Functional Morphology of Endurance Swimming Performance and Gait Transition Strategies in Balistoid Fishes

Triggerfishes and filefishes (Balistoidea) use balistiform locomotion to power slow steady swimming with their dorsal and anal fins and transition to a gait dominated by body and caudal fin (BCF) kinematics at high speeds. Fin and body shapes are predicted to be strong determinants of swimming performance and the biomechanics of gait transitions. The goal of this study was to combine morphometrics and critical swimming tests to explore relationships between balistoid fin and body shapes and swimming performance in a phylogenetic context in order to understand the evolution and diversification of the balistiform swimming mode. Among the 13 species of balistoid fishes examined, fishes with high aspect ratio fins tended to achieve higher critical swimming speeds than fishes with low aspect ratio fins. Species with long, large median fins and wide caudal peduncles tended to use the balistiform gait alone for a larger percentage of their total critical swimming speed than fishes with short, small median fins and narrow caudal peduncles. Fishes on both ends of the aspect ratio spectrum achieved higher swimming speeds using the balistiform gait alone than fishes with median fins of intermediate aspect ratios. Each species is specialized for taking advantage of one gait, with balistiform specialists possessing long, large median fins capable of the large power requirements of swimming at high speeds using the median fins alone, while BCF specialists possess short, small median fins, ill-suited for powering high-speed balistiform locomotion, but narrow caudal peduncles capable of efficient caudal fin oscillations to power high-speed locomotion.\n\nSummary StatementGeometric morphometrics reveal that fin and body shapes are good predictors of endurance swimming performance and gait transition strategies of triggerfishes and filefishes.