| Summary: | Invasive species are one of the foremost damaging environmental problems for biodiversity and conservation, and can affect human health and man-made structures. They pose a great challenge for pest management, with little known about their control and few available success stories. Many crustacean species are invasive and can affect both biodiversity and aquaculture. Controlling invasive Crustacea is a complex and arduous process, but success could lead to increased environmental protection and conservation. Invasive Crustacea also comprise a significant pathway for the introduction of invasive pathogens. If these invaders carry pathogens, parasites or commensals to a new site they may threaten native species. Alternatively, pathogens can control their invasive host and could be utilised in a targeted biological control effort as a biocontrol agent. Looking specifically at one species of invasive brachyuran crab (Carcinus maenas) collected from the UK, Faroes Islands and Atlantic Canada, and several species of invasive amphipod from the UK and Poland, I explore which groups of microorganisms are carried alongside invasions, and if any could be used as biocontrol agents or whether they pose a threat to native wildlife. This thesis involves wide-scale screening of Carcinus maenas and several amphipod species, identifying a range of metazoans, fungi, protozoa, bacteria and viruses; many new to science. Taxonomic descriptions are provided for previously unknown taxa: Parahepatospora carcini; Cucumispora ornata; Cucumispora roeselii; and Aquarickettsiella crustaci. The application of metagenomics to pathogen invasion ecology is also explored, determining that it can be used as an early screening system to detect rare and/or asymptomatic microbial associations. Finally, I used experimental systems to assess the impact of pathogens carried by Dikerogammarus haemobaphes upon both itself and alternate host species (Dikerogammarus villosus and Gammarus pulex), identifying that C. ornata can infect native species and decrease their chance of survival. Overall this thesis describes a research process following through three main steps: i) invasive pathogen detection, ii) taxonomic identification, and iii) host range and pathological risk assessment and impact. Screening invasive and non-native hosts for pathogens is recommended for invasive species entering the UK, to provide a fast and informed risk assessment process for hazardous hitchhiking microbes.
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