Functional morphology of aquatic substrate-based locomotion in walking batfishes (Lophiiformes; Ogcocephalidae)

Most often, paired fin behaviors in fishes are associated with aquatic propulsion, maneuverability, and stability within the water column; however, several species use the paired fins to walk along the bottom. Prior research of benthic fishes reveal different ways in which substrate-based locomotion can be achieved, including movements of either one or both sets of paired fins, along with axial bending. Batfishes, for example, use their paired fins to exert forces on substrates to walk across the ocean floor; their paired fins are morphologically distinct from typical fish fins and have been compared to tetrapod limbs. Furthermore, the tetrapod limb may have evolved first for aquatic locomotion for propulsion off the substrate rather than for terrestrial locomotion. In consideration of these findings, the morphology and locomotion of two batfish species, Halieutichthys aculeatus and Ogcocephalus cubifrons (Lophiiformes; Ogcocephalidae), has been analyzed to better compare benthic fish fins and tetrapod limbs. Results indicate that the morphology of their pectoral and pelvic fins are advantageous for substrate-based locomotion. For instance, the dorsoventral fin position supports the body during walking. And, similar to tetrapod locomotion, appendage rotation is possible owing to the shape of the first radial bone and the attachment points of inserting muscles. During substrate-based locomotion, the paired fins move independently in a diagonal-sequence gait, exhibiting several characteristics of tetrapod locomotion including: appendage retraction, “elbow” extension, and lateral axial movements. This work provides an understanding of the unique and shared traits of fin-based walking and tetrapod locomotion.