Reproductive output of the solitary zooxanthellate coral Balanophyllia europaea (Scleractinia, Dendrophylliidae) along a natural pH gradient at Panarea Island.

Approximately a quarter of the anthropogenic carbon dioxide (CO2) emissions to the atmosphere are absorbed by the ocean. This ocean uptake of CO2 causes a progressive decrease of seawater pH and carbonate ion concentration, also known as ocean acidification and shifts in seawater carbonate chemistry. Ocean acidification will have many negative consequences for marine calcifying organisms, including corals. This thesis examines the short-term potential effects of ocean acidification on reproductive output of the solitary and zooxanthellate coral Balanophyllia europaea, transplanted along a natural pH gradient generated by an underwater volcanic crater near Panarea Island (Sicily, Italy). Cyto-histometric analyses reveal that oocyte and spermary abundance (#/100 mm3), oocyte and spermary gonadal index (%), fertility (#/100 mm3), embryonal index (%) and embryo diameter (μm) of Gonadal development and Maturity period of B. europaea was unaffected by increasing acidification, even under an extreme pH value. Only oocyte and spermary diameter of both reproductive periods varied among sites but without a clear trend. Thus, reproduction of B. europaea seems to be tolerant to decreasing pH, while the azooxanthellate corals, L. pruvoti and A. calycularis, transplanted along the same natural gradient, were negatively affected by increasing acidification. This different result could be explained by the different trophic strategy of B. europaea, as the symbiosis with zooxanthellae could provide additional energy to dedicate in reproduction.