On a summer night in the Bay of Naples, hordes of worms swam upward from the seagrass toward the water\u2019s surface under the light of a waning moon. Not long before, the creatures began a gruesome sexual metamorphosis: Their digestive systems withered, and their swimming muscles grew, while their bodies filled with eggs or sperm. The finger-length creatures, now little more than muscular bags of sex cells, fluttered to the surface in unison and, over a few hours, circled each other in a frantic nuptial dance. They released countless eggs and sperm into the bay \u2014 and then the moonlit waltz ended in the worms\u2019 deaths.<\/p>\n

The marine bristle worm Platynereis dumerilii <\/em>gets only one chance to mate, so its final dance had better not be a solo. To ensure that many worms congregate at the same time, the species synchronizes its reproductive timing with the cycles of the moon.<\/p>\n

How can an undersea worm tell when the moon is at its brightest? Evolution\u2019s answer is a precise celestial clock wound by a molecule that can sense moonbeams and sync the worms\u2019 reproductive lives to lunar phases.<\/p>\n