Co-occurring morphologically distinct algae support a diverse associated fauna in the intertidal zone of Araçá Bay, Brazil

Abstract Species diversity is regulated by historical, neutral and niche processes, with species tolerance, dispersal and productivity guiding diversity at larger scales, while habitat heterogeneity and biotic interactions acts in smaller scales. In rocky shores, several organisms provide secondary substrates for mobile fauna, with macroalgae being the most abundant and diverse ones. The patchiness promoted by different macroalgae hosts enhances small-scale heterogeneity and may increase and maintain the diversity of the mobile organisms, since there is a close relationship between the associated fauna and its hosts. In this study we selected three morphologically different macroalgae that coexist in the same rocky shore height in the Araçá Bay, an area under the threat of the nearby harbor expansion, and evaluated the fauna associated to each algal host. Even under similar abiotic pressure (same rocky shore height), the associated fauna of each algal host varied in number and composition, revealing a close relationship. The poorly branched foliose Ulva lactuca sustained a lower density of organisms and was dominated by isopods, while the heavily branched turf and Bostrychietum community showed a high density of organisms, with a dominance of peracarid crustaceans and annelids on the turf and more resistant groups, such as bivalves, acaris and terrestrial insects on the Bostrychietum. Previous studies in the Araçá Bay already revealed a large spatial heterogeneity in the processes and sessile organisms distribution, and here we highlight that this heterogeneity can be observed in an even smaller scale, with different algal hosts mediating the turnover of species in a scale of centimeters and meters, resulting in diversity maintenance of the associated fauna. Since the harbor expansion may prevent the occurrence of macroalgae as a result of light limitation by suspended platforms, we may expect not only a decrease in algal cover but also in the total diversity of the associated fauna in the Araçá Bay.