Cockle health, disease connectivity and trophic interaction dynamics

Albuixech-Martí, Sara

Cockle health, disease connectivity and trophic interaction dynamics

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Date

2021-11-30

Authors

Albuixech-Martí, Sara

Publisher

University College Cork

Abstract

Parasites and pathogens are an essential part of the community structure, and their transmission and development are affected not only by their host’s immune system and host population density, but also by multiple biological and environmental drivers that impact on the host-parasite dynamics. There are many gaps in the understanding of these processes and how they influence one another. Given their ecological and economic importance and their exposure to fluctuating and stressful coastal environments, common cockles Cerastoderma edule are ideal model organisms to investigate these dynamics within a natural framework. Therefore, this thesis is intended to present C. edule as a model pathosystem to study marine epizootics and their increased risk of occurrence due to the current changing environment. This thesis develops the understanding of the biological and environmental factors that influence the distribution and dynamics of the host-parasite interplay. In this study, single and multiple parasite and pathogen infections plus over a range of study areas were utilised to better understand these dynamics. The initial part of the study describes a previously undocumented geographic range on the northeast and south coasts of Ireland of two haplosporidian species, Minchinia tapetis and Minchinia mercenariae-like, that have been recently characterised in C. edule. In order to understand the Minchinia:cockle disease dynamics, species-specific primers were developed and these parasites were detected by PCR and confirmed by sequencing. The histology, in turn, confirmed the presence of spore-like stages in the connective tissues of C. edule. A range of factors including host condition and environmental drivers affected both spatial and temporal patterns of these haplosporidian infections in C. edule. Based on statistical modelling, high dissolved oxygen in seawater and small cockle size were the main drivers of the haplosporidian prevalence in the study. The detailed examination of a wider microbial community within C. edule by PCR and histology revealed that single infections with Haplosporidia or Vibrio were more common than coinfected individuals, as expected by random chance. During this second study, no microsporidian species nor ostreid herpesvirus-1 microVar (OsHV-1 μVar) or variants were detected. Based on statistical modelling, coinfected individuals with Haplosporidia and Vibrio occurred because the same risk factors (increased seawater temperature, reduced salinity, and poor host condition) promoted their presence and increased the probability of infection by both pathogen groups. Coinfection with both Minchinia species was overrepresented in our samples and a positive association between M. tapetis and M. mercenariae-like was statistically confirmed. Findings emphasise the need for a more holistic approach in pathological studies, considering the diverse pathogen community within the cockle and their relationship with the environment. The transmission of the microbial community within C. edule to shorebird populations via consumption of cockles and the existing connectivity with the environment (sediment) were also assessed by molecular techniques (PCR and Sanger sequencing) during a third study. Identical strains of Vibrio splendidus were identified in C. edule and, for the first time, in shorebird faecal samples. The consumption of C. edule, thus, may be a source of this Vibrio species in shorebirds. Our findings add evidence to the significant role migratory birds may have as carriers of infectious agents, enabling their transport and dispersal. The findings also support the role of the sediment as an environmental reservoir for Vibrio, which may have an impact on the infaunal community within the sediment responsible for different ecosystem services. Ultimately, in a study that spanned the east and west coast of Ireland and the Welsh coast, patterns of occurrence of Minchinia and Vibrio spp. in C. edule were associated with environmental stressors (high concentration of nitrates and warm temperatures), which may have affected host susceptibility and increased pathogen prevalence. These findings highlight the impact of site-specific environmental stress and habitat degradation on pathogen emergence, particularly in coastal marine environments. In conclusion, this thesis develops a greater understanding of C. edule as a pathosystem and its potential for the early detection of emergent infectious agents, which will have important implications for the sustainability of wild and cultured bivalve populations and the functioning of coastal ecosystems in a changing environmental context.

Keywords

Bivalves , Cerastoderma edule , Pathosystems , Emerging pathogens , Host-pathogen ecology , Coinfections , Pathogen transmission , Environmental stressors

Citation

Albuixech-Martí, S. 2021. Cockle health, disease connectivity and trophic interaction dynamics. PhD Thesis, University College Cork.