Abstract
Parasite community studies are an important aspect of community ecology, having contributed much knowledge about patterns and processes of species assembly in nature. However, our understanding of which macro- ecological and evolutionary processes have shaped parasite communities comes from exploring only few host-parasite systems. A thorough understanding of these concepts requires a much more comprehensive investigation of a broader range of host-parasite systems from various ecosystems. The present study focuses on New Zealand grenadiers and their endoparasites on the Chatham Rise, a potentially ideal model system for deep-sea parasitological studies not previously investigated in New Zealand.
The overarching aim of this thesis was to test the effects of host phylogeny and geography on the structure of endoparasite communities in grenadiers, using a distance decay framework. I hypothesised that host phylogeny determined the presence/absence of parasites, whereas both host phylogeny and geographical distances among sampled populations affected the prevalence and mean abundance of parasites. My key predictions were that there would be (i) a decay of similarity in parasite presence/absence as the pairwise phylogenetic distance (the number of base-pair differences of DNA sequences) between grenadier species increased, and (ii) a decay of similarity in community composition based on parasite prevalence and mean abundance as both pairwise phylogenetic and geographic distances increased.
The grenadiers and their parasites involved in this study were part of the collections made during a National Institute of Water and Atmospheric Research (NIWA) trawl survey along the Chatham Rise in January–February 2020. All parasites from sampled grenadiers were identified to the lowest taxonomic level possible with an integrative taxonomic approach. The 28S rRNA genetic marker was used for digenean and cestode identification, and the 18S rRNA marker for nematode identification. To investigate parasite assemblages at both infra-community and component community level, community descriptors were used to compute Jaccard and Bray-Curtis similarity indices.
The study yielded several important findings. First, grenadiers from the Chatham Rise harbored a surprisingly high diversity of digeneans, cestodes and nematodes, with different species of grenadiers having different parasite assemblages. Second, two possible new digenean species in the genus Lepidapedon were also discovered, one of them being a cryptic species. In line with the main aim, I found that similarity in presence/absence of parasites was only affected by the phylogenetic relatedness among grenadier species, whereas both phylogenetic distance among grenadiers and geographic distance between sample locations influenced the similarity of parasite communities based on prevalence and mean abundance. Most importantly, this study highlighted the significant effect of deep-sea host phylogeny in shaping their parasite assemblages, a factor previously neglected in studies of parasite communities in deep-sea systems.
Further, I investigated the possible macroevolutionary processes that have shaped the modern associations between Coelorinchus grenadiers and their Lepidapedon digeneans. The hypothesis tested here is that these host-parasite associations are mainly explained by cospeciation, with additional incomplete host-switching events. Species delimitation for the two cryptic Lepidapedon digeneans were assessed with 28S rRNA and cox1 (cytochrome oxidase subunit I) genetic markers. The phylogenetic relationships among the host and digenean species were established based on the cox1 marker, and a cophylogenetic analysis was subsequently conducted with the PACo software.
The analysis showed greater congruence between host and parasite phylogenies than expected by chance. The macroevolutionary interpretation suggested cospeciation played a major role in shaping modern associations of this host-parasite system, with incomplete host-switching events occurring afterwards. This study highlighted that evolutionary divergence in host ecology and historical dispersal events might have played a critical role in the diversification and distribution pattern of extant parasites in a localized deep-sea system. Overall, the results of this thesis have important implications for the study of both deep-sea parasitology and fish-digenean cophylogeny.