Spatiotemporal analysis of parasitic nematodes preserved in land snail shells.

Understanding how parasite abundance changes over time is inherently challenging, as trematodes, nematodes and cestodes degrade quickly and are poorly preserved. Here, I propose a novel approach for investigating the spatiotemporal dynamics of parasitic nematode infection by exploiting a key component of the snail immune system - the ability to trap and encapsulate these parasites within their shell. By utilising museum and modern collections, I was able to understand how nematode populations fluctuated in two species of snails (Cepaea nemoralis and C. hortensis) from three locations (Point of Ayr in Wales, Formby in England and Strathy beach in Scotland) from 1962 to 2024. I show that in vivo parasite populations collected from Point of Ayr in 1962 were initially low, with 8% of shells infected, but this dramatically increased to 60% in 2016 and fell to 10% in 2024. The number of C. nemoralis shells infected from Formby in 2016 was 18% but increased to 31% in 2024, and 27% of C. hortensis shells from Strathy beach in 2016 were infected by nematodes but plummeted to 3% in 2024. Thus, dramatic spatiotemporal changes in nematode abundance can be seen over several decades. By combining preserved snails in conchology collections with collections of modern specimens, empirical evidence of how parasite numbers change over time can be quantified.