Gamete release by star coral (Orbicella (Montastraea)franksi) during mass coral spawning. Photo: Hickerson/FGBNMS
Every year, 7-10 days after the full moon in August, the reef-building corals of Flower Garden Banks National Marine Sanctuary put on a fantastic spawning display, one of the most abundant displays in the entire Caribbean. In ways that we still don't understand, each coral species times its gamete (egg and sperm) release for maximum benefit.
Polyps of Montastraea cavernosa releasing egg bundles during spawning. This is a gonochoric species of coral. (Image: FGBNMS/Schmahl)
Most scientists agree that these mass spawning events are designed to allow genetic mixing and dispersal of offspring over large distances. And, the sheer volume of the events allow for the fertilization and survival of a significant number of larvae despite the best efforts of predators.
Polyps of Orbicella franksi releasing egg/sperm bundles during spawning. This is a hermaphroditic species of coral. (Image: FGBNMS/Schmahl)
Each year, sanctuary scientists and researchers from other facilities visit the sanctuary to observe this phenomenon in an effort to decipher even more pieces of the puzzle.
Not all species of coral are broadcast spawners (participate in mass spawning events).
Most broadcast spawners are hermaphroditic, meaning that both sexes are represented in each coral polyp.
Hermaphroditic species release bundles that contain both egg and sperm (gametes). These bundles rise to the surface and break open, allowing for mixing and fertilization.
A few broadcast spawners are gonochoric, meaning that they have separate male and female colonies. (e.g. Montastraea cavernosa, photo above)
Coral gametes floating on the surface of the ocean
In gonochoric species, female colonies release bundles that contain only eggs while male colonies release a smoky, white cloud of sperm. Egg bundles float toward the surface then break open to mix with the sperm for fertilization.
Floating gametes and gamete bundles create a sheen on the surface of the water.
There is a distinctive odor above the water's surface during spawning.
Mass spawning occurs in approximately the same sequence each year.
Timing of the mass spawning at Flower Garden Banks is different from that on other reefs around the world.
2007 mass coral spawning. Photo: FGBNMS Volunteer
At Flower Garden Banks National Marine Sanctuary we have observed the following species participate in the annual mass spawning event and we can predict quite accurately on which night(s) and at what time each species will spawn.