Studying One of the Ocean’s Most Magical Phenomenon: Coral Spawning

Carly Stines


Corals are often mistaken as rocks strewn across the ocean floor, simply there to provide habitat for other animals. Although this is one of their significant roles in the marine environment, corals are actually animals themselves! Each coral is made up of thousands of miniscule animals called polyps that are related to anemone or jellyfish. Each individual polyp is made up of a sac-like body and an opening surrounded by stinging tentacles called nematocysts.

To protect its soft body from the harsh environment, the polyps utilize calcium carbonate, or limestone, to build a hard skeleton. While their skeletons are white, their vibrant brown and green colors come from a symbiotic algae called zooxanthellae that lives within their tissues. These tiny animals can live individually or in colonies that make up an entire reef.


To put it simply, coral spawning is the sexual reproduction of coral reefs… although it is not that simple. Spawning is an extremely complex event, relying on various external and internal factors, some of which scientists have not yet figured out. However, this mysterious aura surrounding the affair makes it incredibly fascinating. 

The majority of coral species are hermaphrodites, meaning they are both male and female. To reproduce as a hermaphrodite, the corals synchronously release small fatty bundles of eggs and sperm called gametes. This phenomenon is commonly referred to as an “underwater blizzard” because of the sheer number of gametes released from the corals. The gametes float to the surface and unravel so eggs and compatible sperm can meet, and fertilization can begin.

Because the gametes are only viable for a couple hours, the synchrony of the release is crucial. Spawning synchrony is reliant on many environmental factors such as sea surface temperature, tidal patterns, lunar cycle, pheromones released by other corals, and coral genotype, to name a few. This will only happen about once or twice a year per coral colony. 

Once the egg and sperm join together as an embryo after fertilization, they develop into coral larva called a planula. These float around the ocean for days or even for weeks before dropping down to attach to the substrate on the ocean floor. This kicks off the new beginning of a coral colony that will grow at a slow rate 

When a coral egg and sperm join together as an embryo, they develop into a coral larva, called a planula. Planulae float in the ocean, some for days and some for weeks, before dropping to the ocean floor. Then, depending on seafloor conditions, the planulae may attach to the substrate and grow into a new coral colony at the slow rate of about 2-10 centimeters per year. 


Coral spawning in captivity, in aquarium or laboratory environments, is a relatively new sector of research. It is becoming an increasingly important area of study due to the frequent disappearance of coral reefs and its benefits for coral protection and restoration efforts. It also provides researchers further understanding on the reproductive mechanisms of corals, as well as the developments and stressors that affect the beginning of a coral’s life cycle.  

While spawning in captivity is now possible, after the Horniman Aquarium in the UK became the first institution to purposefully reproduce broadcast coral in captivity in 2013, it is even more complex than spawning naturally. Factors such as temperature and lunar and solar cycles that corals depend on to begin their reproductive cycle have to be replicated by technology in closed aquarium systems and monitored by scientists. 


The Coral Reproduction and Evolution Ecology Fogarty (REEF) Lab is run by Dr. Nicole Fogarty, who is interviewed in the video below, at the University of North Carolina Wilmington. The lab uses a multidimensional approach involving fieldwork, laboratory experiments, and molecular biology to obtain a better understanding of the anthropogenic (human-caused) stressors resulting in increased coral death. The lab will use their research to support efforts of coral conservation, restoration, and management. With their focus on coral reproduction and fertilization, they have conducted many experiments in hopes of successfully spawning corals in captivity, and have been the first to do so with many coral species. 

I had the incredible opportunity to be a part of the coral spawning project as a research volunteer in the UNCW REEF Lab in September of 2020. The spawning monitoring process is full of nights as late as two in the morning, and later if they actually spawn, making sure the lights and set-up is running smoothly, and more. But once you see those little tiny bundles rising up from the coral polyps, and you yell for everyone to come see, it is fully worth it! 

REEF Lab Website: 


9/03/20: 10:45pm- 1am 

  • Took shifts with another student to monitor the coral tanks
  • Meditated and focused on breathing I learned in my yoga class while sitting with the corals- they are quite peaceful and don’t talk much 
  • A bit of odd clumpy fluid coming out of some corals due to stress, which can be common 

9/08/20: 11pm- 1am

  • Took shifts with other students to monitor the coral tanks 
  • Lots more fluid coming out of the corals, still not quite sure the reason but easy to mistake for setting (the step before spawning)
  • When not monitoring, I put together labels for the corals in the temperate tanks because the old labels were falling off of their pedestals- only glued my fingers together twice with the superglue

9/09/20: Not my shift, but they spawned!! (see video) 

9/10/20: 9pm- 1:45am 

  • Got there early to take care of the new coral babies collected from spawning the night before—used plastic wrap on the surface of the water to remove dead eggs and excess cells and transferred the babies to new containers 
  • Took shifts with other students to monitor the coral tanks 
  • Put together more labels for temperate corals when not monitoring 
  • Two corals spawned at different times, but neither were viable gametes for fertilization 

SOURCES: NOAA, Florida Keys National Marine Sanctuary, Coral Reef Alliance, Horniman Aquarium 


Carly Monitoring Corals: credits Carly Stines

Coral Spawning Video: credits Kory Enneking 

Coral Babies Video: credits Sharla Sugierski 

Spawning Stills 1-4: pulled from Kory Enneking’s video