Functional Morphology and Evolution of the Balistiform Swimming Mode

Fishes have evolved a wide variety of fin and body shapes and inhabit nearly every aquatic habitat on Earth. Understanding associations between fish morphologies and functional traits that facilitate their occupation of these diverse ecosystems is a general goal of many evolutionary ichthyologists. Exploring these relationships in the context of swimming can be especially informative, as most fishes rely on swimming performance for nearly all aspects of their lives. This thesis examines the functional morphology of triggerfishes (family: Balistidae) and filefishes (family: Monacanthidae) in the superfamily Balistoidea in the contexts of endurance swimming performance, ecology, and steady swimming biomechanics. Despite high morphological and ecological diversity, all balistoid fishes power slow steady swimming using their median dorsal and anal fins in a swimming mode termed balistiform locomotion and transition to a gait dominated by body and caudal fin undulations at high speeds. First, I use swimming performance experiments and geometric morphometrics of 13 balistoid species to explore relationships among fin and body shapes, gait transition strategies, and endurance swimming performance (George and Westneat 2019). This research reveals that balistoid fishes use several biomechanical strategies to achieve high endurance swimming performance. Balistiform specialists possess long, large median fins capable of powering high-speed locomotion using the median fins alone. Conversely, body/ caudal fin specialists possess short, small median fins, ill-suited for high-speed balistiform locomotion, but narrow caudal peduncles capable of facilitating high-speed caudal fin-powered locomotion. Species with large, high aspect ratio fins exhibit the highest overall endurance swimming performance. In the second study, I explore patterns of morphological evolution, fin asymmetry, and ecomorphology among 80 balistoid species. Morphologies range from deep-bodied forms with high aspect ratio median fins to elongate forms with low aspect ratio, rectangular median fins. Ancestral state estimations reveal an early morphological divergence between families, with the filefish common ancestor exhibiting low dorsal, anal, and caudal fin aspect ratios and the triggerfish ancestor possessing high aspect ratio median fins. Dorsal and anal fin aspect ratios then underwent widespread convergence events within and between families. High aspect ratio fins are associated with pelagic and planktivorous species that benefit from high endurance swimming performance. Finally, this chapter reveals widespread morphological asymmetries between dorsal and anal fins. Next, I use high-speed video to study three-dimensional dorsal and anal fin kinematics during steady balistiform locomotion of the gilded triggerfish, Xanthichthys auromarginatus. This work reveals differences in nearly every kinematic property between dorsal and anal fins and along their lengths, with the exception of all fin rays oscillating at the same frequency. Leading edge fin rays provide nearly half the total propulsive effort, and the dorsal fin provides greater effort than the anal fin. Given the relatively morphologically symmetrical fins of X. auromarginatus, these biomechanical fin asymmetries likely occur in many balistoid species, challenging the long-standing assumption of symmetrical balistiform median fin biomechanics. In the concluding chapter, I review the implications of these results for the evolution of the balistiform swimming mode and discuss an ongoing balistiform swimming maneuverability field project