Effects of ocean acidification on the reproductive output of three Mediterranean corals transplanted at an underwater crater characterized by CO2 emissions

Oceanic uptake of anthropogenic CO2 is causing significant changes in ocean chemistry, reducing pH and increasing sea surface temperature. A decline of 0.3–0.5 pH units and 2.0-4.5 °C of temperature are expected by 2100. Few information are available regarding the effects of ocean acidification on sexual reproduction in corals and none on temperate corals. This study assessed the effects of ocean acidification on sexual reproduction in three Mediterranean scleractinian corals: Balanophyllia europaea (solitary, zooxanthellate), Leptopsammia pruvoti (solitary, azooxanthellate), and Astroides calycularis (colonial, azooxanthellate). The relationship between reproductive parameters (fecundity rate, spermary abundance, gonadal index, reproductive element size) and pCO2 were assessed at an underwater volcanic crater characterized by continuous emissions of CO2 which create a natural pH gradient, located near the island of Panarea (Tyrrhenian Sea, Italy). Samples of the three species were transplanted along the pH gradient in four sites from the center of crater (pH 7.4) to its periphery (control). The biological responses were analyzed according to different reproductive (gonochoric vs hermaphrodite) and metabolic (zooxanthellate vs non zooxanthellate) strategies of each species.