Factors affecting the prevalence of the trematode parasite Echinostephilla patellae (Lebour, 1911) in the limpet Patella vulgata (L.)

- Environmental factors determining the distribution and abundance of parasites are poorly understood.
- There was no difference in infection prevalence between rock pools and emergent rock.
- There was a positive relationship between infection prevalence and limpet (host) size.
- Indeterminable individuals were more likely to be infected than males or females, suggesting castration.

Parasites are ubiquitous throughout nature and can have dramatic effects on their hosts. Although much is known about the pathology of parasites, the environmental factors influencing the distribution and abundance of parasites are poorly understood. Investigations into these factors could help predict the effect of parasites on the functioning of ecosystems. The limpet Patella vulgata is a key grazer on European rocky shores and is the first intermediate host for the trematode parasite, Echinostephilla patellae. This study investigated the spatial distribution of E. patellae in relation to P. vulgata at fourteen rocky shores around Ireland. Despite P. vulgata recruiting into rock pools, adults are more abundant on emergent rock and experience greater mortality in pools compared to emergent rock. It has been suggested that greater parasite prevalence in pools may be a factor driving this disjunct distribution pattern. Prevalence of infection was compared among rock and pool habitats. Size and sex of the host were also investigated in order to determine whether host phenotype influenced levels of infection prevalence. Results varied considerably among study sites, probably due to the heterogeneity of environmental conditions. No clear pattern emerged in relation to infection prevalence between habitats but this may be owing to the low numbers of limpets collected from pools. There was a significant positive relationship between infection prevalence and increasing host size. Individuals for which sex was indeterminate were more likely to be infected than distinct males or females, suggesting that infection may be causing castration and subsequent removal of these individuals from the breeding population. We discuss the importance of considering host-parasite dynamics in a period of rapid environmental change.