Once In A Blue Moon

Every year during months with the warmest average sea temperatures, numerous coral species synchronize their spawning events with the light of a full moon . In a report published today in Science, marine biologist Oren Levy of the University of Queensland in St. Lucia, Australia, and his colleagues discovered that cryptochromes--proteins that enable other organisms to respond to blue light and maintain an internal biological clock--play a critical role in the process.

On the night of spawning, soon after sundown, the individual polyps which make up the living coral colony begin to expand as sperm packets are moved up from a space within the underlying coral skeleton. Soon after, bright orange or red eggs are similarly produced and "glued" to the individual sperm bundles, until the sperm is completely surrounded. This is an important component to the spawning process, as these clusters of 9 - 190 eggs around a central sperm core float to the surface due to the high fat content of the eggs. In this way, eggs and sperm from different colonies have a chance to intermingle at the ocean's surface. If the combination is right, fertilization will take place.

Research has shown coral eggs can distinguish among the different kinds of sperm present during the mass-spawning. Eggs reject sperm which originate from their own parent colony but readily accept sperm from a different individual of the same species. In controlled experiments, researchers have also observed sperm from one species fertilize eggs of related but different species. This process is called hybridization, and such crosses may be one reason for the astounding diversity of coral species.