Living on the rocks: substrate mineralogy and the structure of subtidal rocky substrate communities in the Mediterranean Sea

Distribution patterns of epibenthic assemblages, sea urchins and fishes were assessed in NE Sardinia (Mediterranean Sea) in shallow (4 to 7 m depth) rocky habitats with 2 rock-type substrates, i.e. limestone and granite, to examine possible differences related to the mineralogical composition of rocks. Sessile organisms and sea urchins were sampled in situ within quadrats. Fishes were assessed by visual census transects. Sessile epibenthic assemblages significantly differed between granite and limestone substrates, and the number of epibenthic sessile taxa was greater on limestone. Average cover of the algae mat (unidentified mixture of filamentous and mucilaginous algae) was significantly greater on granite than limestone, while no significant differences were detected in total cover or in the average cover of the remaining most common epibenthic taxa. Densities of sea urchins (i.e. Paracentrotus lividus and Arbacia lixula) did not differ between the 2 rock types. Fish assemblages were significantly different between granite and limestone rock substrates. Labrids of the genus Symphodus and Serranus scriba were more abundant on granite, whereas Serranus cabrilla, Parablennius rouxi, Gobius bucchichi and Thalassoma pavo showed greater densities on limestone. Sarpa salpa (the most important herbivorous fish in the Mediterranean sublittoral) did not show any significant difference between granite and limestone substrates. These results suggest that rock type may have the potential to influence marine assemblages, probably through direct effects on epibenthic organisms (e.g. the presence of quartz and substrate texture, both related to the mineralogical features of rocks), and indirect effects on fishes (e.g. the influence of the structure of the epibenthic cover). These results suggest a possible role of the mineralogical composition of rocks in 'bottom-up' processes influencing marine assemblages in shallow sublittoral rocky habitats in the Mediterranean, where direct effects on sessile organisms (e.g. macroalgae) may cascade up through the entire community and affect higher trophic levels (e.g. carnivorous fishes). However, extensive experimental work is needed before drawing any conclusion about the specific processes determining the patterns we observed.