| dc.description.abstract | Global temperatures are rising and heat waves are becoming more frequent, reaching higher temperatures, and lasting longer. Many animals are living close to their limits for warming tolerance and mass mortality events are recorded in multiple ecosystems. Aquatic ectothermic animals are predicted to be particularly vulnerable to both gradual warming and heat waves because of their limited opportunity to thermoregulate. Differences in warming tolerance may therefore determine which animals can withstand current and future climate warming.
There are, however, several outstanding questions on animal responses to warming, and answering these can help predict ecosystem impacts. For instance, it is still not known which physiological mechanisms are setting the warming limits and whether the same traits are important for enduring heat waves and gradual warming. It is also not known whether the same mechanisms are setting the thermal limits across life stages, and the warming tolerance of early life stages is understudied because of methodological challenges. If a particular life stage is more vulnerable to warming than others, it could act as a bottleneck and determine the climate change impacts on the whole species.
The overarching aim of this thesis was to understand the mechanisms limiting tolerance to rapid heating events in fish. I used multiple strategies to identify underlying mechanisms for warming tolerance. The first objective was to test correlations between warming tolerance and other traits to pinpoint underlying mechanisms. Second, the function of specific organ systems was assessed during warming to test whether these could set the limits for the whole-animal warming tolerance. Finally, we developed comparable methods to establish the vulnerability of fish from different life stages to warming.
Populations with evolved differences in warming tolerance can be used to identify traits that underpinning these differences. We used the seventh generation of zebrafish (Danio rerio) selected for increased or decreased warming tolerance to test a range of traits that have been suggested to limit warming tolerance. Surprisingly, we found that evolution of higher tolerance to acute warming did not result in a reduced tolerance to acute cooling. This goes against predictions based on warm adaptation of proteins or cellular membranes. Furthermore, we found that the aerobic scope of warming-tolerant fish was not higher than in control fish, demonstrating that warming tolerance can evolve independently of aerobic capacity. This leaves mechanisms that can increase tolerance to both acute warming and cooling (e.g., heat shock proteins) as a likely explanation for the evolved warming tolerance in the selected zebrafish lines. We also compared the acclimation capacity and thermal performance in a range of traits in wild-caught and laboratory zebrafish. These results revealed that wild fish maintained a higher performance in a range of traits at nonoptimal temperatures, but that the warming tolerance was largely conserved over 150 generations of domestication in stable temperatures.
Fish lose controlled locomotion during acute warming and central nervous system failure has been proposed as an underlying cause. To test this, we measured the neural activity during warming in genetically modified zebrafish larvae. A reduction in neural activity before the larvae reached the warming tolerance limit suggests that brain function sets the tolerance limits of these larvae. Furthermore, hyperoxia elevated the warming tolerance, and oxygen is therefore limiting the resistance to warming in zebrafish larvae. This result contrasts with our finding that aerobic capacity was not higher in adult zebrafish that had evolved higher warming tolerance. I therefore conclude that the underlying mechanisms for warming tolerance can differ between life stages.
In the final experiment, we develop a method to assess the warming tolerance of fish embryos from wild three-spined sticklebacks (Gasterosteus aculeatus) and black gobies (Gobius niger) and compared the warming tolerance across life stages. Together with the data on zebrafish larvae, these suggest that early life stages of the tested species are not more vulnerable to warming than later life stages. Importantly, comparable methods for assessing warming tolerance across life stages is essential for determining species vulnerability to warming.
The mechanisms that fail and limit the survival of animals under rapid warming, might be much more variable than often assumed, as suggested by these results across life stages, acclimation temperatures and heating rates. However, different underlying mechanisms do not necessarily affect the resilience to warming. The evolution of increased tolerance to rapid warming revealed that specific mechanisms for tolerating acute stressors might be more important under rapid heat waves than those related to increased performance under chronically elevated temperatures. I, therefore, recommend more focus on finding the major patterns in warming tolerance limits between different species, life stages and habitats. This is essential for predicting the ecosystem impacts of acute warming events. | en_US |
