Coral spawning – ‘bundling’ of gametes
Introduction to Coral Spawning
Reef-building corals can reproduce both asexually (via fragmentation) and sexually (via ‘brooding’ or ‘broadcast’). Hermaphroditic broadcast coral species (such as the Acroporidae) reproduce by releasing bundles of gametes (eggs and sperm). These bundles contain fatty lipids and are therefore buoyant, allowing them to travel to the surface of the water column where fertilisation takes place.
This spawning cycle is one of nature’s most spectacular events and only occurs once or twice each year, on cues from water temperature, wind speed and the lunar cycle. A rapid increase in water temperature has been shown as a predictor in coral spawning as it helps mature gametes. Wind speed, notably low wind speed, has also been shown as an indicator to optimise fertilisation.
Coral species are sessile organisms that must synchronise their gamete release to increase chances of fertilisation (gametes only survive for a few hours). Mature colonies develop gametes within their tissues over several months in a process known as gametogenesis, a timely process that utilises a substantial amount of energy and is likely why they only spawn once a year.
This phenomenon has been well-documented around the world (Australia, the Caribbean), however, there is little documentation from the Maldives about species-specific spawning and time of year. It is imperative that we begin to understand synchrony and timing of spawning here in the Maldives, with important implications for the management of coral reef ecosystems and identifying the health of reefs.
In case you missed it, catch up with our October spawning report.
Coral Spawning Updates in the Maldives, November 2021
Following on from October’s Coral Spawning event here in the Maldives, we continued to expand our knowledge on coral spawning synchronicity and timings by researching various historical spawning events around the world. This desk-based research revealed that spawning can take place at various lunar phases, not just the full moon. This information proved highly important as we tracked November coral spawning over two lunar phases.
- Howells et al., (2014) – various Acropora corals spawned around the new moon (Gulf of Oman).
- Bouwmeester et al., (2021) – gamete release occurred one to two days before the new moon.
- Mangubhai & Harrison (2008) – various Acropora corals spawn at different times of the lunar cycle (Kenya).
- Harrison & Hakeem (2007) – asynchronous spawning over multiple lunar cycles (Maldives).
Our Reefscapers team has been tracking coral spawning events here in the Maldives since 2013. We are continually expanding our knowledge of species-specific coral spawning, adding to our long-term temporal database.
You might also be interested in our short reports and photos from previous years, Coral Spawning 2013-2020.
Identifying Mature Coral Colonies
Maturing gametes in Acropora corals – range of colour pigmentation (Kuda Huraa, November 2021)
- 2 November – gametes present within Acropora species (A. secale, A. valida, A. squarrosa, A. retusa, A. plantaginea). Primary observation revealed pale pigmented eggs, located in coral colonies attached to our frames.
- 17 to 19 November – we observed a change in egg pigmentation from pale to highly pigmented and rounded eggs (A. squarrosa, A. plantaginea).
- 23 November – an additional survey revealed highly pigmented eggs (A. humilis and A.tenuis).
- 27 November – on the natural reef, we observed one colony of gemmifera with pale pigmented gametes (this year, we have not witnessed spawning of wild colonies).
Colonies with eggs were tagged with red/white tape for easy identification. HOBO temperature loggers were kept in the same location as October, to use as a direct comparison (one at 4 m and one at 1 m depth, based on low tide).
Collecting Coral In-situ
As Acroporidae coral gametes are buoyant, they float to the surface and can therefore be collected using a variety of methods. Collection devices that were created in October were utilised again this month. Nets were placed in-water between 15:00 – 18:00 (MV time) on tagged frames during predicted spawning dates. Attachment took 10 minutes per net, utilising conventional SCUBA or freediving, and four nets were placed at various locations at our Dive Site. Plastic bottles were labelled brightly, to keep track of the gametes before fertilisation.
- LG2681 (transplanted January 2016) – several mature colonies of A.plantaginea.
- LG0619 (re-transplanted in 2019) – one mature colony of A.squarrossa.
- LG3868, LG3865, LG3890 & LG3907 (transplanted February 2019) – mature A.tenuis colonies.
- LG3908, LG3911, LG3890 (transplanted February 2019) – mature A.humilis colonies.
Each frame had been transplanted with Acropora species of varying genetic diversity (on different dates throughout February 2019), providing a good selection of colonies for ex-situ cross-fertilisation.
- 2 to 11 November – nightly snorkels took place around the New Moon (after observation of coral gametes).
- 17 to 27 November – nightly snorkels around the Full Moon.
Each night, we snorkelled from 18:00 – 22:00 to check for ‘bundling’ and the presence of eggs in our collection devices. Tidal charts identified the lowest point of the outgoing tide. From October’s observations, we predicted bundling would be seen 30 minutes prior to the lowest point of the tide, a key indicator that the coral colonies would spawn that evening.
