Patterns of movement of the limpet Cellana tramoserica on rocky shores and retaining seawalls

Artificial structures are becoming common features of the landscape in shallow waters of urban areas, but very little is known about their ecology. In Sydney Harbour (NSW, Australia), intertidal seawalls replace considerable portions of natural habitats, including rocky shores. Previous studies have shown that seawalls and rocky shores generally support similar assemblages of plants and animals, with important differences in relative abundances of some species. The behaviour of key consumers can have a fundamental role in structuring benthic assemblages, and it is, therefore, necessary to assess whether it is changed by the replacement of natural habitats with artificial structures. This study compares patterns of movement of the common patellid limpet Cellana tramoserica between seawalls and vertical rocky shores. The distances displaced, the orientation of movement and the homing behaviour of marked individuals were recorded from day to day, over a period of several tidal cycles (14 d) and for a longer period (ca. 3 mo), on several seawalls and rocky shores. Short-term patterns of movement (1 to 14 d) did not differ between structures. In contrast, after 3 mo, although the orientation of the movement remained random on both kinds of structures, limpets on seawalls had dispersed longer distances and tended not to retain their original positions. These results suggest that long-term dispersal of C. tramoserica, rather than foraging bouts, could be altered by the replacement of rocky shores with artificial structures. Differences in use of habitat by key consumers should be incorporated in models aimed at identifying the mechanisms responsible for the occurrence of different assemblages on natural and artificial structures.