The relative role of intrinsic and extrinsic factors driving alongshore movement of swash-riding smooth plough shells, Bullia rhodostoma

Animal movement underpins many ecological processes, and thus plays a key role in maintaining functional ecosystems. Sandy shores are an interesting model system on which to study animal movement because they are physically dominated, and the resident animals are generally very small (< 1 mm-5 cm). However, beach macrofauna have a number of key adaptations that may equip them to harness the movement potential presented by the physical environment. Therefore, we aimed to quantify alongshore movement of the smooth plough shell snail, Bullia rhodostoma, and identify whether intrinsic (animal size) or extrinsic (beach morphology/swash climate) factors were stronger drivers of this movement. We captured, measured, and marked > 2000 snails in each of six occasions (three on neap tides; three on spring tides), released them from a single fixed point, and recaptured them for four consecutive days following release. Each day, a standard suite of physical variables was also measured at low tide. The majority of snails maintained their position along the shore and remained close to the release point, however, some moved up to several hundred meters alongshore per day in both directions away from the release point. Overall, the average maximum daily displacement (daily “step-length”) was 100 m; this was independent of animal size and tide state. However, when considering the maximum cumulative distance travelled (furthest distance attained relative to the release point per day), tide state and time were significant drivers of B. rhodostoma movement, with animal size playing a secondary role. The latter may be because of differences in animal density and foot size with increasing snail size, translating into differences in tidal migrations and alongshore displacement. Overall B. rhodostoma, like other beach macrofauna species, largely maintain their alongshore position at a local scale, but also have the potential to disperse much further even as adults. Of the variables measured, it seems that external forcing (beach morphology and wave/swash climate) is more important for driving B. rhodostoma movement than are intrinsic drivers (size). However, there are other intrinsic variables not tested here, e.g., state of hunger, that may be important as well. The limitations of the study notwithstanding, we present a low-cost, low-technology method for estimating beach-animal movement that could be easily applied to test other related questions.