Movement patterns of the sea urchin Paracentrotus lividus in a marine reserve and an unprotected area in the NW Mediterranean

Sea urchins can alter the composition and dynamics of algal communities by grazing. Changes in their displacement capability can influence their grazing and thus their effect on algal communities. The daily and monthly movement of Paracentrotus lividus inside and outside a marine reserve in the NW Mediterranean Sea was studied in order to determine the role of predation in sea urchin movements and its potential grazing impacts. During the 3-month study, the total distance travelled by sea urchins ranged from 71.8 to 673.6 cm (mean = 379.2 cm, SD = 221.3). Home ranges (distances from the initial position) were between 50 and 302 cm (mean = 150.7 cm, SD = 99.4). Movements were not directional but random, with a mean of 150 cm around the initial position. The activity of Paracentrotus lividus was significantly higher at night than during daylight. Total displacement at the end of the experiment was significantly higher in the unprotected area than in the reserve, as was the home range. The linear distance travelled in a 24-h period ranged between 6 and 220 cm with a mean home range of 51 cm. These results agree with those of other authors, and allow us to rule out major migratory processes of sea urchin populations in this area. Nevertheless, these results underline the role of predatory fishes in reducing herbivory pressure by trophic cascades, reinforcing the evidence that sea urchin escape behaviour may be an important factor in structuring algal communities.