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CORAL SPAWNING at FGBNMS

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, Orbicella, Pseudodiploria 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.

A ruby brittle star crawls across a spawning coral head

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.

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Coral Spawning Observations

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.

Coral Spawning Schedule
Species 6th Night after Full Moon 7th Night after Full Moon 8th Night after Full Moon 9th Night after Full Moon 10th Night after Full Moon
Colpophyllia natans       2020-
2132
2030-
2110
Pseudodiploria strigosa   2100-
2215
1935-
2230
1920-
2230
 

Orbicella (Montastraea) annularis

  2223-
2256
2246   2230
Montastraea cavernosa (female) 1950-
2115
2059-
2138
2040-
2200
2104-
2230
 
Montastraea cavernosa (male) 2100 1830-
2127
2030-
2145
2140-
2200
 
Orbicella (Montastraea) faveolata   2221-
2320
2113-
2330
2300-
2600
 
Orbicella (Montastraea) franksi 2145-
2210
2100-
2245
2030-
2245
2100-
2250
 
Stephanocoenia intersepta (female)   2250 2208-
2300
2200-
2240
2215-
2245
Stephanocoenia intersepta (male)   2200-
2220
2218-
2300
2200-
2300
2215-
2245

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7th Night After the Full Moon

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.

Two coral colonies on the reef. The one on the left is spewing a milky substance into the water.
A male colony of Montastraea cavernosa releasing
sperm during the annual mass spawning event.

That same evening Orbicella (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).

O. 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”.

A star coral species releaseing small white bundles of eggs and sperm.
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.  

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8th Night After the Full Moon

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, Orbicella (Montastraea) faveolata, releases its gamete bundles beginning soon after 10 p.m.  The separation in the timing between O. franksi and O. faveolata demonstrates the differing reproductive strategies of these two species.

A coral colony spawning
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. 

A group of ruby brittle stars intertwined on top of a coral head

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.

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9th Night After the Full Moon

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
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.

White sponge spawn settled onto a reef
A "drift" of giant barrel sponge spawning material.

Sponge spawning video is available in our Video Library.

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.

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10th Night After the Full Moon

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.

click on the > to play

A transcript of this coral spawning video is available in our Video Library.

Other Observations

Orbicella (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.

Diver holding onto a blue rope at the surface of the water amid a sea of floating coral spawn
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.

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Coral Spawning Literature

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.).

For more information about these publications, please contact the sanctuary Research Coordinator.

Coral Spawning Basics

For general information about coral spawning, please visit the Coral Spawning page in the Education section of our website.

Spawning Expeditions

Coral Spawning 2010

Coral Spawning 2009

Coral Spawning 2008

Coral Spawning 2007

Coral Spawning 2006

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Orange, branching gorgonian (soft coral) anchored in a bed of sponges and other sea life.
   
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