Small-scale spatial heterogeneity in infection levels by symbionts of the amphipod Talorchestia quoyana (Talitridae)

The beach hopper amphipod Talorchestia quoyana is not distributed homogeneously in its sandy beach habitat, but instead occurs in dense patches under large pieces of tidal debris. This marked patchiness on a scale of just a few metres may create a higher level of aggregation among hosts for the symbionts and parasites of beach hoppers. Prevalence of infection by a mermithid nematode parasite varied significantly among 5 beach hopper patches, located within a few hundred metres of one another, on a New Zealand sandy beach. Similarly, the prevalence and mean intensity of infection by rhabditid nematodes and mites also varied significantly among beach hopper patches. This variation is not due to differences in amphipod sizes among patches, as the effect of size was controlled in the analysis. The symbionts and parasites were aggregated among individual amphipods within patches, with rhabditid nematodes also aggregated among patches, reflecting a higher, independent scale of aggregation. There were also positive associations between rhabditid nematodes and both mermithid nematodes and mites across host individuals, suggesting that individual variation among amphipods in susceptibility to symbionts may account for their aggregation within patches. These results indicate that the loosely fragmented distribution of beach hoppers is linked to a higher scale aggregation of their parasites and symbionts, a phenomenon that may impact on their population biology and that of the host.