"Slow sperm ... now that’s a problem,” said Jonathan Havenhand, his British accent compounding the gravity of the message. “That means fewer fertilized eggs, fewer babies and smaller populations.” We were sharing a hilly cab ride along the glistening northern coast of Spain to attend an international symposium about the effects of climate change and excess atmospheric carbon dioxide on the world’s oceans. As researchers, we were concerned about the underappreciated effects of changing ocean chemistry on the cells, tissues and organs of marine species. In laboratory experiments at the University of Gothenburg in Sweden, Havenhand had demonstrated that such changes could seriously impede the most fundamental strategy of survival: sex.