Coral Spawning: the Success of Coral Restoration

As you might have seen on our various social media channels, our teams witnessed another coral spawning event! This time, at our partner resort, the Sheraton Full Moon Resort & Spa (North Malé Atoll, Maldives) where we observed three different species of Acropora corals spawning over two days!

Coral spawning Acropora Reefscapers Sheraton Maldives (c) Kate Sheridan

Mass coral spawning on 20.11.21 in the North Malé Atoll, Maldives [photo by Kate Sheridan]

So, how exactly do corals reproduce?
And why is coral spawning so exciting?

Most corals are hermaphrodites and produce both male and female reproductive cells. The most common method of reproduction among corals is spawning. Most coral colonies only spawn once a year, making it incredibly rare to witness.

Corals are what is known as ‘broadcast spawners’ meaning they reproduce by releasing great numbers of sperm and eggs in unison, into the ocean to distribute their offspring over a large geographic area. Coral spawning can occur as a mass, synchronised event whereby several species spawn on the same night around the same time, and so thousands of coral eggs and sperm can be seen being released into the water.

The sperm and eggs then make their way to the surface where they join, forming free-floating larvae called planulae. Mortality of eggs, sperm and planulae during this time is very high. They encounter several threats, including predators or strong currents separating the sperm and eggs. However, mass spawning events increase the likelihood of successful fertilisation because gametes (reproductive cells) tend to only be viable for a few hours.

Coral spawning Acropora Reefscapers Sheraton Maldives (c) Amélie Carraut

Mass coral spawning on 20.11.21 in the North Malé Atoll, Maldives [photo by Amélie Carraut]

The next stage coral spawning stage is ‘settlement’

Planulae can float at the surface for several days, until returning to the bottom where they settle, if conditions are right. In most species, planulae settle within a couple of days, but the longest record of planulae free-floating was 3 months! Following successful settlement, planulae metamorphose into polyps, form colonies, and begin to grow.

Reefscapers’ coral restoration work increases the abundance of coral in the area, thus increasing the likelihood of successful spawning. Hence, we were incredibly excited to find eggs in our colonies.

The team at the Sheraton Full Moon Resort and Spa first noticed eggs in their colonies in October. When we plant a new coral frame, we take a coral fragment from one of our older, larger colonies. This method does not damage the existing colonies and allows for a new colony to grow on a different frame. One day while collecting coral fragments, we noticed the presence of eggs.

As eggs grow and the time of spawning gets closer, they develop darker pigmentation. When our teams first noticed eggs, they were relatively white. However, by the time of spawning they were mature and pink.

Coral eggs Reefscapers Sheraton Maldives (c) Amélie Carraut
Coral eggs Reefscapers Sheraton Maldives (c) Kate Sheridan

1) Our team first noticed white eggs inside fragments of Acropora secale [photo by Amélie Carraut]
2) About a month later, the eggs had developed darker pigmentation, ready to spawn [photo by Kate Sheridan]

Coral spawning events are timed with precision

Corals utilise various environmental cues to know when conditions are favourable for spawning. These cues include the lunar cycle and spawning typically occurs following a full moon. Spawning also tends to occur around low tide, especially where the difference between low and high tide is larger. Spawning typically occurs at night, and water temperature and winds also play a role in coral spawning. Furthermore, some species rely on chemical cues as opposed to environmental cues. There is a lot to consider when trying to predict coral spawning, and the process if complex. However, this makes it even more special to witness!

When corals are ready to spawn, they start bundling. Before releasing their sperm and egg bundles, they become visible inside the polyps. Bundling can last for a couple hours, where the bundles become more and more visible until you notice they have started to be released.

Overall, across various Atolls, our Reefscapers teams have witnessed several spawning events over the last two months. We are delighted to have had such an incredible experience, and to know that our corals are healthy and thriving. 💙

Coral bundling Reefscapers Sheraton Maldives (c) Amélie Carraut

Bundling occurring. Bundling occurred for over 2 hours on both evenings before spawning started [photo by Amélie Carraut]

Experiments with Coral Larvae Settlement
(November 2021)

During the last few weeks, our Reefscapers coral biologists have been fortunate to have witnessed a total of 9 separate coral spawning events (of 7 different coral species) at different sites across the Maldives. Our team at Landaa Giraavaru successfully harvested Acropora spawn from several of our mature coral frames. In the Fish Lab, we promoted fertilisation of the gametes, and achieved settlement of coral larvae that have now successfully grown into tiny coral polyps!

We think this is the first time that settlement in Acropora species has been recorded in the Maldives! Alongside this achievement, we have also fully documented embryogenesis, and perfected techniques in ex-situ fertilisation.