Growth and demography of the Mediterranean endemic solitary coral Balanophyllia europaea living along a natural pCO2 gradient

Increasing research effort on marine calcifying organisms is important and urgent, given the projected CO2-driven ocean acidification for the coming century. Here we assess the growth and population dynamics of the Mediterranean endemic zooxanthellate solitary coral B. europaea at three sites along a natural pCO2 gradient of the volcanic vents off Panarea Island (Italy), in the Mediterranean Sea. B. europaea was the only scleractinian found along the gradient and its abundance decreased with increasing pCO2. The age-length relationships, obtained for each site by sclerochronology through computerized tomography, resulted homogeneous and then data were pooled to obtain a general age-length relationship. Skeletal linear extension rate and bulk skeletal density were measured and net calcification rate was calculated for each sample at each site. While linear extension rate did not vary with pCO2, bulk skeletal density and net calcification rate decreased with increasing pCO2. The population age structure was obtained by analyzing photoquadrats and several observed and theoretical demographic parameters were derived (age structure stability, instantaneous rate of mortality, average age, % of immature individuals, age at maximum biomass, average age of biomass) and compared among sites. Populations showed a deficiency of young individuals with increasing pCO2. The species seems sensitive to ocean acidification even if the photosynthetic efficiency of its symbionts increases with pCO2, suggesting an even higher sensitivity for shallow water non-zooxanthellate scleractinians.