Comparative performance of Crassostrea species in Portugal and Brazil : climate change implications

Oysters are a diverse group of marine bivalves that present major ecological and socio-economic importance in coastal ecosystems worldwide. However, oyster species are becoming increasingly threatened by climate change related stressors (e.g. seawater acidification, salinity shifts and temperature rise) as well as anthropogenic pollution (e.g. arsenic). Therefore, it is important to increase the baseline knowledge on the stress response capacity of different oyster species towards environmental stress, in order to help predict and manage the fate of these important biological resources in the context of global change. Hence, the aim of the present thesis was to investigate the comparative performance of different oyster species under several climate change related scenarios, and to project on the future of these species performance in light of global change. To achieve this goal, a series of laboratory based experiments were performed to simulate scenarios of seawater acidification, salinity shifts and temperature rise, as well as the combined exposure to arsenic (As), on different oyster species and life stages (embryo-larvae, juveniles and adults), followed by the assessment of embryo-larvae development, biochemical performance, and proteomic analysis. Because different regions of the globe will face different climate change insult, this study focused on the comparison of the performance of oysters species from populations currently present in temperate (C. angulata, Portugal) and subtropical (C. brasiliana, Brazil) ecoregions, with that of a worldwide distributed species (C. gigas), to understand how climate driven stressors may define species competitive advantages in a changing environment. Comparative performance of C. angulata and C. gigas, revealed that each stressor (seawater acidification, salinity and As exposures) induced similar stress response mechanisms in each species, although with differentiated capacities. Differences between species were more evident at the embryo-larvae stage, for which C. angulata was clearly more sensitive to changes in ambient salinity, temperature and As than C. gigas. At the juvenile life stage, differences in each species stress response signatures were revealed considering both oxidative stress response capacity and alterations at the proteome level. At this life stage, proteomic data suggested higher phenotypic plasticity of C. angulata in response to the investigated scenarios, thus likely attributing higher response capacity to seawater acidification in this species. In contrast to juveniles, adult C. angulata and C. gigas were less responsive to seawater acidification and As exposure, despite that data indicated that biomineralization capacity was impaired. As a corollary, data on juvenile and adults suggested that both C. gigas and C. angulata presented capacity to tolerate seawater acidification scenarios, despite that the stress response mechanisms involved present associated energetic and biomineralization costs that may have repercutions at the population levels. On the other hand, salinity showed to markedly influence oysters oxidative, metabolic and energetic status, with C. angulata likely presenting higher capacity to sustain osmotic stress. Hence, early development stages might represent the bottleneck that may define species competitive advantages towards one another. Comparative performance of C. brasiliana and C. gigas (Brazil), revealed that each stressor (seawater acidification and temperature) induced different stress response strategies in each species. Differences between species were more evident at the juvenile stage, for which C. gigas presented higher capacity to sustain acidification than C. brasiliana. On the contrary, C. brasiliana presented better physiological fitness and oxidative status under thermal stress than C. gigas. Adult oysters were less susceptible to tested scenarios in both species, despite presenting overall similar response patterns than juveniles. The present data suggest that C. brasiliana is more tolerant to thermal stress than C. gigas, but in turn may be more sensitive to seawater acidification. Generally, both species presented stress response mechanisms that enabled for survival towards the tested stressors, however data obtained suggests that the time of duration and the intensity of climate change related phenomena such as seawater acidification and thermal stress, will have a crucial influence on both species performance.