In Response to the Wonder that is Coral Spawning
On the 11th and 12th nights of September, the CIEE team and I were fortunate enough to experience the spectacular phenomenon of coral spawning. Around 6:30 each night we splashed at Cliff diving site in Kralendijk Bonaire, and then again around 10:00 pm. As the sun went down and the coral polyps came out to play; the magic started to happen around the reef. The colonies synchronize the release of their gamete bundles, resulting in a breathtaking display of nature on the reef at its finest.
On our particular dives, we saw what Dr. Franziska Elmer (a seasoned diver) referred to as “the best coral spawn she had ever seen”. This spectacular display included the spawning of massive corals Orbicella annularis, Orbicella faveolata, and Montrastrea cavernosa. Montrastrea cavernosa is a dioecious colony, meaning that they are either male or female from colony to colony. The spawning of these dioecious colonies that we observed were from male colonies which resulted in the large polyps appearing like mini volcanoes as they released their sperm into the water. The Orbicellacorals, on the other hand, are hermaphroditic corals meaning that they release gamete bundles that have the egg and sperm packaged within them. These gamete bundles pop out from the coral polyps and float up into the water column. For me, the spawning of the Orbicella spp. could best be described as an underwater version of the Loi Krathong Lantern festival in Thailand (similar to the lantern scene from Disney’s Tangled). All in all, for lack of better words I would describe the coral spawning event as a perfect mix of science, nature, and magic.
Rachel Fuller, Student of CIEE Bonaire Fall 2017
So What is it and Why does Coral Spawning Occur?
Coral spawning events are not only a spectacular sight for any diver lucky enough to take part but are also an essential and intricate process necessary for a healthy reef ecosystem. It is a feat of timing as coral colonies across the reef synchronize to release their gametes on the same night. This synchronized release increases the chances of reproductive success for corals and is tied to the lunar cycle and yearly temperature patterns.
Coral spawning is a form of sexual reproduction utilized by a wide range of corals. The parent colony produces gametes, this process ranges in cost to the organism as sperm requires only a few weeks preparation to produce while eggs can require upwards of 10 months (Richmond & Hunter 1990). Corals can be hermaphroditic, producing both eggs and sperm within the same colony, or gonorchoristic, in which a colony produces only eggs or sperm and relies on neighboring colonies to produce the other gamete (Richmond & Hunter 1990). In either case, the mixing and subsequent fertilization of gametes may take place within the coral or externally in the water column. For our dives broadcast spawners, that release their gametes into the water column, create a spectacular show. With a bit of luck, male and female gametes will meet in the water column, fertilize, and begin zygote formation to become free-floating coral larvae. Each larvae has a unique recombination of their parents DNA. This genetic recombination offers evolutionary advantages, as genetic variability allows a population to respond and adapt to changing environmental conditions over time. However, a tradeoff is that it will take decades for the coral larvae to grow into a colony large enough to reproduce again.
By John DeBuysser, CIEE Fall Intern
Richmond RH, Hunter CL (1990) Reproduction and recruitment of corals: comparisons among the Caribbean, the Tropical Pacific, and the Red Sea. Mar Ecol Prog Ser. 60: 185-203.