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Physiological responses of developing Port Jackson sharks to predation and elevated temperatures
thesisposted on 2022-03-28, 13:01 authored by Connor Robert Gervai
Elasmobranchs are especially vulnerable to perturbation given their life history characteristic including slow growth rates and relatively long gest ation time. Species which develop in eggs are thought to be at a heightened risk to predation and the most vulnerable to changes in environmental conditions, such as increased temperatures, given their lack of capacity to move and evade these situations. R elative to other taxa, elasmobranchs are relatively under studied and lack much of the baseline information regarding their embryonic development and responses to these pervasive conditions. I n this thesis, I investigate d the physiological and behavioural implications of predation and elevated temperature over embryonic and juvenile life stages of the Port Jackson shark, Heterodontus portusjacksoni. Specifically, the first two data chapters explore d the respiration strategies of embryos in response to chemi cal cues from a known predator during development, and how conspecific necromones influence these traits in juvenile sharks. Secondly, the final two data chapters investigated how rearing temperature impacts the physiology and development of embryos collec ted from two distinct populations, and the knock - on effects this may have on the behaviour and physiology after hatching. Intermittent flow - respirometry techniques were utilized to measure the physiological changes in oxygen uptake rates with exposure to d ifferent stimuli (predator cues or temperature). This methodology can be useful at determining changes in an individual's energetic need (proxy for metabolic rates), as well as determining strategies linked to changes in respiratory function. Furthermore, to determine upper thermal limits and activity patterns across temperatures, standard critical thermal methodologies and a step - wise thermal stress methodology were utilized. Predation can shape marine communities, influencing behaviour and physiology of prey. Even without the physical presence of a predator, predat or - associated cues can elicit anti - predator strategies. While most species simply show enhanced or pre - emptive escape responses to evade predatory situations, young life stages may have limited options. Specifically, during embryonic development species are restricted within their egg capsule and may be limited in their sensory capabilities. Therefore, different strategies may be utilized during these early life - history stages to allow them to ev ade predator detection. In Chapter I, I investigated the capacity of developing embryos to detect and respond to predatory odours. Embryos were exposed to predatory cues (a horn shark) and non - predator cues (a teleost fish) while in a respirometry chambers to measure changes in oxygen uptake rates. Embryos displayed variable responses both dependent on odour and on developmental stage. During development their response was strongest to non - predatory cues, in which embryos depressed oxygen uptake rates, simi lar to that of 'crypsis' responses. However, this response diminished over development which may indicate that embryos have the capacity to assess predation risk and thus regulate their response based on prior experience. At hatching, the embryo s response s w here highes 10 towards predatory cues in which they increased oxygen uptake which may be similar to that of 'fight or flight' responses in which sharks were pre - emptively trying to relocate away from predation cues. Unlike embryonic individuals, hatchling s are able to move and utilize different anti - predator strategies to that of embryos. No longer are they restrained to one place, but they can use energetically costly escape responses to evade capture. The scent of a dead conspecific (necromones) is often enough to elicit anti - predatory strategies in other species such as teleost fish and amphibians; however, it is un known if these cues would elicit any response from sharks. Given that juvenile Port Jackson sharks are found within loose aggregation in the wild, in Chapter II I explored the anti - predatory strategies of juvenile sharks and their response to conspecific necromones. Juveniles were exposed to necromones, chased (to simulate a predator attack) or both. Sharks depressed their oxygen uptake rates b y 18% in response to necromones, indicative of 'crypsis' response. Unsurprisingly, when chased, regardless of if exposed to necromones, oxygen uptake rates increased to meet the energetic demand of exercise, and any other associated processes ...