Interestingly, November’s New Moon was a ‘Super New Moon’, meaning the Moon is close to the Earth during its elliptical orbit (382,500 km) and appears slightly larger in size when viewed from Earth. On 4 December, a ‘Super New Moon’ is even closer to the Earth (359,856 km). By contrast, the 19 November Full Moon was a ‘Micromoon’ which coincides with apogee, the furthest point away from Earth during orbit.
Coral ‘bundling’ of gametes
Coral spawning – gametes released
Coral spawning – collecting the floating gametes, released from mature colonies growing on our coral frames.
Photo shows Acropora humilis releasing gametes in the lagoon at Landaa Giraavaru, Baa Atoll, Maldives (November 2021)
- 5 November (New Moon) – spawning event, possibly Acropora secale.
We observed gamete bundles all along the surface and in the water column close to the beach. We collected the gametes to fertilise ex-situ and observe the embryogenesis cellular divisions prior to settlement. Unfortunately, we were too late to observe which coral species had spawned, (however, the next day, A.secale colonies no longer had gametes, whereas the other species did, and were not at the bundling stage).
- 23 November (four days after Full Moon) – bundling observed.
Bundling was observed in in A. plantaginea. Spawning lasted 10-15 mins; we successfully harvested gametes in our collection devices.
- 25 November (six days after Full Moon) – bundling observed.
Bundling was observed in A. humilis and A. squarrossa (the latter spawned early). Again, we were able to successfully collect gametes.
Fertilisation & Embryogenesis
- 5 November (New Moon) – unknown Sceleractinian, possibly Acropora secale.
Gamete bundles were mixed in a glass beaker to rinse away the sperm and separate from the eggs. Next, the eggs were placed into a beaker with fresh seawater. We proceeded to watch and track cellular division and embryogenesis. The following day, aeration and CCA were added into the tank to provide planulae chemical cues to settle on substrate.
- 23 November (four days after Full Moon) – Acropora plantaginea.
Gamete bundles from four coral frames (consisting of nine colonies) were mixed ex-situ into four separate buckets of fresh seawater. They were then mixed in a glass beaker to rinse away the sperm and separate it from the eggs. It is imperative to stir gently to break up egg/sperm bundles and allow fertilisation to begin. Post-mixing, eggs were separated from the sperm mixture and rinsed, then placed into four open flow tank systems (adjusted to drain from the base of the tank). To accurately track embryogenesis developmental stages, we worked overnight from 18.45 to 07.30. On 26 November, we observed the planulae stage.
- 25 November (six days after Full Moon) – Acropora humilis.
Gamete bundles were collected in-water using collection devices. Two frames consisting of four colonies were mixed ex-situ into two separate buckets that contained fresh seawater. Post-mixing, eggs were separated from the sperm mixture and rinsed, then placed into an open flow tank system (adjusted to drain from the base of the tank).
We proceeded to track the cellular embryogenesis, and on 29 November, we observed the planulae stage.
Acropora coral embryogenesis
Coral Larvae Settlement Success
- A. secale gametes (collected 26 October) successfully fertilised ex-situ, and subsequently settled four days later (30 October).
- The unknown sub-samples (collected 5 November) were successfully fertilised; on 9 November we added CCA and pre-conditioned rubble to the open flow tank system. Larvae settled on 11 November, two days after reaching planulae larval stage.
- A. plantaginea successfully reached the planulae larval stage on 26 November; we added CCA, pre-conditioned rubble, a mini coral frame, and unconditioned tiles to the open flow tank system.
Two days later (28 November), thousands of planulae successfully settled on all substrates (including the glass and PVC pipe), increasing our knowledge on settlement success on a variety of substrates (video, below).
- A. humilis successfully reached planulae larval stage on 29 November. CCA and pre-conditioned rubble were added to the open flow tank system; planulae settled on 30 November.
Video: After successfully collecting and fertilising coral gametes gametes, this video shows the coral planulae searching for settlement locations on our substrates. This is the first time that ex-situ settlement (Acropora species) has been achieved within the Maldives.
Improvements to our Open Flow System
Following the successful settlement of October’s coral larvae, we decided to mitigate labour time with regard to water changes and filtration systems within our aquaria. We created an ideal standpipe to ensure drainage from the bottom of the tank, and thus maintain a high level of water quality to optimise survival of gametes. This in turn minimised the detrimental impact on the buoyant coral eggs during the extremely fragile cellular division stages.
To create a standpipe, we placed PVC piping over our open flow system, drilled holes at the base, and sealed with 80-micron plankton mesh (glue-gunned to the pipe).
