Physiological and Immunological Plasticity of Crassostrea gigas and Ostrea edulis in Response to Ocean Warming, Acidification and Species Interactions: A Mesocosm Study

Supervisor: Chris Hauton (Univ. Southampton)
This study assesses the physiological and immunological effect responses in two species of oyster; the native European oyster, Ostrea edulis and the invasive pacific oyster, Crassostrea gigas, to the ocean acidification, warming and species competition. Global climate change is underway, and has profound implications for many marine ecosystems; oceanic warming causes shifts in species distribution and induces pervasive metabolic effects, and ocean acidification alters seawater carbonate chemistry, reducing ocean pH, which will have unprecedented consequences for calcifying organisms. O. edulis has already been cited as a priority marine species in the UK Biodiversity Action Plan due to recent population decline. For restoration, it is critical to know the limits of their thermal tolerance, sensitivity to hypercapnia, any synergistic effects, and if their population is threatened as a result of C. gigas. C. gigas is the most cultivated oyster species, however, both species make up a substantial proportion of the global aquaculture industry. Oysters are ecologically important in many coastal ecosystems. Many sessile organism rely on oysters for substrate and as a community their reef structures provide important spawning grounds for other marine species. Oysters also play a key role in terms of ecosystem functioning through their filter feeding which provides the ecosystem service of water purification as well as being particularly important with regards to nutrient cycling. Few studies have investigated sub lethal effects of warming and OA in adult molluscs. Identifying the underlying principles at cellular and tissue levels will provide new insight on how higher level biological mechanisms and functions will be affected at the organism-level. Impairments in biological functioning could illicit ecosystem shifts as more resilient species exploit competitive coastal niches. The analysis of their respiration, filtration rate, total haemocyte count, phagocytic activity, glucose and total protein in tissue will provide proxies that can be used to infer the metabolic state, immunological capacities and stress at the organism-level and provide important insight to their overall physiological plasticity. Two temperatures (12ºC and 16ºC) and two CO2 levels (400ppm and 1000ppm) were selected to represent present day conditions, and the IPCC IS92 projections for 2100. A full-factorial experimental design was applied to test all environmental and species combination scenarios (n = 6). The experimental ran for 3 months. Taken together, the results showed carbon dioxide to be the main climate change driver. More specifically, CO2 was found to have a negative effect on total haemocyte counts (P = 0.005). O. edulis demonstrated greater CO2 sensitivity leading to a reduction in phagocytic activity (P = 0.026). The filtration rates for O. edulis in a competitive situation also decreased significantly (P <0.001). Temperature was found to have a minor effect on the protein levels in C. gigas when in competition (P = 0.032). The results here, illustrate clear differences between the two species which may be exacerbated in the future to the detriment of O. edulis. Historical mass mortalities of both species have been associated with parasitic epizooites and viruses, with these results showing decreased immunological capacity, further research is needed to elucidate how pathogen resistant sub populations will respond. There are gaps in the knowledge concerning multiple climate change stressors which urgently needs addressing. The next step in climate change research is life cycle analysis to test if any resiliencies in juveniles manifest into a stronger adult population and vice versa and establish an accurate long-term critical threshold for CO2 on C. gigas and O. edulis.