Ocean acidification, due to the increase of carbon dioxide (CO 2 ) concentration in the atmosphere and its absorption by the oceans, affects many aspects of marine calcifying organisms' biology, including reproduction. Most of the available studies on low pH effects on coral reproduction have been conducted on tropical species under controlled conditions, while little information is reported for either tropical or temperate species in the field. This study describes the influence of decreasing pH on sexual reproduction of the temperate nonzooxanthellate colonial scleractinian Astroides calycularis, transplanted in four sites along a natural pH gradient at the underwater volcanic crater of Panarea Island (Tyrrhenian Sea, Italy). The average pH values of each site (range: pH TS 8.07-7.40) match different scenarios of the Intergovernmental Panel on Climate Change (IPCC) for the end of the century. After 3 months under experimental conditions, the reproductive parameters of both oocytes and spermaries (abundance, gonadal index, and diameters) seem to be unaffected by low pH. However, a delay in spermary development in the pre-fertilization period and a persistence of mature oocytes in the fertilization period were observed in the most acidic site. Furthermore, no embryos were found in colonies from the two most acidic sites, suggesting a delay or an interruption of the fertilization process due to acidified conditions. These findings suggest a negative effect of low pH on A. calycularis sexual reproduction. However, longterm experiments, including the synergistic impact of pH and temperature, are needed to predict if this species will be able to adapt to climate change over the next century.Carbon dioxide (CO 2 ) is the main greenhouse gas produced by human activities, and over the last decades, its concentration in the atmosphere has followed an exponential growth (Stocker et al. 2013). About 25% of atmospheric CO 2 is absorbed by the ocean (Friedlingstein et al. 2020), with a consequent decrease in marine pH and carbonate ions concentration, in a phenomenon known as ocean acidification (Feely et al. 2004). Carbonate ions are essential for the calcification process of several marine organisms, including corals (Kleypas et al. 1999;Al-Horani et al. 2003;Cohen and McConnaughey 2003). Scleractinian corals (i.e., stony corals) are essential in the maintenance of ecosystems and their biodiversity (Bellwood and Hughes 2001;Wild et al. 2011). Studies on the effects of ocean acidification on calcification of both tropical and temperate corals reveal variable responses, suggesting that the calcification response to increasing ocean acidity may be rather complex (Chan and Connolly 2013;Prada et al. 2017;Teixid o et al. 2020). Since scleractinian corals are among the most sensitive taxa to changes in environmental conditions, they play an essential role in understanding the evolution of life cycles (Harrison 2011) and how these changes may influence the reproductive biology of calcifying marine organisms (Richmond et al. 2018;...
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