Influence of elevated temperature on juvenile damselfish performance and their scope for adjustment through developmental plasticity

Understanding how individuals and populations respond to higher temperatures is crucial for making predictions about the vulnerability of communities to global climate change. Studies within a climate-change context commonly expose adult animals to acutely elevated temperatures, but seldom account for the inherent capacity of individual and populations to change on longer time scales. Tropical ectotherms, like those inhabiting coral reefs, are expected to be particularly sensitive to thermal change as they have evolved in relatively stable thermal environments. However, recent evidence suggests that coral reef fishes are physiologically plastic if exposure to thermal change occurs during early life stages. Yet, it is unknown whether this thermal plasticity extends to individual performance in ecological processes and whether the link between physiological and ecological performance is temperature sensitive. This study aimed to determine the effects of projected future temperatures from climate change on the competitive performance, antipredator escape performance, and physiology of juvenile coral reef damselfishes, and to explore the extent of thermal plasticity individuals have for these processes. To determine the ability of species to cope with long-term elevations in average temperature, a baseline of how individuals respond to elevated temperature in the shortterm is required. This measure can be thought of as the thermal sensitivity of currentday populations for rising temperature. To address this aim, newly settling juveniles of Pomacentrus nagasakiensis and P. chrysurus were collected using light traps at Lizard Island, northern Great Barrier Reef. Juveniles were then randomly allocated to one of four temperature treatments for seven days (Chapter 2). Size-matched intraspecific pairs were then placed into competitive arenas where aggressive interactions for a coral shelter were video recorded. There was little effect of temperature on aggressive interactions for P. chrysurus, whereas P. nagasakiensis displayed substantial increases in aggression levels with temperature. Furthermore, temperature changed the dynamics of contests of P. nagasakiensis, leading to fiercer contests. The differences in thermal sensitivity between species suggests reef assemblages may be restructured under warmer conditions. The next aim was to determine the extent of thermal plasticity for competitive interactions through longer-term exposure to rising temperatures. This chapter used juveniles of Pomacentrus amboinensis and P. moluccensis collected from the reefs near the Cairns region, Australia. Individuals were divided into ambient temperature for the collection season (29 °C) and two elevated temperature treatments (30 and 31 °C) for either 4d or 90d exposure periods. Chapter 3 followed methods similar to chapter 2 to record competitive performance, however with the added comparison of performance between the 4d and 90d exposure periods to test for thermal plasticity. Both intra- and inter-specific competitive contests were used to link differences in species thermal performance with interspecific competitive dominance. When comparing the control with 4d exposure to elevated temperatures, there were opposing responses by species, with P. moluccensis significantly increasing aggressive interactions whereas P. amboinensis exhibited a decreasing trend in aggression at higher temperatures. After 90d, P. moluccensis showed signs of beneficial plasticity as aggression trended back to control levels while P. amboinensis showed a further decrease in aggression. Interspecific contests showed a trend for P. moluccensis to win more contests in elevated temperatures. These results indicate that differences in species thermal performance may impact dominance hierarchies in fish assemblages. Chapter 4 used a similar experimental design of elevated temperature and extended exposure to explore the extent of thermal plasticity in antipredator escape performance for P. amboinensis and P. moluccensis. Many fish undertake a C-start escape response when faced with the threat of predation. Recordings from a high-speed camera show temperature effected behavioural decision-making more than physical ability to escape. Higher temperature altered the directionality of escape responses with both species exhibiting more reactions towards the stimulus. Only P. moluccensis showed a beneficial response after 90d, with directionality of response returning to control levels. There was a slight increase in responsiveness to the stimulus at elevated temperatures, suggesting a decrease in the stimulus threshold to trigger a C-start. However, this was reversed after 90d as both species showed significant decreases in responsiveness and even overcompensated as levels fell below control. Finally, I tested the plasticity of physiology through varying exposure durations to elevated temperature and explored the link between physiological and competitive performance (Chapter 5). Previous work has attempted to correlate metabolic rates with aggression and competitive dominance to better understand the ecological relevance of physiological performance; however, there is uncertainty about how these traits relate to one another. Individual oxygen consumption was recorded via closed chamber respirometry as a measure of metabolic rate and then paired with competitive data from chapter 3. Maximum metabolic rate (MO₂ Max), routine metabolic rate (MO₂ Routine), and aerobic scope (MO₂ Max – MO₂ Routine; AS) significantly increased with shortterm exposure to elevated temperature. After 90d, all three metabolic traits for P. moluccensis had virtually returned to control levels, indicating plasticity fully compensated the effects of temperature while P. amboinensis only showed partial plasticity. Metabolic performance was then matched with competitive data from chapter 3 and individuals were categorised as dominant or subordinate. There was no association of individuals with greater metabolic rates becoming dominant necessarily. However, individuals more thermally sensitive with greater changes in metabolism tended to become subordinate in contests. With extended exposure, individuals that displayed greater signs of plasticity by returning to control metabolic rates were more likely to become dominant. This research showed projected elevated temperature will alter competitive ability, antipredator escape responses, and metabolic rates for several species of damselfish. After prolonged exposure to temperature, there were signs of beneficial plasticity of some traits for certain species. The extent of the effects and plasticity observed varied species to species. Such species-specific responses will have implications at the community level, potentially restructuring dominance hierarchies within and between species, and altering species assemblages