Future Oceanic Warming and Acidification Alter Immune Response and Disease Status in a Commercial Shellfish Species, Mytilus edulis L. Clara L. Mackenzie 1 *, Sharon A. Lynch 2 , Sarah C. Culloty 2 , Shelagh K. Malham 3 1 Centre for Marine Biodiversity and Biotechnology, School of Life Sciences, Heriot-Watt University, Edinburgh, United Kingdom, 2 Aquaculture and Fisheries Development Centre, School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland, 3 Centre for Applied Marine Sciences, Bangor University, Menai Bridge, Anglesey, United Kingdom Abstract Increases in atmospheric carbon dioxide are leading to physical changes in marine environments including parallel decreases in ocean pH and increases in seawater temperature. This study examined the impacts of a six month exposure to combined decreased pH and increased temperature on the immune response and disease status in the blue mussel, Mytilus edulis L. Results provide the first confirmation that exposure to future acidification and warming conditions via aquarium- based simulation may have parallel implications for bivalve health. Collectively, the data suggests that temperature more than pH may be the key driver affecting immune response in M. edulis. Data also suggests that both increases in temperature and/or lowered pH conditions may lead to changes in parasite abundance and diversity, pathological conditions, and bacterial incidence in M. edulis. These results have implications for future management of shellfish under a predicted climate change scenario and future sustainability of shellfisheries. Examination of the combined effects of two stressors over an extended exposure period provides key preliminary data and thus, this work represents a unique and vital contribution to current research efforts towards a collective understanding of expected near-future impacts of climate change on marine environments. Citation: Mackenzie CL, Lynch SA, Culloty SC, Malham SK (2014) Future Oceanic Warming and Acidification Alter Immune Response and Disease Status in a Commercial Shellfish Species, Mytilus edulis L.. PLoS ONE 9(6): e99712. doi:10.1371/journal.pone.0099712 Editor: Pikul Jiravanichpaisal, Fish Vet Group, Thailand Received January 30, 2014; Accepted May 16, 2014; Published June 13, 2014 Copyright: ß 2014 Mackenzie et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was funded by the project entitled: Shellfish Productivity in the Irish Sea (SUSFISH). SUSFISH is partially funded by the European Regional Development Fund (ERDF) through the Ireland Wales Territorial Co-operation (INTERREG 4A) Programme 2007-2013. INTERREG 4A is managed in Ireland by the Southern and Eastern Regional Assembly on behalf of the Irish Government, the Welsh Government and the European Commission (http://www.irelandwales.ie). The funders had no role in study design, data collection and analysis, decision to publish,or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: clm32@hw.ac.uk Introduction Increases in atmospheric carbon dioxide are leading to physical changes in marine environments. Under the IPCC IS92a CO 2 emission scenario, ocean pH is expected to decrease by 0.2–0.4 units by the year 2100 [1,2]. In addition to changes in ocean pH, organisms will also have to contend with simultaneous increases in seawater temperature with the current average sea surface temperature of 19.7uC predicted to rise to 22.7uC by the end of the century [2,3]. The ability of marine organisms to adjust to future climate change conditions will be critical to their health and ultimate survival. Bivalves are unable to thermo-regulate and are consid- ered poor regulators of haemolymph acid-base balance [4,5]. Consequently, pH and temperature changes in the environment will likely have direct influence on bivalve haemolymph pH and temperature. Additionally, any physiological mechanism associat- ed with the bivalve circulatory system is liable to be affected under hypercapnic conditions. The bivalve immune system, for example, exists as an integrative part of the circulation system [6,7]. The circulating cells, or haemocytes, represent the major cellular component of the animal’s immune response and are responsible for a number of defence activities including phagocytosis or encapsulation of foreign or diseased cells, release of reactive oxygen metabolites and enzymes, and secretion of cytotoxic molecules [6,8,9,10]. While prior investigations of the consequences of near-future changes in ocean pH and temperature have demonstrated a number of effects on marine organism physiology including impacts on growth, calcification and acid-base status [4,11–18], less confirmed is how coinciding stressors might influence immune response in organisms [19]. Changes to single environmental factors, particularly temperature [20–24] but also salinity [25], pH [26] and dissolved oxygen [27] have demonstrated impacts on various immunological parameters including haemocyte numbers, phagocytosis and oxidative burst response. The immune system is a major physiological mechanism ensuring host survival in the battle with pathogenic or parasitic organisms [28]. As stress impacts immune function and such mechanisms are the primary line of defence against pathogens, there is likely a strong link between immune response and the outbreak of disease in shellfish culture [29,30]. Thus, climate change impacts on immunological aspects of physiology may be paralleled by associated changes in bivalve disease status. Global climate change has increased pathogen development and survival, disease transmission and host susceptibility in the last PLOS ONE | www.plosone.org 1 June 2014 | Volume 9 | Issue 6 | e99712