Adrian Gleiss

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.

Temperature affects most physiological processes which in turn impact animal behaviour and ecology. In ectotherms, both short- and long-term variations in temperature impact physiological (i.e. locomotor and metabolic capacity) and ecological (i.e. body condition and growth) performance and thus affect survival. It is therefore critical to understand the mechanisms driving these relationships. Here, we investigated the impact of seasonally changing temperature on the ecological and physiological performance of juvenile free-ranging largetooth sawfish (Pristis pristis) in its riverine nursery in North Western Australia. Animal-attached accelerometers, revealed that despite a 10°C increase in temperature, sawfish were active and displayed substantial burst-swimming capacity. Physiological performance, as ascertained by locomotory capacity, increased in the warmer late dry season conditions, whereas timing and duration of activity did not. Contrary to the physiological performance, late season sawfish were poorer condition that those in the early season. Activity was primarily crepuscular, irrespective of the season. This suggests that even though locomotory performance of juvenile sawfish increased at greater temperature, foraging activity and thus energy intake was not sufficient to maintain body condition, resulting in declining growth. Contrary to popular belief, seeking warmer temperatures can represent a disadvantage for juveniles under certain scenarios. Especially for individuals that are intake limited, greater temperatures and associated metabolic rates are disadvantageous and can result in lower growth rates and potentially starvation. Physiological and ecological performances may thus respond differently to warming temperatures, emphasizing that ‘optimal’ temperatures may be highly context dependent. Ecological scenarios responsible for mediating growth performance are discussed and integrated into a classic bio-energetics framework.