Micro-scale distribution of Chthamalus stellatus Poli, 1791 is driven by the mineralogical properties of the substrate

Biotic and abiotic factors and their interactions can shape the structure of marine benthic communities in several ways, acting at different spatial, temporal, and biological scales. Among them, the role of the mineral composition and surface texture of the substrate in driving recruitment, settlement, and survival phases of sessile marine species has been recently investigated in different contexts. Thanks to their lifestyle, Chthamalus species are a good proxy to test for the role of substrate features in affecting their distribution and population structure, in energetic splash zones. The present study aimed to describe the density and size of Chthamalus stellatus occurring on different mineral phases/rocks intermixed in the same bedrock and visually detectable on the surface. This micro-scale approach was used in two case studies: Alisu (Western Corse, Tyrrhenian Sea) where the paragneiss bedrock presents large quartz veins and nodules and Sant’Andrea Cape (Elba Is., Tyrrhenian Sea) where a monzogranite hosts large K-feldspar macro-crystals and basalt inclusions. The results indicated that barnacles completely avoid large crystals (quartz and K-feldspar), independently from their composition, selecting substrate portions with a fine—grain texture. The comparison of granite vs basalt, intermixing in the same rocky substrate, showed that barnacle density was lower on basalt while the individual size was lower on granite. We hypothesize that the known toxic properties of quartz crystals, abundant in granites, can negatively affect barnacle growth.