Coral Colony Ecological Volume
The mature spawning coral colonies were measured using callipers (height, length, width) to determine ecological volume. Next year, it will be interesting to note percentage growth rates of colonies, and to look at coral egg density to identify redundancies in size and maturity.
Eyewitness Reports of Coral Spawning
- 26 November – Our resort colleagues reported a large coral slick of floating gametes at Jaavakara reef (Lhaviyani Atoll).
- 26 November – Colleagues on Four Seasons Explorer observed a coral slick in Rasdhoo (North Ari Atoll).
Settlement of Coral Larvae in our Laboratory
Over the course of the last two months, our Reefscapers teams have observed nine separate coral spawning events, in seven different species, at various locations across the Maldives. At Landaa Giraavaru, we successfully collected and fertilised the released gametes in our laboratory, and we are very happy to say that, on 30 October, we achieved settlement of the resulting larvae!
This is the first time that settlement in a species of Acropora has been recorded in the Maldives! Alongside this ground-breaking advancement, we have also documented the stages of embryogenesis, and perfected the ex-situ fertilisation technique!
tiny juvenile coral polyp (Acropora secale) successfully settled on substrate in our laboratory.
tiny juvenile coral polyp (Acropora secale) successfully settled on substrate in our laboratory.
Uptake of Symbiodinium Zooxanthellae in our Juvenile Coral Polyps
During October and November, we successfully collected, fertilised and settled three species of Acropora (A. secale, A. plantaginea, A. humilis, plus one unknown). These broadcast spawners do not obtain their symbionts from vertical transmission from the parent colonies, but from the environment through horizontal transmission.
This demonstrates the ability of Acropora species to acquire and maintain zooxanthellae that differ from those found in their adult populations (Abrego et al., 2009).
The October settlers were able to establish their algal symbionts from the water column, however, the November juvenile polyps were not displaying signs of zooxanthellae (symbiodinium) uptake within their tissues after a few weeks of being in our coral tanks.
We therefore began to feed the corals (every other day) and introduced a few coral fragments of the same species into tanks, to identify whether juveniles could uptake symbiodinium from adult counterparts if not readily available from the water column (due to filtration).
pale-colured juvenile coral polyps without symbiodinium
Uptake of symbiodinium in juvenile coral polyps (Acropora humilis)
Uptake of symbiodinium in juvenile polyps (Acropora plantaginea)
Coral Spawning Database
We have created a new database to record and study spawning data, including a variety of environmental factors such as lunar cycle, tidal information, wind data, temperature, and exact spawning times.
- This will help us to identify trends over time, which will expand our knowledge on coral spawning patterns in the Maldives (currently little-studied).
- We can compare data between locations, to distinguish differences in species synchronicity.
- We hope to better estimate spawning events (within a 1-3 day window) by identifying the initiating cues. It will be interesting to compare the natural reefs with our coral frames.
To maintain data continuity, environmental information will be extracted from the same websites each year:
Key Findings in Coral Spawning
As Sceleractinian (stony) corals are sessile organisms, synchronisation of their spawn release hugely increases chances of fertilisation (coral gametes only survive for a few hours). Mature colonies develop gametes within their tissues over several months in a process known as gametogenesis, which utilises a substantial amount of energy, and likely explains why they only spawn once or twice per year.
This phenomenon has been well documented around the world (including the Caribbean and Australia), however, little is known about species-specific spawning and across Atolls in the Maldives. It is imperative that we begin to understand synchrony and timing of spawning here in the Maldives, as there are important implications for the management of coral reef ecosystems and identifying the health of reefs.
This year, we have expanded our long-term temporal database of species-specific coral spawning in the Maldives. Here at Four Seasons Landaa Giraavaru (Baa Atoll) we have tracked spawning against key environmental factors of nine species of Acroporidae (Acropora aspera, A.secale, A. plantaginea, A. humilis, A. valida, A. gemmifera, A.retusa, A.tenuis, A. squarrossa). All coral species observed spawning were growing on our coral frames, initially propagated from coral fragments that had successfully grown into mature colonies. (We did observe eggs in one wild colony,)
For three of these coral species at Landaa Giravaaru, we have successfully enhanced genetic diversity through the collection of gametes in situ, fertilisation ex-situ, and settlement ex-situ. Moreover, this is the first recorded research for the successful settlement of Acropora species here in the Maldives. We have had incredible settlement success of thousands of coral larvae on a variety of substrates. Additionally, we successfully collected, fertilised, and settled a species of Acropora (likely A. secale) during a collection of gametes from the water column.
Healthy coral juvenile polyps (Acropora plantaginea) successfully settled on substrate, and growing in our laboratory