Near-future levels of ocean acidification reduce fertilization success in a sea urchin

Although it is widely believed that seawater is chemically well-buffered, CO₂-induced acidification of the world's oceans threatens the viability of many species [1-3]. Research to date has focused on the responses of adult stages of calcifying taxa to gross pH changes relevant for the years 2200-2400 [3,4]. We investigated the consequences of exposure of gametes and larvae of the sea urchin Heliocidaris erythrogramma to CO₂-induced acidification by -0.4 pH units (the upper limit of predictions for the year 2100 [5]), and found statistically significant reductions in sperm swimming speed and percent sperm motility. We predicted the effects of these changes using an established model [6], and tested fertilization success experimentally in assays using the same gametes and pH treatments. Observed reductions in fertilization success corresponded closely to model predictions (24% reduction). If general, these findings have important implications for the reproductive and population viability of broadcast spawning marine species in the future acidified ocean.