Effects of ocean acidification on skeletal characteristics of a temperate coral at a CO2 vent system

Ocean Acidification (OA) is predicted to have profound impacts on marine ecosystems because carbonate ions are an essential substrate for the biomineralization of shells and skeletons of calcifying marine organisms, from phytoplankton and corals to fishes1,2,3. Volcanic CO2 vent systems, where seawater is naturally acidified, offer a unique opportunity to investigate the response of benthic organisms and habitats to OA. The Ischia Island (Tyrrhenian Sea, Italy) offers a natural laboratory for OA studies, allowing us to investigate how a suite of habitats and species responds to acidification. A. calycularis is a Mediterranean endemic azooxanthellate coral. It is long-lived species and commonly found in dim light shallow rocky habitats of the south-western Mediterranean Sea4. It broods its larvae5 and thus has relatively low dispersal capacities and high potential for local adaptation. This coral is reported as vulnerable in the IUCN red list6. There is one population of A. calycularis that naturally occurs in a semi-submersed cave (Grotta del Mago) affected by CO2 venting, where is highly abundant (70% cover at 1 m depth). Here, we assess population structure and the skeletal characteristics of A. calycularis originating from different sites (naturally acidified and ambient pH sites) along the coast of Ischia Island . We hypothesize that the population thriving under naturally acidified conditions shows higher population dynamics and differences in biomineralization process than the populations studied from other reference sites with ambient pH.Colonies in the Grotta del Mago have encrusting morphology, with smaller size and consequent, lower weight and lower number of polyps compared to conspecifics from sites at normal pH conditions. With increasing acidification (lower pH), the skeletal porosity decreased while the bulk and micro- density increased. Given the reduced calcification rate that may be expected in acidified waters, the observed increase in skeletal density may be counterbalanced by a strong decrease in linear extension rate.