Oscillatory wave tank used in Experiment 2.

<p>(A) Position of the experimental area [3×4 grid of concrete tiles of 0.3×0.3×0.05 m each], and (B) relative positions of the kelp (<i>Alaria esculenta</i>) line, zone of maximum canopy cover [Canopy zone], and zone to which green sea urchins (<i>Strongylocentrotus droebachiensis</i>) were introduced prior to the onset of trials [Start zone]. The sequence at the bottom shows urchins at (C) t = 0 [prior to introducing the kelp line], (D) t = 1 h, and (E) t = 6 h [end] of a trial at a wave velocity of 0.1 m s^-1 (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0118583#sec002" target="_blank">Materials and methods</a> for details).