Elevated pCO 2 increases sperm limitation and risk of polyspermy in the red sea urchin Strongylocentrotus franciscanus KIM E. REUTER *, KATIE E. LOTTERHOS *, RYAN N. CRIM w , CATHERINE A. THOMPSON z and CHRISTOPHER D. G. HARLEY w *Department of Biological Sciences, Florida State University, Tallahassee, FL 32306, USA, wDepartment of Zoology, University of British Columbia, Vancouver, BC, Canada V6T1Z4, zDepartment of Biological Science, Simon Fraser University, Burnaby, BC, Canada V5A1S6 Abstract Anthropogenic carbon dioxide (CO 2 ) emissions and the resultant acidification of surface ocean waters are predicted to have far-reaching consequences for biological processes in the marine environment. For example, because changes in pH and pCO 2 can alter sperm performance, ocean acidification may be accompanied by reductions in the success of fertilization in marine broadcast spawners. Several studies have attempted to determine the effects of elevated pCO 2 on marine invertebrate fertilization success, albeit with differing results. These conflicts may stem from the use of inappropriate sperm–egg contact times and, in several cases, the lack of measurements over a range of sperm concentrations extending from sperm-limited conditions to polyspermy scenarios. In our study, we used biologically realistic sperm–egg contact times and a full range of sperm concentrations to assess the effect of elevated pCO 2 on fertilization in the broadcast spawning sea urchin, Strongylocentrotus franciscanus. Fertilization experiments were carried out in seawater bubbled with CO 2 to 400 (control), 800, and 1800 ppm. Using a fertilization kinetics model, we estimate that elevated pCO 2 levels both increased sperm limitation and reduced the efficiency of fast blocks to polyspermy. Thus, elevated pCO 2 decreased the range of sperm concentrations over which high fertilization success was likely. Given the inherent difficulties in achieving high fertilization success in broadcast spawners, raised pCO 2 levels are likely to exacerbate low fertilization success in low-density populations or in areas with high water turbulence. Keywords: anthropogenic climate change, carbon dioxide, echinoid, fertilization, marine invertebrates, ocean acidification Received 18 December 2009 and accepted 9 March 2010 Introduction Rising carbon dioxide (CO 2 ) levels in the ocean are contributing to a decrease in both oceanic pH and carbonate ion (CO 3 2À ) availability, a process termed ocean acidification (Feely et al., 2004; Doney et al., 2009). These oceanic changes are predicted to have far-reaching impacts in marine ecosystems (Broecker et al., 1971; Royal-Society, 2005; Harley et al., 2006; Andersson et al., 2008; Wootton et al., 2008), and the negative effects of ocean acidification are increasingly well-documented for a large diversity of marine orga- nisms (Anderson, 1996; Leclercq et al., 2000; Ingermann et al., 2002; Bibby et al., 2007, 2008; Gazeau et al., 2007; Andersson et al., 2008; Dupont et al., 2008; Ebert, 2008; Kurihara, 2008; Rost et al., 2008; Wood et al., 2008; Dey et al., 2009; Mcdonald et al., 2009; Parker et al., 2009). Although changing ocean chemistry could affect any part of the life cycle of a marine organism, the earlier life history processes (fertilization, larval development and metamorphosis) may be the most susceptible (re- viewed in Kurihara, 2008). For broadcast spawners that release gametes directly into the water column, fertili- zation may be particularly sensitive to ocean acidifica- tion due to the vulnerability of both sperm and eggs to changes in seawater chemistry. Sperm motility depends in part on sperm density, sperm age, water temperature, oxygen-availability, pH and pCO 2 levels (Levitan et al., 1991; Levitan, 1993; Ingermann et al., 2002). Raised pCO 2 or acidic conditions have decreased sperm motility in sea urchins (Havenhand et al., 2008), fish (Ingermann et al., 2002; Dey et al., 2009), and sea cucumbers and corals (Morita et al., 2009). Lower acidity can directly affect the intracellular pH in sperm (Christen et al., 1983), decreasing its ability to move towards and collide with an unfertilized egg. In these cases, decreased pH negatively affect sperm motility and possibly gametic compatibility, which could result in decreased fertiliza- tion success. In sea urchins, egg susceptibility to fertilization may be affected by elevated pCO 2 by two mechanisms. Correspondence: Katie E. Lotterhos, tel. 1 1 850 645 8577, fax 1 1 850 645 8447, e-mail: klotterhos@bio.fsu.edu Global Change Biology (2011) 17, 163–171, doi: 10.1111/j.1365-2486.2010.02216.x r 2010 Blackwell Publishing Ltd 163