Species and gamete-specific fertilization success of two sea urchins under near future levels of pCO2

•We investigated sea urchin Strongylocentrotus nudus and Hemicentrotus pulcherrimus fertilization success to ocean acidification.•Our study revealed that exposure of S. nudus sperm to elevated pCO2, even at 450 ppmv, dramatically decreased fertilization rates.•When seawater pH was decreased using HCl, fertilization success did not decrease significantly until pH reached 7.3.•Fertilization success of S. nudus eggs did not changed until pCO2 to which eggs were wxposed became 3000 ppmv.•This study provided no indication that S. nudus sperm motility or speed was influenced by elevated pCO2 within our range of investigation.

Since the Industrial Revolution, rising atmospheric CO2 concentration has driven an increase in the partial pressure of CO2 in seawater (pCO2), thus lowering ocean pH. We examined the separate effects of exposure of gametes to elevated pCO2 and low pH on fertilization success of the sea urchin Strongylocentrotus nudus. Sperm and eggs were independently exposed to seawater with pCO2 levels ranging from 380 (pH 7.96–8.3) to 6000 ppmv (pH 7.15–7.20). When sperm were exposed, fertilization rate decreased drastically with increased pCO2, even at a concentration of 450 ppmv (pH range: 7.94 to 7.96). Conversely, fertilization of Hemicentrotus pulcherrimus was not significantly changed even when sperm was exposed to pCO2 concentrations as high as 750 ppmv. Exposure of S. nudus eggs to seawater with high pCO2 did not affect fertilization success, suggesting that the effect of increased pCO2 on sperm is responsible for reduced fertilization success. Surprisingly, this result was not related to sperm motility, which was insensitive to pCO2. When seawater was acidified using HCl, leaving pCO2 constant, fertilization success in S. nudus remained high (> 80%) until pH decreased to 7.3. While further studies are required to elucidate the physiological mechanism by which elevated pCO2 impairs sperm and reduces S. nudus fertilization, this study suggests that in the foreseeable future, sea urchin survival may be threatened due to lower fertilization success driven by elevated pCO2 rather than by decreased pH in seawater.