The effects of substrate structural complexity on distribution patterns of sea urchins in the barren of Ustica Island MPA

We investigated the distribution patterns of the sea urchins Paracentotus lividus (Lamark) and Arbacia lixula (L.) in three substrates characterized by different degrees of physical structural complexity present in the barren of Ustica Island MPA. Sea urchin densities and test size structure were scored in two areas (aprox. 500 m distance apart), chosen randomly within the no-take zone of the MPA. Four sites, two per each area (aprox. 50 m apart) were sampled. Measurements were carried out selectively on smooth rocky bottom, wrinkled hard surface and rugged surface capped with round lava boulders. The average density of P. lividus was greater on wrinkled surface and lower on smooth surface, in all sites of the two areas. The smallest individuals, included in size classes 1-2 and 2-3 cm, were found only on the wrinkled surface where they hyde in crevices and holes. On the contrary, the average density of A. lixula did not significantly change between substrates. Although their life habits are very close and they co-occur in the barren habitat, the response of the two sea urchins to the structural complexity of the substrate is different. Patterns in space usage of the two species are probably related to differences in morphofunctional features and consequently on motion ability, water turbolence resistance and predation sensibility on the three substrates.