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

Abstract

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&#65533 8.3) to 6000 ppmv (pH 7.15&#65533 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 spermmotility,whichwas insensitive to pCO2.When seawaterwas acidified using HCl, leaving pCO2 constant, fertilization success in S. nudus remained high (N80%) until pH decreased to 7.3. While further studies are required to elucidate the physiological mechanismbywhich elevated pCO2 impairs spermand reduces S. nudus fertilization, this study suggests that in the foreseeable future, sea urchin survival may be threatened due to 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&#65533 8.3) to 6000 ppmv (pH 7.15&#65533 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 spermmotility,whichwas insensitive to pCO2.When seawaterwas acidified using HCl, leaving pCO2 constant, fertilization success in S. nudus remained high (N80%) until pH decreased to 7.3. While further studies are required to elucidate the physiological mechanismbywhich elevated pCO2 impairs spermand reduces S. nudus fertilization, this study suggests that in the foreseeable future, sea urchin survival may be threatened due to