Meeting Abstract
Buoyancy control is a fundamental aspect of aquatic life that has major implications for locomotor performance and ecological niche. Unlike terrestrial animals, the densities of aquatic animals are similar to the supporting fluid, thus even small changes in body density may have profound effects on the energetic costs of locomotion. Here we analyzed the evolution of body composition in 32 shark species to study buoyancy control and its effects on locomotor performance. Our comparative phylogenetic analyses indicate that although lean tissue is isometric, liver volume exhibits positive allometry, suggesting that larger sharks evolved bulkier body compositions by adding lipids to lean tissue rather than replacing lean for lipid. Furthermore, we revealed a continuum of buoyancy control strategies that ranged from more buoyant sharks in deeper ecosystems to relatively denser sharks with small livers in epipelagic habitats. Across this eco-morphological spectrum, our hydrodynamic analyses suggest that steady swimming drag and swimming economy is reduced for animals closer to neutral buoyancy and drag against unsteady swimming is reduced for sharks with greater negative buoyancy, resulting in greater burst swimming capacity and agility. This suggests that the selection for locomotor capacity to be relaxed in deeper habitats and/or selection for greater economy of movement to be increased. Moreover, the hydrodynamics of both steady and unsteady swimming appear independent of scale, implying that changes in locomotor behavior with size alter selective forces shaping body composition. These physical trade-offs associated with buoyancy may have played a major role in shaping the evolution of body condition, locomotor performance, and ecological niche in this diverse clade of marine fishes.
