| Summary: | 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 ...
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