomotion of elongate fishes.

Fish live in a complex world and must actively adapt their swimming behavior to a range of environments. Most studies of swimming kinematics focus on two-dimensional properties related to the bending wave that passes from head to tail. However, fish also twist their bodies three dimensionally around their longitudinal axis as the bending wave passes down the body. We measured and characterized this movement, which we call 'wobble', in six species of elongate fishes (Anoplarchus insignis, Xiphister mucosus, Lumpenus sagitta, Pholis laeta, Apodichthys flavidus and Ronquilus jordani) from three different habitats (intertidal, nearshore and subtidal) using custom video analysis software. Wobble and bending are synchronized, with a phase shift between the wobble wave and bending wave. We found that species from the same habitats swim in similar ways, even if they are more closely related to species from different habitats. In nearshore species, the tail wobbles the most but, in subtidal and intertidal species, the head wobbles more than or the same as the tail. We also wanted to understand the relationship between wobble and the passive mechanics of the fish bodies. Therefore, we measured torsional stiffness and modulus along the body and found that modulus increases from head to tail in all six species. As wobble does not correlate with the passive properties of the body, it may play a different role in swimming behavior of fishes from different habitats.