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The stable oxygen (δ18Oshell) and carbon (δ13Cshell) isotope ratios retrieved from the carbonate shell of terrestrial gastropods can be used as an environmental proxy and are thought to reflect dietary composition and ambient climatic conditions (e.g. precipitation amount, humidity, temperature). Here, we generate high-resolution isotopic profiles of nine modern land snails of the genus Placostylus, collected from two locations in New Caledonia and one location in New Zealand. We found that snails from New Zealand had, on average, higher δ18Oshell values than their counterparts in New Caledonia, which surprisingly runs counter to the expected relationship based on the isotopic composition of rainwater between these two regions. Specimens from New Caledonia exhibit ephemeral decreases in their δ18Oshell values, which could be linked to extreme precipitation events in this region, while snails from New Zealand have less variation in their δ18Oshell values. Snails from New Zealand had, on average, slightly higher δ13Cshell than their counterparts in New Caledonia, but a large difference in carbon isotopes was sometimes observed between snails collected at the same location. Most snails exhibit a temporal trend in their δ13Cshell values, indicating potential shifts in diet through to maturity.

Human activities during the Holocene have induced a sixth biodiversity crisis and initiated rapid changes in the climate. The anthropogenic pressures put on ecosystems can result in direct or indirect environmental degradation, fragmentation and defaunation. Understanding local patterns of wildlife population structure, species interactions and initial biodiversity are all crucial to making well-informed decisions that leads to population sustainability and conservation of global biodiversity. This thesis is focused on the genus of giant land snail Placostylus and seeks to improve our overall knowledge of the genus and its potential to store information about the local environment (such as temperature and humidity) during shell formation. Placostylus is a genus endemic to the southwest Pacific and the many species present a valuable opportunity to integrate studies of ecology and environment at a scale relevant to current anthropogenic climate change. The characteristics of Placostylus shells can be used to investigate extant and extinct morphological variation within the genus, and their chemical composition can be used to track the environmental conditions in which the snails lived. In parallel to shell analysis the generation of genetic data can be used to infer phylogenetic relationships between distant taxa, and at a fine-scale patterns of population structure allow us to infer gene flow and differentiation. Understanding the extent to which shell shape and size is controlled by genetic differences and how much phenotypic plasticity leads to differences is essential if we are to correctly interpret the significant of phenotypic variation. For example, arid conditions can lead to Placostylus snails maturing when much smaller in size. Potentially, intraspecific shell shape and size variation and shell chemistry can all inform us about the local environmental conditions that existed as snail shells were formed. Three main axes are developed throughout the thesis. First the diversity of Placostylus and extended species of the super-family Orthalicoidea are introduced using a phylogenetic investigation. Evolutionary relationships are inferred from DNA sequences of mitochondrial and nuclear genetic datasets. Second, morphological variation is examined in detail where two Placostylus snail species are sympatric (the Isle of Pines, New Caledonia). The variation in shell shape of taxa living and growing in the same environment must represent genetic differences rather than phenotypic plasticity. However, genetic data from the Placostylus species present on the Isle of Pines was needed when a third snail morphotype was discovered. On the Isle of Pines giant land snails of the species P. fibratus are harvested for food, where iii they are sympatric with the vulnerable species P. porphyrostomus. Understanding local population structure of both species and their interaction will inform management decisions for both species. Third, the stable isotopic composition of extant Placostylus shells is analysed from Placostylus shells from New Zealand and New Caledonia. This works has the aim to establish a climate proxy system which through the analysis of fossil shells could inform us about past environmental conditions. A protocol to sample high-resolution isotopic signatures from Placostylus shells is developed and the stable isotopic composition of shells are examined in light of the environmental variables of the snail collection locations.

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