Written by Ayla Sage
In my last post, “Love is in the Water – Coral Spawning Explained”, I addressed the methods corals use to reproduce and touched on the annual phenomenon that is mass coral spawning. During a mass spawning event, corals of the same and different species release their eggs and sperm into the water column. Once fertilized at the ocean surface, free-swimming larvae form and fall back into the water column to find a place to permanently settle and begin producing a colony. However, you may ask yourself, how do they know when to simultaneously release these gametes?
Mechanics behind mass coral spawning
It is still not fully understood how mature corals are able to release their gametes all at the same time. So far, it’s thought that corals know when to release their tiny egg and sperm bundles on cues from the lunar cycle and surrounding water temperature [1]. They typically spawn during the warmer months of the summer season and correspond by species to different phases of the lunar cycle. The spawning months differ through the year depending on the geographic region. Other factors that have been found to most likely affect the timing include solar irradiance, wind, and time of sunset [3]. What’s left in the end is a blizzard of colorful flakes cascading in white, yellow, red, and orange.
Why synchroneity is important
The synchrony of gamete release is crucial for corals. The gametes of most coral species are only viable for a few hours, so when they enter the “blizzard” they are more likely to become fertilized quicker [1]. Since the gametes are full of fatty substances called lipids, they are able to float up slowly to the ocean surface and meet to begin the fertilization process. If these gametes are not fertilized and turned into larvae in time, they will be eaten by a number of other organisms and become part of the food web.
Effects of climate change
As we know, destructive human activities are beginning to drastically change the environment we live in. Some negative effects of climate change are readily apparent, however others not so much, especially those harming the marine environment. The changes are altering reproductive phenology and species viability for many marine organisms, but here we are focusing on corals. Coral reproductive phenology has not only shifted to a different time period relative to what has been historically known but also has lost its pronounced synchrony [3]. This dramatically reduces the success of fertilization and impacts subsequent generations of corals, further reducing coral cover on our reefs.
As was briefly mentioned, corals rely heavily on environmental cues, such as temperature to know when to release their eggs and sperm. Temperature allows corals to determine the exact month of when to spawn [3], and since seawater temperatures are now continuing to rise, corals are beginning to spawn earlier than what was typically seen. Moreover, the lunar cycle is believed to cue the exact night, and the sunset to cue the exact hour [3]. It is very difficult to replicate these factors in a lab setting to promote corals to spawn ex situ, but it has been done before.
Besides rising temperature, there are other human induced factors that may be altering the effectiveness of the cues mentioned above. It’s possible that light pollution may be “confusing” the corals. This light coming from nearby coastal cities may be causing the corals to think that the moon is in a different period of the lunar cycle than it actually is. Inducing a pseudo lunar phase would cause corals to spawn at different months and nights rather than specific hours which is what has been observed in studies [3]. Another plausible driver altering synchrony may be hormonal pollutants released into the water. These anthropogenic chemicals can disrupt the endocrine system of these animals. It’s not distinctly determined how these fluxes of herbicides, pesticides, plastic, sewage, etc. are altering corals reproductive systems, but it is very likely that they are [3]. Lastly, there is noise pollution. Although noise pollution doesn’t affect the synchronous release of gametes, it does affect the settlement behavior of the free-swimming larvae. It’s been found that coral larvae respond to acoustic cues possibly allowing them to detect preferred settlement locations from far distances [4]. The sounds they hear are commonly produced by fish and crustaceans moving about the reef. Noises coming from boats, jet skis, construction, and even terrestrial sources could be disrupting these sound frequencies and making it more difficult for these coral larvae to find their forever homes. As coral cover continues to decrease, the alleviation of these pollutants may gain further urgency.
So what can we do?
There are small things that passionate readers like you can do to help make a change! A good starting point is getting educated on the matter and understanding the impact the individual can make such as reducing your carbon footprint (i.e. carpooling, no single use plastics, reusable grocery bags, etc.) and becoming an “Ocean Steward” [7]. Besides being an ocean steward and making environmentally conscious choices in our daily lives, there are non-government organizations you can support through volunteering and donations that are making moves to help assist coral reef conservation and longevity. SECORE International is an example that is actively working towards restoration efforts [6]. Their name SECORE stems from the phrase “SExual COral REproduction” and they have chosen it because this is what their efforts are majorly focused on. Teams will go out on dives at night when they predict coral spawning will occur. While in the field, they collect gametes from various sites in the Atlantic/Caribbean and take them back to the lab to ensure fertilization. Once fertilized, they allow the free-swimming larvae to settle onto their uniquely patented tri-shaped stones which can be seen in Figure 3 below. These stones have the ability to be out-planted back onto coral reef sites which not only increases coral cover, but heightens genetic diversity as well. Depending on the initial conditions of the settling larvae, some may be shaped to have resistance to coral mortality factors such as thermal stress and disease.
Conclusion
All in all, regardless of the exact cause leading to declines in spawning synchrony, it is only going to become increasingly important that we as a human community begin to notice these subtle changes. With bleaching events occurring more regularly, coral reefs are facing a difficult time in their attempts to recover since they are such slow growing animals. So as spawning synchroneity decreases, a dearth of new coral recruits will lead to aging populations that lack vital juvenile replenishment. As studies enhance our understanding of coral conservation, we may be able to find more ways to further assist coral evolution and advancement. Fortunately there are methods that are well studied and are in their beginning stages of implementation to ecologically assist coral growth.
SECORE International is an incredible example of a group that shows us that not all hope is lost for the survival of our coral reefs. They, and groups are alike, are doing everything they can to increase the likelihood of survival for our coral reefs. Just remember, you do not have to be members of these organizations to make a difference. It really takes a village, so if you can, try to go out there and do some good today!
References
[1] NOAA. What is coral spawning? National Ocean Service Website.
https://oceanservice.noaa.gov/facts/coral-spawning.html
[2] Espitia, Paola. “Moon’s Over Miami: Coral Spawning Under the Stars.” Ola’pi Creative, Ola’pi Creative, 29 Aug. 2017, olapicreative.com/blog/2016/8/22/moons-over-miami-coral-spawning-under-the-stars
[3] Shlesinger, Loya_2019_Breakdown in spawning synchrony A silent threat to coral persistence
[4] Vermeij MJA, Marhaver KL, Huijbers CM, Nagelkerken I, Simpson SD (2010) Coral Larvae Move toward Reef Sounds. PLoS ONE 5(5): e10660. https://doi.org/10.1371/journal.pone.0010660
[5] Lirman and Schopmeyer (2016), Ecological solutions to reef degradation: optimizing coral reef restoration in the Caribbean and Western Atlantic. PeerJ 4:e2597; DOI 10.7717/peerj.2597
[6] “SECORE International: Series 04: Why I Care for Coral Reefs.” SECORE, 17 Nov. 2020, www.secore.org/site/newsroom/article/series-04-why-i-care-for-coral-reefs.226.html.
[7] “Ocean Steward and Marine Ecologist.” Ocean Steward, 2020,
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