Coral Spawning in the Florida Keys and the Bahamas

Coral spawning is perhaps the most spectacular natural event I have ever witnessed.

During only a few nights each summer, many of the Caribbean’s important reef-building corals coordinate to reproduce simultaneously through mass spawning. Using cues from the environment around them such as the full moon, water temperature, and day length, sexually mature colonies release their gametes (eggs and sperm) into the water column in perfect synchronization with other colonies of the same species. 

By releasing their gametes all at once, these adults increase the chances that their eggs will be successfully fertilized.

Each summer since starting my Ph.D., I have had the privilege of witnessing and studying this incredible phenomenon with diverse, multi-disciplinary research teams. In September 2018, I spent a week at the Cape Eleuthera Institute in the Bahamas for coral spawning work with researchers from SECORE International, The Perry Institute for Marine Science, the Nature Conservancy, and other organizations. In July 2017 and 2018, I joined a team of coral researchers from the National Oceanic and Atmospheric Administration (NOAA) for coral spawning work in Key Largo.

Coral spawning research is hard work that requires all hands on deck. Every night for approximately a week after the full moon, the team dives to observe the colonies that spawn, collects gametes from as many of them as possible, cross-fertilizes gametes from different colonies to create a variety of genotypes, raises the larvae until they settle, and uses them for restoration efforts or experiments.

The process begins about an hour before sunset each evening, when our team of scientific divers enters the water to identify the colonies we suspect might spawn later that evening. We mark each colony we intend to monitor with different colored buoys, so that later when collecting gametes we can use corresponding colored jars to help keep each parent colony’s gametes separate from one another. Depending on the location, we focus on a few key reef-building species: Acropora palmata (commonly known as elkhorn coral), Orbicella faveolata (mountainous star coral), and Pseudodiploria strigosa (symmetrical brain coral). Each dive buddy pair also takes advantage of the daylight to orient ourselves within the reef so that we will feel comfortable and familiar with its layout while navigating in the dark a few hours later. 

Then, we return to the boat, set up our dive gear on new tanks, munch on sandwiches, and wait until night falls; that's when the real show begins.

At approximately 10:00 PM, when the sky has completely darkened and stars twinkle above us, we splash again. Equipped with dive lights and glowing cyalumes, we penetrate the pitch darkness of the reef to find the marked colonies. Once we locate them, we watch the polyps closely with our underwater flashlights to look for signs that they are ready to spawn.

Most corals are hermaphrodites, producing both male and female gametes (eggs and sperm). A. palmata, O. faveolata, and P. strigosa polyps all produce both eggs and sperm, packaging them together and releasing them into the water column in single bundles.

Corals aren't the only organisms active at night on the reefs. Bioluminescent ctenophores drift like spaceships through the dark water, nurse sharks munch crustaceans off the sea floor, dinner-plate-sized reef spider crabs crawl atop massive coral colonies, and schools of barracuda circle and watch us silently as we work.

When we begin to see the round bulges appear in the mouths of the polyps (pictured above), we know the gamete bundles are “setting” and the coral is ready to spawn! We race to place “collectors” - conical mesh nets with lead line at the base and an inverted jar at the apex - over the colonies. All at once, only a few minutes after the bundles set, the polyps release thousands of them into the water column. Positively buoyant, they float lazily upward while rocking side-to-side in the gentle surge. One by one, the tiny pinkish bundles trickle into the jar at the net’s apex.

Once the bundles fill about 20% of the jar, we cap it off, replace it with a new jar, and bring the bundles to our team members on the boat. We are careful to keep jars of gametes from colonies of different genotypes separate until we are ready to cross-fertilize them on the boat or back on land. A few hours after being released, these bundles will break up into individual gametes and fertilize the gametes of other colonies.  

Just like that, as suddenly as the bundles appeared and were released, the spawning event is over. We collect our nets and colony markers, haul ourselves back onto the boat, and speed back to land to assess fertilization success and begin experiments with the larvae that will start developing soon.

Back on land, we perform crosses of gametes from the colonies of different genotypes. We stay up late into the night (often until 4:00 am), watching under microscopes to quantify fertilization success and watch as new coral embryos begin to develop into larvae.

Over the next days and weeks, we monitor the survival of the larvae, as corals generally experience high mortality during early life stages. We provide the developing larvae with fresh seawater multiple times each day and remove dead larvae from tanks. We also expose some batches of larvae to a variety of temperatures, sediments, chemicals, and substrate types to compare their performance under different environmental conditions.

Over a period of days, the larvae progress from passively drifting and ball-shaped to actively swimming and oval-shaped. We provide them with terra-cotta or ceramic tiles at the bottom of their tanks for them to settle on. These tiles were preconditioned on local reefs for weeks or months before being presented to the larvae, so that they could develop the biofilm of micro-organisms found in a natural reef environment that serve as positive settlement cues for coral larvae. Finally, the swimming larvae sink out of the water column and down to the tiles, where they select their permanent homes and metamorphose into polyps with mouths and tentacles.

Once the majority of the larvae have settled, I transport batches of these new recruits back to Miami where I maintain them in the Coral Reef Future Lab’s indoor recirculating seawater tanks at the University of Miami. Here, I conduct experiments to track their acquisition of symbiotic algal partners and encourage them to preferentially take up thermally-tolerant Durusdinium by manipulating biotic and abiotic conditions around them. Read about these experiments here.

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