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

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 (pH7.96-8.3) to 6000ppmv (pH7.15-7.20). When sperm were exposed, fertilization rate decreased drastically with increased pCO₂, even at a concentration of 450ppmv (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 750ppmv. 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.