Invasive species and aquaculture pathogens in the Irish and Celtic Seas

Invasive species represent a major threat to biodiversity and ecosystem functioning, however research into the interactions between invasive species and their parasites is lagging far behind research into general invasion biology. This thesis explores the relationship between invasive species, specifically those which impact the aquaculture sector through biofouling or predation on commercial species, and the parasites and pathogens with which they interact. Focus is paid to bivalve aquaculture, since species such as the Pacific cupped oyster Crassostrea gigas and the blue mussel Mytilus edulis are heavily cultured within the study regions. The first research chapter takes the form of a review which synthesises invasive host-parasite interactions using marine bivalves as a model group. The global aquaculture industry is discussed in detail, as often it is this industry that facilitates the spread of both invasive species and disease, but it is also this industry that is adversely impacted by subsequent disease outbreaks. The chapter then provides recommendations to enhance our understanding of marine diseases, and also addresses how climate change might influence invasive host-parasite complexes. The second data chapter investigates the impact of one particular group of invasive species - tunicates that can have a significant impact on aquaculture. The study looks at the impact of these tunicates on the maintenance of select pathogens that affect commercial bivalves, including the ostreid herpesvirus OsHV-1 μVar, the bacterium Vibrio aestuarianus and the haplosporidia Bonamia ostreae and Minchinia spp. PCR, Sanger sequencing and histology confirmed the presence of B. ostreae and Minchinia mercenariae-like in the leathery/club tunicate Styela clava, and V. aestuarianus was confirmed by qPCR in the orange sheath tunicate Botrylloides violaceus and the carpet sea squirt Didemnum vexillum. Furthermore, histology confirmed M. mercenariae-like sporonts in S. clava suggesting that the tunicate can facilitate replication of this species. The results indicate that tunicates can act as reservoirs of infection in areas where disease occurs and potentially transport diseases to uninfected sites. Microbial diversity in nearshore bivalve culture environments is well-documented, but less is known about offshore environments. The third data chapter of this thesis examined 65 plankton samples collected from the Irish and Celtic Seas in May 2018 to investigate zooplankton-associated microbial communities. Bacteriome sequencing was used to characterise the bacterial community structure and identify any potential pathogens, and PCR was also used to further screen for haplosporidian and viral pathogens. The bacterium Vibrio splendidus was detected, as was a haplosporidian species (18_Haplo_BMVA_WEY) first detected off Weymouth, England between 2011-2012. The results also revealed distinct bacterial profiles arising from the Irish Sea, Celtic Front, Eastern Celtic and Southern Celtic Sea areas, suggesting that oceanic currents and fronts may act as barriers or facilitators to microbial dispersal thereby providing pathways for pathogens. The final empirical chapter of this thesis takes the form of a horizon scanning exercise utilising cargo shipping records from 2018-2019 (n = 9,291). The chapter focuses on the connectivity between four major ports in Ireland (Dublin, Cork, Rosslare and New Ross) and global shipping ports. Ballast water and hull fouling are vectors for invasive species movements, and regional management may be strengthened by identifying shipping networks as this will allow for targeted inspections. A targeted horizon scanning exercise for invasive species likely to arrive in Ireland was included, and seventeen incoming ports were highlighted as having high connectivity to Ireland. Furthermore, the focal invasive species are present in these ports, suggesting there is strong potential for invasion. Shipping routes within Ireland also demonstrate high connectivity, meaning the potential for secondary spread is strong. The thesis concludes with a discussion highlighting the main findings and emphasising the importance of integrating the fields of parasitology and invasion ecology to enhance our understanding of pathogen dispersal and transmission