Canopy structure at the edge of seagrass affects sea urchin distribution

The purpose of this study was to add insights to the understanding of seagrass Posi- donia oceanica –sea urchin Paracentrotus lividus interactions, evaluating whether the canopy structure at the edge of meadows next to rocks affects the distribution of P. lividus . The experi- ment consisted of the manipulation of seagrass structure by modifying the shoot density (20, 50, and 100%) and leaf length (natural and reduced). Plastic seagrass was used to test for the mecha- nism of attraction to the seagrass edge. Thus, for each treatment combination (shoot density × canopy height), 3 types of Posidonia were considered: natural seagrass, artificial seagrass, and natural seagrass + frame as a procedural control. The results highlighted that P. oceanica canopy structure influences the distribution of P. lividus , as the mean abundance of the sea urchin was directly proportional to the shoot density at both canopy heights. Since the type of Posidonia did not influence seagrass density, grazing is not the mechanism of attraction of the sea urchins towards the seagrass edge — a more plausible reason, thus, is the search for shelter. Although these findings add to the understanding of P. oceanica–P. lividus interactions at the edge of the seagrass, space use patterns in sea urchins and the structure of the seascape over which they tra- verse must be quantified so that a more complete understanding of species–habitat relationships and seascape connectivity can be developed.