Towards the yin and yang of fish locomotion: linking energetics, ecology and mechanics through field and lab approaches.

Most of our understanding of fish locomotion has focused on elementary behaviors such as steady swimming and escape responses in simple environments. As the field matures, increasing attention is being paid to transient and unsteady behaviors that characterize more complex interactions with the environment. This Commentary advocates for an ecologically relevant approach to lab studies. Specific examples have brought new understanding to the energetic consequences of fish swimming, such as (1) station holding around bluff bodies, which departs drastically from steady swimming in almost all aspects of kinematics, muscle activity and energetics, and (2) transient behaviors such as acceleration and feeding, which are critical to survival but often neglected because of challenges in measuring costs. Beyond the lab, a far richer diversity of behaviors is available when fish are given enough space and time to move. Mesocosm studies are poised to reveal new insights into fish swimming that are inaccessible in laboratory settings. Next-generation biologgers that incorporate neural recordings will usher in a new era for understanding biomechanics in the wild and open the door for a more mechanistic understanding of how changing environments affect animal movement. These advances promise to allow insights into animal locomotion in ways that will mutually complement and accelerate laboratory and field studies in the years to come.