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 such as lunar cycle, water temperature, and day length, sexually mature colonies release their gametes (eggs and sperm) into the water column in near 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 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. 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, Shedd Aquarium, the Nature Conservancy, and other organizations.
Coral spawning research is very hard work that requires all hands on deck. The team dives every night for approximately a week after the full moon 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, including Acropora palmata (commonly known as elkhorn coral), Orbicella faveolata (mountainous star coral), and Pseudodiploria strigosa (symmetrical brain coral). Each pair of dive buddies 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, switch our dive gear over to new tanks, snack on sandwiches, chit-chat and wait until night falls; that's when the real show begins.
At approximately 10:30 PM, when the sky has completely darkened and stars twinkle above us, we splash again. Equipped with underwater flashlights and multicolored glow sticks fastened to our gear, we penetrate the pitch darkness of the reef to find the marked colonies. Once we locate them, we watch the polyps closely with our flashlights to look for signs that they are ready to spawn.
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 shimmering silver barracuda circle and watch us silently as we work.
Most Caribbean corals are simultaneous hermaphrodites, producing both male and female gametes (eggs and sperm), packaging them together, and releasing them into the water column in single bundles. After over half an hour in the water, I finally see what I’ve been waiting for; as I hover over a colony, the beam of light from my underwater flashlight reveals unmistakable round bulges in the mouths of each polyp (pictured above). This means that the gamete bundles are “setting” and the coral is ready to spawn! With the help of my dive buddy, I race to place a “collector” – a conical mesh net with lead line at the base and an inverted jar at the apex – over the colony just in time. 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.
In the darkness around us, flashlights illuminate nets over other colonies, all releasing their bundles and spawning in synchrony. Some colonies are too large for the nets to completely cover them, so some of their gamete bundles escape collection and waft around us in the water column. The effect is strangely beautiful, like being inside of a dark snow globe. Although exhausted and cold from repetitive dives late at night, I always feel profoundly grateful to view this natural phenomenon that so few people will ever see. Despite the damage that Caribbean corals have endured due to climate change, pollution, overfishing, and other stressors, these threatened animals successfully interpret cues from nature to reproduce and give reefs a future. Witnessing the start of new life in this important ecosystem, which supports a vast diversity of marine life and sustains growing Caribbean communities, I feel that our research can make a difference. In those moments, I know that I had made the right decision to dedicate my career to studying and conserving coral reefs.
Once the bundles fill about 20% of the jar, we cap it off, label which colony it came from, replace it with a new jar, and bring the bundles to the boat. Onboard, team members on dedicated “surface support” duty are careful to keep jars of gametes from colonies of different genotypes separate until they are ready to cross-fertilize them, diligently tracking which batches of bundles they eventually mix. Not long after being released, these bundles will break up into individual eggs and sperm and fertilize the gametes of other colonies. We want to keep track of which parent colonies created each batch of larvae in order to compare the relative fitness of their offspring over the following days and months.
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 distribute our crossed gamete batches into new containers at lower concentrations and give them fresh, clean seawater. The team stays up late into the night – often past 4:00 am – squinting into the lenses of microscopes to quantify fertilization success and watch as new coral embryos begin to develop into larvae.
Over the next few days, we monitor the developing larvae closely, as corals generally experience high mortality during early life stages. Part of this mortality can be density-dependent; as larvae die, they break apart and release lipids that can trap and suffocate other larvae (Graham et al., 2008). To avoid this problem, we disperse the larvae as much as possible. Researchers at SECORE International have pioneered a method for culturing a large portion of the larvae they collect in floating in-water pools (see below for photos), which provides and a constant flow of fresh seawater and washes out contamination from dying larvae. For the larvae that remain in our land-based lab, we maintain low densities by distributing them among many different containers. We provide fresh seawater and remove dead larvae from these containers multiple times each day.
Over a period of days, the larvae progress from ball-shaped passive drifters to oblong active swimmers. We provide the larvae in SECORE’s floating pools and in the lab with terra-cotta or ceramic substrates for them to settle on (see above for photos). These substrates, usually in the shape of small tiles, plugs, or tetrapods, were preconditioned on local reefs for weeks before being presented to the larvae so they could develop the biofilm of micro-organisms found in a natural reef environment that serve as positive settlement cues for coral larvae. In addition, the substrate surfaces have been designed with small grooves that larvae prefer to settle in to avoid damage from organisms that scrape and graze the reef floor. After a few days, the swimming larvae sink out of the water column, select their permanent homes by attaching to the substrates we provide them, and metamorphose into polyps with mouths and tentacles.
Finally, after the SECORE and NOAA teams have cared for settlers for several weeks, they are outplanted on their substrates back to a local reef to rejoin the native population. Outplanting sexually-produced coral recruits help to seed degraded reefs with new individuals and new genetic diversity, ultimately helping combat reef decline.
So why do we do all of this? Why do we deprive ourselves of sleep for a week or more at a time to collect and rear things we can barely see with the naked eye? Read Part 2 of this article - “Starting Them Young: Enhancing Survival and Success During Coral Early Life Stages” - to find out!
(All photographs in this article provided by Liv Williamson unless otherwise noted.)