GRUBICH, JR; WESTNEAT, MW; The Field Museum of Natural History; The Field Museum of Natural History: Bichirs, Bowfins, and Gars, Oh My! Functional Morphology of Pectoral Fin Locomotion in Basal Actinopterygian Fishes

Pectoral fin locomotion has been investigated across a phylogenetically broad range of fishes including elasmobranchs, acipenseriformes, and teleosts; yet, there are several actinopterygian groups that remain to be analyzed. This study seeks to fill in the gaps in the evolution of paired fin function by comparing the functional morphology and kinematics of pectoral locomotion in three basal lineages of actinopterygians: Polypterus palmas (Polypteriformes), Amia calva (Amiiformes), and Lepisosteus platostomus (Semionotiformes). Musculoskeletal architecture and fin shape showed considerable morphological variation, with Polypterus exhibiting the simplest muscular organization with a single abductor and adductor muscle mass controlling fin ray movement. In contrast, both Amia and Lepisosteus have multiple muscle subdivisions controlling fin motion. Fin shapes in all three lineages are paddle-like with low aspect ratios where the fins broaden distally indicative of drag-based propulsion. Behaviorally, Polypterus and Amia use their pectoral fins predominantly for synchronous forward locomotion at slow swimming speeds. In contrast, Lepsisosteus oscillate opposing fins out of phase for midwater hovering and reverse locomotion at slow swimming speeds. Kinematic data reveal dramatic differences in fin displacement, stroke plane, and fin beat frequency, which can range from less than 1hz in Lepisosteus to greater than 4 Hz in Polypterus. Results of EMG experiments analyzing motor pattern activity in homologous muscles will also be discussed within the context of evolutionary shifts in neuromuscular control for forward and reverse swimming behaviors in pectoral fin function.