Solitary snails in search of a mate put off parenthood as long as possible in the hopes that a partner will appear. But when Physa acuta snails smell predators, they don't wait as long for a mate.

Normally, it takes the waxing and waning of the moon to coax certain worms from hiding on the seafloor to mate. Out in the open, sex-inducing chemicals kick off a swirling dance that culminates in a ...