A large and erected sponge assemblage on granite outcrops in a Mediterranean Marine Protected Area (NE Sardinia)

Abstract Coralligenous reefs represent one of the key ecosystems of the Mediterranean Sea in terms of both species richness and habitat heterogeneity. Their main characteristic is the massive development of crustose macroalgae, the corallines, which form a bio-constructed basal layer. However, when growing on granitic substrates, these algae show a lower capacity to build thick concretions than those present on limestone. Assemblages developing on isolated granitic outcrops were investigated by scuba diving at 73 sites down to a depth of 56 m within the Tavolara-Punta Coda Cavallo Marine Protected Area (NE Sardinia, Mediterranean Sea). These outcrops were characterised by a scarce development of crustose corallines, due to the widespread presence of the brown algae Carpomitra costata and, to a lesser extent, Cystoseira zosteroides. Animal dominance increased with depth, mainly by sponges, in particular Axinella spp. and several species of Keratosa (Dysidea spp., Sarcotragus foetidus, Spongia lamella and S. officinalis). Gorgonians, which typically characterise the upper layer of coralligenous assemblages, were scarcely represented: Eunicella cavolini and E. verrucosa were generally rare, E. singularis was widespread, while Paramuricea clavata was comparatively frequent only at depth. All these features determined a peculiar community that was markedly different from the coralligenous assemblages occurring on neighbouring limestone rocks. Such a community, clearly characterised by the dearth of the typical biogenic hard substrate made by coralline algae, uneven distribution of gorgonians and, especially, the dominance of large and erect sponges, constitutes a distinctive facies of Mediterranean circalittoral communities worthy of conservation.