Inter- and intra-specific competition between Paracentrotus lividus and Arbacia lixula in resource-limited barren areas

The sea urchins Paracentrotus lividus and Arbacia lixula coexist in the infralittoral zone along the Mediterranean and north-east Atlantic coasts. Through their grazing activity they can produce and maintain barren grounds, habitat characterised by low algal diversity and productivity. This study addresses the coexistence of these species within severely limited barren grounds by assessing differences in population structure, feeding preferences and conditions. Density, size structure, feeding conditions and gut contents of the two species, along with the associated algal assemblage were analysed in a hierarchical sampling design (Locations: thousands of meters apart; Sites: hundreds of meters apart). The algal assemblage, dominated by encrusting corallines (EC), was depauperate with bare rocks representing around 67% of the total percent cover, and homogeneous both at the Site and at the Location level. Sea urchins showed significant variability in density and gut conditions only at the Site level. No compensative mechanisms have been observed (no inverse correlation between the two species). The gut contents of the two species were quite different: A. lixula fed mainly on encrusting corallines, while P. lividus fed on non-encrusting macrophytes. Food limitation was detected for P. lividus, as a consequence of intra-specific competition. Our results highlight that in barren conditions a large trophic niche differentiation occurs, demonstrating that the two urchins play complementary and synergic roles in the maintenance of barrens in the Mediterranean.