Diversity, ecology and evolution of feather mites in seabirds

Feather mites are among the most abundant and commonly occurring bird ectosymbionts. They live permanently on the host body, are adapted to inhabit well-defined host microhabitats and are found in almost all avian groups. Although they have been extensively studied at the taxonomic level, much diversity remains undescribed and there are many open questions on the biology, ecology and evolution of feather mites. There is also an ongoing debate on the type of symbiosis, that is, whether bird-feather mite interactions are parasitic, commensalistic or even mutualistic. Therefore, better knowledge of feather mite communities at different levels of their biological organization (i.e., among host individuals, host populations and host species) can contribute to our understanding of the evolutionary ecology of feather mite-bird interactions, and of the evolution of parasite biodiversity in general. In this context, the main goal of this thesis was to investigate the evolutionary and ecological factors driving the diversity and community structure of feather mites inhabiting seabirds of the north-eastern Atlantic Ocean and Mediterranean Sea. More specifically, using a multi-host and multi-mite species community approach, I quantify feather mite morphological and genetic diversity within different seabird species and populations; evaluate the relative importance of host versus geographic structure in influencing the evolution and population structure of seabird feather mites; investigate the spatial distribution and trophic structure within a host individual to identify the driving mechanisms and proximate factors shaping symbiont infra-community structure. Based on morphological criteria, the results show that seabird species harbour a diverse and unique mite fauna composed of 38 species belonging to 10 genera and three families. All seabird species hosted at least three feather mite species, while the richest community was composed of nine mite species. Each seabird genus harboured a distinct feather mite community and only three mite species were shared by two related shearwater genera. Seventeen of the 38 mite species found were new, undescribed species; official descriptions of six are included in this thesis. Overall, molecular data correlated well with morphological species descriptions, but also revealed the presence of six putative cryptic species belonging to four mite genera. The patterns of mite genetic structure were variable both among different sympatric hosts and among the same host species in different geographic regions. Thus, the great majority of mite species from sympatric seabirds exhibited strong host-associated patterns of genetic structure. When comparing mite communities among sibling host species from the same genus, some mite species were genetically unstructured among hosts and localities, whereas other mite species showed higher degree of genetic diversity and among population differentiation. These observed differences correspond to microhabitat use on the host body. No genetic differentiation among localities was found for feather mites, suggesting that mite dispersal regularly occurs between host populations. The distribution of two widely abundant mite species that co-occur on the flight feathers of a single seabird species, Calonectris borealis, showed clear spatial segregation among feathers; one species preferred the central primaries, whereas the other was restricted to the outermost primaries. This pattern resulted from a combination of habitat-specific adaptations and ongoing competition. In addition, isotopic analyses of mite diet indicated that the two mite species share the same host food resources, probably preen gland oil complemented with exogenous material. Altogether, this thesis highlights the vast and largely unrecognized diversity of feather mites harboured by seabirds, provides a comprehensive characterization of the patterns of seabird-feather mite species/lineages associations and contributes to a better understanding of the ecological and evolutionary processes that have lead to their high diversity.