Tardigrades exhibit robust interlimb coordination across walking speeds and terrains

Significance As perhaps the smallest legged animal and one of the only known soft-bodied walkers, tardigrades possess a uniquely versatile set of locomotor tools. Tardigrades have evolved to move through a vast array of environments—freshwater, marine, and terrestrial—using a strongly conserved body plan. We characterize tardigrade interleg coordination patterns using high-speed video of animals walking on gel substrates. Tardigrades utilize a tetrapod-like stepping pattern remarkably similar to that observed in insects, despite significant disparities in size and skeletal structure between the two groups. We find that tardigrades adapt their locomotion to a “galloping” coordination pattern when walking on softer substrates. This strategy has also been observed in arthropods to move efficiently on flowing or granular substrates. Tardigrades must negotiate heterogeneous, fluctuating environments and accordingly utilize locomotive strategies capable of dealing with variable terrain. We analyze the kinematics and interleg coordination of freely walking tardigrades (species: Hypsibius exemplaris). We find that tardigrade walking replicates several key features of walking in insects despite disparities in size, skeleton, and habitat. To test the effect of environmental changes on tardigrade locomotor control circuits we measure kinematics and interleg coordination during walking on two substrates of different stiffnesses. We find that the phase offset between contralateral leg pairs is flexible, while ipsilateral coordination is preserved across environmental conditions. This mirrors similar results in insects and crustaceans. We propose that these functional similarities in walking coordination between tardigrades and arthropods is either due to a generalized locomotor control circuit common to panarthropods or to independent convergence onto an optimal strategy for robust multilegged control in small animals with simple circuitry. Our results highlight the value of tardigrades as a comparative system toward understanding the mechanisms—neural and/or mechanical—underlying coordination in panarthropod locomotion.