On certain full moons each year, blue waters above reefs turn milky as corals release eggs and sperm in massive outbursts of reproductive activity. Genetic studies now show that these simple animals depend on some of the same light-sensing proteins as insects and vertebrates to keep their spawning on schedule. The discovery of the proteins in corals pushes back by millions of years the existence of these timekeeping molecules.
Some relatives of coral, such as jellyfish, have simple "eyes" that enable them to respond to light, but corals lack any such specialized organ. Yet hundreds of coral species somehow time their spawning with the light of the moon. This maximizes their chances for fertilization by having all the corals release their gametes at the same time and also improves the resulting larvae's chances for dispersal because of the full moon's large tides. These larvae eventually settle down and transform into sedentary reef-builders. Marine biologist Oren Levy of the University of Queensland in St. Lucia, Australia, and his colleagues wondered if cryptochromes--proteins that enable other organisms to respond to blue light and maintain an internal biological clock--somehow played a role in the process.
The team used the sequences of cryptochrome genes from mice, fruit flies, fish, and frogs to probe the genome of the fingerlike coral Acropora millepora. They found four cryptochrome genes and have now studied two in depth. The sequences indicate that these two are the ancient relatives of cryptochromes in other organisms. What's more, they may represent an innovation that likely enabled the early ancestors of corals and jellyfish to migrate deeper into the water as needed to avoid damage from the sun's potentially lethal ultraviolet light.
To see if these two cryptochrome genes followed a particular schedule, the researchers monitored their activity over the course of the day and during a lunar cycle. On a daily basis, one gene was an early riser, revving up to express protein at first light, whereas the other peaked about noon; then both quieted down at night, the group reports in the 19 October issue of Science. When kept in the dark for 24 hours, the genes' activity fluctuated randomly, confirming light's influence. Just one gene turned on full blast during the full moon but not during the new moon, suggesting that it helps set the spawning clock. "By monitoring the phase of the moon, corals are essentially tuning themselves to the tides and the ability of predators to detect their offspring as they float in the ocean," says Levy.
The study "provides an important mechanistic link in how mass spawning in corals is cued," says Margaret Miller, a marine biologist with the U.S. National Oceanic and Atmospheric Administration. She hopes the discovery will provide clues about whether the inability of some elkhorn coral in the Caribbean to spawn en masse is natural or a consequence of environmental stress. Marine biologist James Guest of Newcastle University in the U.K. says the findings may aid coral reef conservation. "Sexual reproduction is possibly the most important process in the recovery of degraded reefs," he says, "so understanding the mechanisms underlying reproductive timing is very important."