By: Sam Gilchrist
Once a year during August, many hard corals such as staghorn and elkhorn, under-go a natural phenomenon releasing their gametes into the water column known as coral spawning. During this process different genotypes from multiple coral colonies are able to sexually reproduce with the hopes of seeding downstream reefs with brand new coral recruits. This year’s spawning event attracted nearly 50 scientists and researchers to Coral Restoration Foundation’s Coral Nurseries to observe and take part in the process. The exact time of the release of gametes is synchronized to a specific time that can be predicted fairly accurately. But what is it that triggers these massive spawning events?
There are multiple environmental cues that cause these corals to reproduce. First of all, they require a slow and steady increase in water temperature. This triggers the eggs and sperm cells to develop within a mature coral. Secondly, for species such as Acropora and Orbicella, annual spawning will occur around or during the time of the full moon in August. There is currently a large amount of research examining why it is that the full moon causes the corals to spawn. Although the specific reason is still mostly unknown, it has been theorized that the corals have photoreceptors that can detect specifically when other corals are releasing their gametes. The full moon theoretically triggers a chemical release that neighboring corals sense. The synchronicity of the release is essential to the success of the reproduction. The greater the time frame of coral colonies releasing gametes, the less of a chance of survival. If the hard corals can sense this through the water, this suggests that proper water quality, free of excess nutrients and pollution, will also contribute to the success of reproduction. Additionally, the nightfall enables spawning among many corals. Since they only spawn during the nighttime, this suggests that the time of day which in turn affects the semidiurnal tides is linked to the timing of spawning.
The type of spawning that elkhorn (Acropora palmata) and staghorn (Acropora cervicornis) corals as well as massive corals such as Orbicella annularis and Orbicella faveolata demonstrate is called broadcast spawning. Broadcast spawning is a magnificent sight that entails coral polyps releasing egg and sperm gametes to float through the water and mix. After they settle on the reef and begin to grow into larger organisms. It takes a couple of weeks of drifting with the water currents for the larvae to settle on the substrate. Also, not all gametes that react will successfully grow into corals. There are many other factors that play a role in survival such as sea temperature, ocean acidification, predation, pollution, substrate suitability, and degraded water quality.
Another type of coral that Coral Restoration Foundation works with is blade fire coral. Although fire coral can reproduce both sexually and asexually, it’s method of spawning is a bit more complex than these other hard corals and is not correlated to the full moon or any lunar cycles. Fire coral, with the scientific name of Millepora, possesses cup like structures called ampullae. These ampullae release small planktonic animals called medusa that can swim on their own. Medusa only live for a few hours but can produce both eggs and sperm. The gametes react in the water and grow into larger organisms usually only taking about 20 to 30 days!
Although observing and measuring spawning events can take a great deal of patience and timing, it is a beautiful work of nature that is essential to many populations.
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Gambino, Megan. “A Coral Reef’s Mass Spawning.” (2009): n. pag.Smithsonian.com. Web. 23 Aug. 2016.
Thomas, Abbey. “Once a Year, Tiny Cells Spew from Corals Causing a Thick, Pink Slick in Tropical Waters All around the World in an Amazing Natural Phenomena Called Coral Spawning.” Hard Core Spawn (2002): n. pag.Abc.net. Web. 23 Aug. 2016.