Grazing by the sea urchins Arbacia lixula L. and Paracentrotus lividus Lam. in the Northwest Mediterranean

The sea urchins Arbacia lixula and Paracentrotus lividus are common on shallow subtidal reefs in the Mediterranean. Previous studies on the ecology of these species reported that P. lividus is generally more abundant on horizontal or gently sloping substrata, where it forages mainly on erect algae. In contrast, A. lixula is more common on vertical substrata and it is considered a main grazer of encrusting coralline algae. Observations on some rocky shores in the Ligurian sea indicated that P. lividus occurs mainly in crevices at the bottom of the vertical walls, and that neither species is present on horizontal or sub-horizontal substrata. In this study we investigated the distribution and abundance of the two species of sea urchins on vertical substrata at different spatial scales and through time. A field experiment was used to test whether A. lixula constrained the distribution of P. lividus on vertical substrata, and to test for the predicted differences in the effects of the 2 species on assemblages of algae and invertebrates. Four treatments were used: (1) control (sea urchins left untouched); (2) A. lixula removed, P. lividus present; (3) A. lixula present, P. lividus removed, and (4) both species removed. The effects of sea urchins on colonising algae and invertebrates were examined after 6, 12 and 18 months. A. lixula was consistently more abundant than P. lividus on patches of vertical substrata, but the removal of A. lixula did not affect the abundance of P. lividus. There was no effect of A. lixula on encrusting corallines, but the cover erect algae increased and the number of limpets decreased significantly where A. lixula was removed. P. lividus had only a minor impact on the assemblage, probably due to its low abundance. A. lixula had effects close to those predicted for P. lividus, suggesting that the ecological role of these herbivores may be more similar than previously thought. The implications of these results for the management of the edible sea urchin, P. lividus, are discussed