| Summary: | Ocean acidification (OA) is predicted to cause major changes in marine ecosystem structure and function over the next century. Stressors associated with OA can negatively affect the physiological performance of individual marine organisms, disrupting inter- and intra-specific interactions such as competition, predation, and parasitism. Parasitic infection can also have wide-ranging negative effects on marine organisms, and has been found to interact synergistically with some abiotic stressors. This thesis investigated the potential impact of OA on host-parasite associations by exposing the transmission stages of trematode parasites, and infected and uninfected marine gastropods, to acidified seawater. An observational study of the seawater chemistry of the habitat of both snail and parasite species showed a high degree of abiotic variability, including pH values that were more extreme than the conditions predicted to occur over the next century. These data suggest that intertidal organisms may be currently operating at their physiological limits, and could prove sensitive to future changes in seawater pH. An OA simulation system was designed specifically for small-bodied, calcifying marine organisms and free-living parasite life stages. This system provided a highly controlled environment in which to expose parasites and their hosts to acidified seawater. Exposure to acidified seawater caused a significant decrease in the longevity of the free-living transmission stages of trematode parasites, although the magnitude of this effect varied between species and appeared to correlate with their life histories. The physiological performance of infected and uninfected snails was significantly and negatively affected by stressors associated with OA. Shell growth rates were significantly reduced, and shell dissolution significantly increased, in an acidified environment. The metabolic status of the host snails was also significantly altered by exposure to acidified seawater. Changes in oxygen consumption rates and tissue glucose concentrations of host snails indicated an increase in metabolic stress followed by metabolic depression as seawater pH was reduced. In all measured physiological parameters, the effects of parasitic infection significantly modified the response of infected snails relative to uninfected individuals, while there was a high degree of variability between snails infected with different species of parasite. Overall, the association between marine trematodes and their gastropod host was significantly altered by exposure to simulated OA conditions, highlighting the need to incorporate parasitology into the field of OA research.
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