Benthic walking, bounding, and maneuvering in flatfishes (Pleuronectiformes: Pleuronectidae): New vertebrate gaits

•Flatfishes converge dorsal and anal fin rays to form functional “fin-feet”.•Dorsal and anal fin rays move in a metachronal wave to produce a walking gait.•Flatfish benthic locomotion is structurally and functionally unique in vertebrates. Video-based observations of voluntary movements reveal that six species of pleuronectid flatfishes use sequential portions of long-based dorsal and anal fins as “feet” (hereafter, fin-feet) to move on the substrate. All six species used a gait that we term “walking,” which produced constant forward movement, and several of these species also used a second gait that we call “bounding” for intermittent movements over the substrate. We selected Pacific Sand Sole, Psettichthys melanostictus, and English Sole, Parophrys vetulus, for kinematic analyses of these two gaits. Psettichthys melanostictus consistently used walking for benthic locomotion; Parophrys vetulus primarily used a bounding gait. During forward walking, a fin ray swings up off the substrate, protracts and converges with neighboring fin rays to contribute to a fin-foot. The fin-foot pushes down on the substrate and rotates posteriorly by sequential recruitment of fin rays, a pattern known as a metachronal wave. As one fin-foot passes off the posterior end of the fin, a new fin-foot forms anteriorly. During bounding, undulations of the body and tail assist one or two waves of fin-feet, producing rapid but intermittent forward acceleration of the body. Flatfishes also use fin-feet to maneuver on the substrate. The Starry Flounder, Platichthys stellatus, performs near zero displacement rotation by running waves of fin-feet in opposing directions along the dorsal and anal fins. Although other teleosts use specialized pectoral fin rays for bottom walking (e.g., Sea Robins: Triglidae), the duplication of structures and patterns of movement in the median fins of flatfishes more closely resembles metachronal motions of millipede feet or the parapodia of polychaete worms. Sequential use of median fin rays in flatfishes resembles that of other teleosts that swim with elongate median fins, including Amiiformes, Gymnotiformes, and some Tetraodontiformes, but flatfishes offer a novel form of substrate locomotion based on dorsal and anal fins.