| Summary: | As anthropogenic CO2 levels continue to rise, the oceans are becoming warmer and more acidic. Organisms need to adjust to such environmental changes and display a variety of mechanisms to maintain their fitness in novel conditions. These adjustments can operate at various levels of biological organisation: from cellular levels to organismal physiology and behaviour. Such adaptive responses of species will determine their persistence under future ocean warming and acidification conditions. If organisms are capable of maintaining fitness after long-term exposure to a stressor this can be indicative of acclimation potential. However, their sensitivity to stressors is linked to life stage. Early life phases are considered to be the most vulnerable to fluctuations in the environment. If detrimental effects occur during an organism’s early life this could modify its capability to handle stress at later life stages. The physiological and behavioural adjustments that are triggered in response to changing conditions can lead to modifications in the phenotypic distributions of traits within a population. Analysing the variation of phenotypical traits offers an insight into the capacity of populations to persist by acclimating to their environment. In this thesis I evaluated the sensitivity of marine organisms to ocean warming and acidification and their various coping mechanisms. I reveal that ocean acidification and warming can alter the behaviour of fish species by increasing their anxiety (chapter 2), boldness (chapter 3 and 5), or feeding rates (chapter 2). Modifications in feeding behaviour were linked to physiological and to changing environmental conditions, creating a feedback mechanism between their cellular and behavioural responses that helped organisms maintain their fitness (chapter 3). However, altered behaviours in a population are not always accompanied by physiological changes, as in chapter 5 I also found changes in risk taking behaviours that did not alter the body condition of temperate or tropical ...
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