Factors contributing to spatial heterogeneity in the abundance of the common periwinkle Littorina littorea (L.)

Although Littorina littorea (L.) exhibits a relatively consistent pattern of vertical distribution throughout the North Atlantic, ranging from the mid-intertidal to the shallow subtidal zone, its horizontal distribution and abundance are highly variable. In this study, we first described the snail's horizontal distribution patterns on Appledore Island, ME, USA and then asked whether wave exposure, rugosity, predator density (e.g. Carcinus maenas and Cancer borealis), the percentage of the substrate covered by Ascophyllum nodosum, Chondrus crispus, Polysiphonia spp., and ephemeral green algae, or the percentage of uncovered substrate (bare rock) were correlated with L. littorea abundance in the intertidal zone (0.6 to 0.0 m relative to Mean Lower Low Water [MLLW]) and the shallow subtidal zone (-1.5 to -3.0 m MLLW) at nine study sites. Intertidal densities of L. littorea were highly variable across sites, ranging from 0 to > 600 m(-2). In this zone, L. littorea density showed a significant positive correlation with rugosity and percentage of bare rock. Densities were very low in the subtidal zone (range: 0-19 m(-2)) and varied little among sites. Exploratory multiple regression analysis suggested that L. littorea density was positively correlated with the density of C. maenas in the shallow subtidal zone. Additionally, snails in the subtidal zone had thicker shells than snails of the same size in the intertidal zone. Our results suggest that structural elements of the habitat, such as rugosity and percentage of uncovered substrate, are among the most important predictors of L. littorea abundance on moderately protected, rocky intertidal shores.