The biology and ecology of mesopredatory sharks revealed through a multidisciplinary approach

Intense fishing pressure has resulted in exceptional declines of shark populations around the globe. Sharks facilitate important roles in marine systems as predators, prey, competitors and vectors connecting ecosystems. Despite our growing understanding of certain sharks’ roles in their ecosystems, there is still a considerable gap in our understanding of mesopredatory sharks. We took a multidisciplinary, holistic approach to derive a broad scale understanding of the biology and ecology of a poorly understood group of mesopredators, the sawsharks (Pristiophoridae). We compared traditional (stomach content analysis) and novel (stable isotope analysis, fatty acid analysis, DNA metabarcoding) trophic methodologies using the common sawshark. This work highlights the benefits of multi-methodological approaches to trophic ecology and describes how the complimentary nature of these methods allows for a deeper understanding of an animal’s ecological role. Next, using a suite of traditional and biochemical approaches, we describe the trophic ecology and resource use of the common sawshark and southern sawshark. We identified resource partitioning between species and describe a previously unidentified linkage in pelagic food webs and we discuss potential implications for nutrient transport. Next, we took a neurobiological approach to investigate the visual systems of the southern sawshark through an anatomical investigation of the retina and a transcriptome analysis to better understand opsin expression. We identified a nonuniform distribution of ganglion cells across the retina, relatively low visual acuity, and visual pigments that may support specialisations into benthopelagic foraging. The ecological consequences of which are discussed and related to trophic and movement ecology observed in the southern sawshark. Next, we identified headfirst ingestion foraging tactics of the common sawshark through conebeam CT scans and discuss possible foraging behaviour. Lastly, we provide a literature review on the biotelemetry applications in small-bodied elasmobranchs and directions for future research. This thesis provides a multi-dimensional description of sawshark biology and ecology and illustrates the benefits multi-disciplinary research in describing an animal’s ecological role.