Rocky shore snails and their trematodes: new systems, new parasites, new insights

The number of parasite species being described is still rising sharply, with little sign of slowing down. This illustrates a limitation in our current understanding of host-parasite interactions. The need for improved knowledge of the biodiversity of parasites and of their effects is especially important due to the realised and potential impacts of parasites across a wide variety of domains, including in clinical, environmental and agricultural practices. Concurrently, as data mounts on specific biological systems, from a more macroecological viewpoint there is an intensifying search for common patterns across systems, with the aim of providing clearer insights into the general properties of host-parasite interactions. This thesis encompasses several of these challenges, namely, parasite species descriptions, parasite impacts and the search for general properties affecting host-parasite interactions. The novel study systems investigated were Australasian rocky shore periwinkle or littorinid snails, and their trematode parasites. Previously, parasites of periwinkle snails were well studied in the Northern Hemisphere, though nothing is known about Southern Hemisphere periwinkles. In New Zealand, two snail species were studied and a total of five parasite species found (Chapter Two). In Australia, a single snail species was found to host five parasites also (Chapter Three). An important knowledge gap was filled with the finding of one of these parasites in Australia providing the first record of a snail host for the trematode family Gorgocephalidae. All parasite species were described using an integrative taxonomical approach, which incorporated line drawings, scanning electron microscopy and molecular analyses. Low prevalence was recorded for most parasites in New Zealand, therefore further experimental work focused on the most abundant parasite, Parorchis sp. NZ (family Philophthalmidae). This parasite displayed an unusual biology whereby it can abbreviate its development, possibly as an adaptation to stressful conditions. To investigate this, the incidence of the abbreviated life cycle (precocious encystment) was studied in response to host stress due to desiccation. Although the results confirmed desiccation as a host stress, it appeared to have little effect on the development pattern of the parasite (Chapter Four). The focus then turned to the effect of the parasite on its host and specifically to the impact of infection on microhabitat choice of snails (Chapter Five). The choice of microhabitat on a rocky shore may influence a snail’s success in withstanding dislodgement, due to wave exposure or predation. Interestingly, the results showed that infected snails spend more time on the rock surface than in crevices, possibly leading to enhanced parasite transmission, and inevitably to a higher risk of mortality for infected snails. Finally, to gain a broader understanding of the ecological drivers underpinning interspecific variation in parasite species richness among periwinkles, a comparative analysis was carried out using data compiled from the literature (Chapter Six). I quantified the contribution of both evolutionary history and ecology to observed patterns of parasite species richness, and found that both latitude and host phylogeny accounted for much of the variation in how many trematode species exploit particular periwinkle species