James E. Byers, April M. H. Blakeslee, Ernst Linder, Andrew B. Cooper, and Timothy J. Maguire. 2008. Controls of spatial variation in the prevalence of trematode parasites infecting a marine snail. Ecology 89:439–451.

Appendix A. Methodologies for field measurements of physical and biological variables.

We recorded bird abundance and diversity immediately upon arrival at a site just before low tide. We only counted those species that were on the ground in the intertidal zone at the site, typically within a 50-m radius of the center of our snail collections, and these counts were performed at least twice (and up to five times) at each site during the study months. The most common birds that are known to be definitive hosts for trematodes that infect Littorina littorea included Herring Gulls (Larus argentatus), Great Black-backed Gulls (Larus marinus), Double-crested Cormorants (Phalacrocorax auritus), and Common Eiders (Somateria mollisima) (e.g., Harris 1964, Bishop and Threlfall 1975, Pohley 1976, Lauckner 1985).

Subtidal diversity and relative abundance of crabs (and some fish) were obtained by deploying two baited traps (FTA-Fish Trap, Aquatic Eco-systems; dimensions: 24" × 18" × 8", 1.25-cm mesh) approximately 5–10 m from shore (below MLLW). Traps were left undisturbed for two hours, and the overall abundance and diversity were recorded for each trap and then summed. Intertidal crab abundance and diversity were quantified using 0.5-m² quadrats, which were placed at 6–8 systematically spaced locations along a 50-m transect in both the high and low intertidal zone. Nonnative European green crabs (Carcinus maenas) dominated the samples. In addition to density measurements of all crab species, for both the subtidal and intertidal counts, crabs were classified as one of four size categories. To compute total crab biomass for each site, we multiplied the number of crabs in each of the four categories by the respective average weight for the size category, which we determined from a quantified size–weight relationship of carapace length and mass for C. maenas and mud crabs (Dyspanopeus sayi) sampled at Adams Point, New Hampshire, USA (wet weight = 0.00034*width^2.9; n = 82; R² = 0.99).

Fish abundance and diversity were also quantified using two baited minnow traps that were placed approximately 10 m from shore. Traps were left undisturbed for two hours, and abundance and diversity were recorded for both traps combined.

Snail abundance and diversity were determined by placing a 0.05-m² quadrat at 8–12 systematically spaced locations along a transect in both the high and low intertidal. All snails within the quadrat were counted by species. Additionally, within one randomly selected quarter of the quadrat we measured the lengths of L. littorea to the nearest 0.1 mm.

Salinity was measured with a hand-held refractometer 2–5 times per site during the study months. Although this replication is low, there was little variation in salinity within a given site over the summer sampling season. Based on orientation of beach, fetch, associated biota, and known information about exposure, we qualitatively assessed exposure and wave energy for each site, categorized as high, intermediate, or low. To measure differences in topography (rugosity) at each site, a 10-m chain was laid parallel to the shoreline and allowed to conform to the microtopography of the intertidal terrain. The straight-line end-to-end distance covered by the chain was measured for three replicate placements at each tidal height. Temperature was measured by affixing a temperature logger (ONSET, Bourne, Massachusetts, USA) to rocks in the low intertidal tide zone at each site. The loggers recorded temperature every four minutes. To characterize summer water temperatures for each site during standardized tidal inundation conditions, measurements logged within one hour of either side of daytime high tides during the month of August were averaged.

LITERATURE CITED

Bishop, C. A., and W. Threlfall. 1975. Helminth parasites of the common eider duck, Somateria mollissima (L.), in Newfoundland and Labrador. Proc. Helminthol. Soc. Wash. 41:25–35.

Harris, M. P. 1964. The incidence of some species of trematoda in three species of Larus gulls in Wales. Ibis 106:532–537.

Lauckner, G. 1985. Diseases of Aves (Marine Birds) Pages 627–637 in O. Kinne, editor. Diseases of Marine Animals. Biologische Anstalt Helgoland, Hamburg, Germany.

Pohley, W. 1976. Relationships among three species of Littorina and their larval Digenea. Marine Biology 37:179–186.