Thermal adaptation and acclimation in an intertidal goby (Bathygobius cocosensis)

Abstract

This thesis explores how short- and long-term thermal variability affects ecologically important performance traits and links how developmental traits alter juvenile acute thermal performance. Temperature is the most important abiotic factor that affects the functioning of ectothermic organism physiology and performance. Organisms experience variation in temperature at multiple time scales, from daily and seasonal fluctuations to among generation thermal changes. It is important to understand how thermal variability and developmental traits alter performance traits that are directly linked to survival and fitness. I aimed to assess if an organism can acclimate and maintain wide thermal performance curves in an environment that experiences equal daily and seasonal thermal variability; to assess if rate of luminance change for camouflage can acclimate with temperature; and to assess how larval traits such as growth rate and size affect juvenile acute thermal performance.Daily and seasonal thermal variability plays an important role in the evolution of both the shape of thermal performance curves (TPC) and acclimation capacity. Acclimation occurs when an ectotherm changes their underlying physiology to maintain performance under changed environmental conditions. In theory, ectotherms in environments that experience small daily thermal fluctuations and large seasonal variation are predicted to have narrow TPCs and the capacity to acclimate. When daily thermal fluctuations are as great as seasonal thermal variability, however, ectotherms are expected to have wide TPCs and a limited capacity to acclimate. Few studies have assessed how the combination of daily and seasonal thermal variation affects the shape of TPCs and acclimation capacity in subtropical environments where there is relatively equal daily and seasonal thermal variability. I aimed to assess if an intertidal goby (Bathygobius cocosensis) that experiences equal magnitudes of daily and seasonal thermal variation has the capacity to acclimate. I found that although B. cocosensis experience large amounts of daily variability and have wide TPCs, B. cocosensis possess the ability to acclimate to seasonal conditions.The responses of animals to temperature change have typically been explored in the context of energetics and locomotor performance, and these types of traits are likely to be important for individual fitness. The effect of temperature on physiological processes, however, can have broad reaching implications for other aspects of organismal behaviour and predator avoidance other than locomotion and energy expenditure. For example, intertidal environments are heterogeneous not only in terms of temperature but also substrate and background type/colour. Gobies have the ability to change luminance (perceived brightness) to match their backgrounds as a predator and prey avoidance mechanism. The rate at which animals can change luminance is acutely affected by temperature, and the rate of background matching may therefore be affected by climate change. No previous studies, however, have examined if rate of luminance change has the potential to acclimate to different longer-term thermal conditions. In this thesis, I demonstrate that rate of luminance change can acclimate with thermal change.Many organisms, including marine fish, have complex life-cycles with distinct larval and post-metamorphic phases. Larval traits, such as size and growth rate, have the potential to affect juvenile thermal performance. Bathygobius cocosensis have a planktonic larval stage meaning that larvae are swept out to sea for 15-30 days. Both genetic and environmental sources of variation mean that individual larvae can grow at different rates and reach different settlement sizes for juvenile metamorphosis. No previous studies have assessed how wild larval growth rates and settlement sizes are correlated with post-metamorphic performance across a range of temperatures. While I found no effect of larval trait variation on the thermal sensitivity of post-metamorphic traits, I did find that larval growth rate and settlement size were negatively correlated with routine metabolic rate and burst swimming speed overall across test temperature. Therefore, slow growing larvae had faster post-metamorphic metabolic rates and burst swimming speeds than fast growing larvae, independently of the effect of temperature on those traits. I also found that juvenile body mass was positively correlated with their critical thermal maximum, but no larval traits were correlated with critical thermal tolerance.I have explored how organisms in thermally variable environments respond to short- and long-term thermal change and have linked how developmental traits effect juvenile thermal performance. Interestingly, across studies, I found that thermal performance curve shape was not altered by thermal acclimation, or variation in larval growth rates. These results suggest that although performance is variable among individuals, and individuals can shift their thermal optimums with longer-term thermal change, it appears that the way that performance varies with short-term thermal variation may be constrained in B. cocosensis. These findings are important for improving the understanding of the co-evolution of thermal performance curve shape and acclimation capacity, and how ectotherms will respond to future changes in climate.