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Coral Spawning: Romance Under a Starlit Sky

Participating in a night dive over a coral reef during the height of the summer is an ideal way to spend an evening during one of the hottest months of the year. But once a year, for a very brief period of time, the coral reefs of the Caribbean become truly magical. On the eighth day following August's full moon, many species of coral will spawn. Very few people have witnessed this phenomenon, which was only discovered in 1982 on Australia’s Great Barrier Reef. Paul Humaan, a renowned underwater photographer, describes a spawning experience off Key Largo as "...an upside-down snow storm of iridescent orange, white and red egg sacs and sperm floating towards the surface for a chance rendezvous."

Credit: Florida Fish and Wildlife Conservation Commission

Since this discovery, coral researchers have been able to document that most of the large, reef-building, boulder corals use this strategy of precise, simultaneous release of sperm and eggs. Although corals reproduce by many other means, mass spawnings are probably the most unusual and certainly the most exciting method to observe. Biologists believe that corals have developed this cooperative approach for a number of reasons. A mass-spawning event allows all of the colonies of one species to mix genetically, maximizing the chances for fertilization. Although many fish take advantage of the spawning event to feed unmercifully on the released sperm and eggs, there is such an immense amount of food available during a spawning event that it is believed the predators become overwhelmed with more food than they could ever consume.

No one really understands what factors contribute to triggering a spawning event or how corals synchronize to spawn all at the same time. Moon phase is undoubtedly an important influence because spawning events can be effectively predicted from closely observing the various phases of the moon. Scientists also believe that water temperature, tidal fluctuations, and length of the daylight period may contribute to corals spawning on cue.

Most of the corals that reproduce by mass spawning are hermaphrodites, meaning they have sperm and egg in each individual polyp. When a hermaphroditic coral begins to spawn, each polyp will release both sperm and egg in a bundle that resembles a BB or a small seed. Once this fragile bundle is released, it floats free, slowly traveling towards the surface. Upon reaching the surface, it easily ruptures and breaks apart, hopefully joining a genetic mix with adjacent corals. Although hermaphroditic corals are the most numerically abundant among mass-spawning corals, there are some mass spawners that are gonochoric. Unlike their hermaphroditic cousins, gonochoric corals will release either sperm or eggs, but not both. Gonochoric corals must depend on a neighboring colony of the opposite sex to complete the fertilization process.

Coral colonies sustain some losses every year, either from natural causes or the activities of humans or both. Natural causes can be hurricanes; periods of extreme water temperatures, either unusually warm or unusually cold; algal blooms; and the feeding activities of other reef-dwelling animals. There are many human activities that influence the health of coral reefs, including coastal runoff that affects water quality, damage from anchors, and vessel groundings. Spawning, fertilization, and settlement of the coral polyps must all be successful to offset the losses and maintain the reefs.

Although a spawning period may last for only a few hours each year, the event is critically important to the viability of coral reefs around the world. Future generations of corals are dependent on the success of these spawning events. Tens, or, perhaps, hundreds of years ago, the large parent corals also began life somewhere in warm ocean waters under the light of a setting August moon. Their legacy will be found gently floating to the surface each summer, in a cloud of eggs and sperm, with the potential to become the reefs of tomorrow.