The coral reefs of Flower Garden Banks National Marine Sanctuary produce one of the most visually prolific mass coral spawning events in the entire Caribbean. This is due to the high density and cover of broadcast spawning corals Montastraea, Diploria and Colpophyllia. Multiple species within these groups release gametes (eggs and sperm) into the water over a period of several days, at specific times, separated by species and sometimes by sex.
This event typically occurs 7-10 days after the August full moon. However, when a full moon occurs in early August or late July, or if two full moons occur in August, mass spawning can happen in September instead of, or in addition to August. In some years, prolific spawning events have occurred in both months.
Coral spawning at the Flower Garden Banks was first reported by a recreational diver in 1990. Since then, sanctuary researchers have been observing and documenting the mass coral spawning event and accumulating more precise data on timing and participant species. A summary of these observations is presented in the table below with times in military format.
The primary mass spawning event typically begins on the 7th night after the full moon, in August (and/or September), with the release of gametes by the giant star coral Montastraea cavernosa, beginning shortly after dark, at around 8:30 p.m. (although release has been observed as early as 6:30 p.m.). This species has separate sexes with male and female colonies releasing gametes within a similar time frame.
A male colony of Montastraea cavernosa releasing
sperm during the annual mass spawning event.
That same evening Montastraea franksi and Diploria strigosa release gametes beginning around 9:00 p.m. Both of these species are hermaphroditic, with eggs and sperm packaged together in bundles (Figure 31).
M. franksi can display a particularly spectacular reproductive behavior, where gamete release occurs in a wave-like progression over a colony. Impressive video of this type of gamete release was featured in Howard and Michelle Hall’s IMAX film “Deep Sea 3D”.
Montastraea franksi spawning.
After about 10:00 pm, male and female colonies of the blushing star coral (Stephanocoenia intersepta) begin releasing their sperm and eggs. The females release eggs that look like champagne bubbles to a diver. As the eggs release, the chocolate brown tentacles of the coral polyps withdraw making the colony appears to “blush” due to the white color of the underlying skeleton.
On the 8th night after the full moon, the spawning sequence of the previous evening repeats itself, but with significantly more colonies releasing gametes. This is the peak night of the coral spawning event, and has often been described as an “underwater snowstorm”.
Also on the 8th night, the smooth star coral, Montastraea faveolata, releases its gamete bundles beginning soon after 10 p.m. The separation in the timing between M. franksi and M. faveolata demonstrates the differing reproductive strategies of these two species.
Montastraea faveolata spawning.
Corals are not the only group to spawn at this time of year. Beginning at around 8:30 p.m. on the 8th night, ruby brittle stars (Ophioderma rubicundum) release their gametes. The males aggregate in piles of a dozen or more on top of coral heads for a synchronized release of red smoky sperm.
At about the same time, individual female brittle stars climb to the tops of coral heads and stand on the very tips of their arms, releasing bright red eggs.
Not yet well understood or well documented is what appears to be a mass spawning event of certain sponges. On several occasions, on the morning of the 9th day after the full moon, mass spawning of sponges has been documented at around 9:30 a.m.
Giant barrel sponges spawning.
In 2006, in both August and September, the barrel sponge, Xestospongia muta, was observed spawning in large numbers. Females forcibly release eggs into the water column through their excurrent canals, which end up in “drifts” around the sponge colony. Males release so much sperm that the local visibility in the water column is reduced from well over 30 meters (100 ft) to about 3 meters (10 ft) in a short period of time.
A "drift" of giant barrel sponge spawning material.
On the 9th night after the full moon, gamete release by all of the previously mentioned coral species continues, but at greatly reduced levels, with the exception of the blushing star coral, Stephanocoenia intersepta, which appears to peak on this night.
A second, much larger species of brittle star, Ophioderma squamosissimum, also spawns on this particular evening, from about 8:30-9:30 p.m.
The final major episode of the mass coral spawning event occurs on the 10th night after the full moon. On this night, the brain coral Colpophyllia natans releases egg and sperm bundles beginning around 8:30 p.m. This appears to be the primary night for this species. By this time, most of the other coral species, with the exception of Stephanocoenia intersepta, appear to have completed their spawning effort.
Montastraea annularis also is a participant in the mass spawning event, however, it releases its gamete bundles intermittently throughout the spawning window, and the specific timing of this species is less obvious.
Overall the mass spawning event produces an enormous amount of the gametic material, but there has been no attempt to quantify the amount of reproductive material to date.
Emma Hickerson floating at the surface amid a sea of
coral spawn following a dive.
As odd as it seems, very little predation of the of gametic material has been observed. Besides the occasional observations of brittle stars (O. rubicundum) collecting gamete bundles with their arms, no other predation has been observed.
The first published account of coral spawning was written by Dr. Tom Bright in 1991, and was published in a popular magazine, Texas Shores. Since then, numerous scientific and popular publications have been produced reporting the spawning event.
A case study of the mass coral spawning at Flower Garden Banks National Marine Sanctuary was included in Coral Reefs of the USA published in 2008 by B.M. Riegl and R.E. Dodge (eds.).