Ocean acidification takes sperm back in time

Ocean acidification as a result of an enriched carbon dioxide atmosphere threatens ecosystem health and marine biodiversity. The reproductive and early life-stages of animals have been suggested as among the most vulnerable to ocean acidification perturbations. To explore this theory further we applied computer assisted sperm analysis (CASA) to investigate the combined effect of pH (8.06-7.67) and temperature (14-20 degrees C) on sea urchin sperm motility. Previous studies have either observed no or inhibitory impacts of ocean acidification on sperm swimming. Surprisingly, we observed a substantial improvement in swimming speed at reduced seawater pH compared with performance at current pH levels. This suggests that current levels may be suboptimal for maximal sperm swimming speeds. Temperature was found to affect swimming speed but not percent motility. Our observations suggest that swimming speed may be improved as seawater pH approaches conditions resembling the paleo-ocean. However, this does not necessarily equate to an improvement in reproductive fitness due to a trade-off between sperm-swimming speed and longevity. This indicates that ocean acidification may benefit certain aspects of the reproductive biology of some marine animals.