juvenile coral polyps (Acropora humilis) uptake of symbiodinium Reefscapers Maldives

Uptake of symbiodinium in our juvenile coral polyps (Acropora humilis)

Coral Spawning in the Maldives 2022 Q1-Q2 Updates

Please also see our main Coral Spawning reports page, from 2013 onwards, including our pioneering lab work in Oct-Nov 2021.

Acropora millepora coral pale pigmented eggs

Pale pigmented eggs in Acropora millepora

Acropora secale coral pigmented eggs

Pigmented eggs in Acropora secale

Tracking Gametogenesis 2022 Q1

  • During January at Landaa, we observed white eggs in three species of both wild and frame colonies(A. nasuta, A. digitifera, A. millepora) and one frame colony with pigmented A. valida eggs.
  • 16 January – we observed immature oocytes (white pigmentation) and continue to track gametogenesis.
  • 20 January – highly pigmented eggs were seen in Acropora valida on a medium-sized coral frame (LG3859, transplanted May 2019).
  • 22 January – white eggs in Acropora digitifera on a Water Villas frame at Kuda Huraa.
  • 27 January – white eggs in Acropora nasuta (frame KH2444) and A. digitifera (KH2555), Kuda Huraa.
  • 5 February (five days after the New Moon, Waxing Crescent) – A. valida spawned. Unfortunately, we did not witness this event, however, the loss of gametes upon checking the next day confirms the spawning date.
  • 24 February – mature, pigmented gametes were located on one heart frame (LG3505) of Acopora secale, transplanted in March 2018. Spawning will be tracked after the New Moon in early March.
  • 28 February to 10 March – we observed oocyte maturation (pale orange pigmentation). During the first few weeks of March, we conducted ‘inspection’ snorkels along the House Reef, while simultaneously collecting for sponsored frames. This allowed the identification of mature colonies while eliminating the risk of fragmenting unnecessarily.
  • 10 to 23 March – we observed very mature oocytes (deeply pigmented) on various species, including one colony of A. digitifera (LG3455); one wild and one frame colony of A. gemmifera; A. humilis (LG2629 in Blue Hole West) and multiple A. millepora colonies, both wild and frame.
Coral eggs in Acropora millepora Reefscapers

Eggs in Acropora millepora (1) white …

Coral eggs in Acropora millepora Reefscapers

… (2) pale pigment

Coral eggs in Acropora millepora Reefscapers

… (3) deeply pigmented

Coral eggs in Acropora humilis Reefscapers

Pigmented eggs in Acropora humilis

Coral eggs in Acropora digitifera Reefscapers

Pigmented eggs in Acropora digitifera

mature oocytes in Acropora gemmifera Reefscapers

Mature oocytes in Acropora gemmifera

Coral eggs in Acropora humilis Reefscapers

Deeply pigmented eggs in Acropora humilis

Coral eggs in Acropora millepora Reefscapers

Deeply pigmented eggs in Acropora millepora

Uptake of Symbiodinium Zooxanthellae in our Juvenile Coral Polyps

On 5 January 2022 (4 to 6 weeks post-settlement), our juvenile corals (Acropora humilis and A.plantaginea) acquired their zooxanthellae through horizontal transmission (Baker, 2003; Quigley, 2017). Photosynthetic algae (symbiodinium) provide corals with up to 90% of their food source. Due to the high filtration system within our aquaria, we introduced fragments of the same species into the tanks, which we believe was the key to the uptake of symbiodinium. We will test this theory through a controlled set-up during the next coral spawning event, to determine the true uptake capacity of symbiodinium in our aquaria. In addition, we lowered light intensity to reduce opportunistic algal growth that can inhibit coral survivorship by shading from light (Box, 2007).

Each individual coral polyp can begin cellular division at a size of 0.2cm (Vermeij, 2008) as evidenced in our juvenile coral polyps, dividing from one to two/three polyps. Raymundo (2004) described juvenile polyps that fuse within eight months of settlement are seen to increase survivorship, in comparison to those that did not fuse. Cruz (2017) demonstrated the fusion of 31 colonies grown from settled larvae grew into one large colony that was sexually mature after just three years. The high settlement/fusion rates within our aquaria are also evident, suggesting a strategy to obtain larger mature coral colonies more swiftly. This coupled with our asexual propagation technique would enhance genetic diversity, complexity, and abundance of Scleractinian coral within a Maldivian reef bionetwork.


juvenile coral polyps (Acropora humilis) uptake of symbiodinium Reefscapers Maldives

Uptake of symbiodinium in juvenile coral polyps (Acropora humilis)

juvenile coral polyps (Acropora plantaginea) uptake of symbiodinium Reefscapers Maldives

Uptake of symbiodinium in juvenile polyps (Acropora plantaginea)

Juvenile coral polyps now deeply pigmented

Juvenile coral polyps now deeply pigmented (February 2022)

Coral Spawning March 2022

Tidal charts were examined, and lunar cycles were closely monitored. Average wind data and precipitation was compiled (zero precipitation during March). Average sea surface temperature was determined; we will later check our HOBO temperature data (two loggers, located close to the frames, at two different depths).

  • 2 March (New moon) – we conducted nightly snorkels for three days but did not witness any coral spawning.
  • 18 March (Full moon) – at the dive site, coral spawning nets were placed over mature colonies of millepora and A. gemmifera.
  • 18 to 23 March (moon in the ‘Waning Gibbous’ phase, from 99% to 50% brightness) – nightly snorkels were conducted to determine time of bundling and coral spawning. A total of six species were observed to spawn over four consecutive days. In addition, we placed one wild colony of A. humilis into our aquarium to witness spawning within a controlled environment.
Same colony of Goniastrea spawning in different places on 2 consecutive days

Same Goniastrea colony spawning in different places on 2 consecutive days (20-21 March 2022); bundling through to spawning (below)

Coral Spawning Collection & Fertilisation

From our dive site spawning nets, the eggs collected from A. gemmifera did not fully fertilise. This is due to one colony producing a substantial number of eggs, whereas the second colony produced very few.

Previous research has described low levels of fertilisation success based on self-fertilisation (Hayward & Babcock, 1989; Chui et al., 2014).

We placed our coral spawn capture devices over three different frames and one wild colony (total of seven colonies of A. millepora with a minimum of four colonies with varying genetic diversity). We witnessed spawning in both the wild and frame colonies of A. millepora.

  • Following the spawning event, our collection devices were removed and placed into three separate buckets of fresh seawater (to remove excess sperm).
  • The bundles were mixed to dissociate egg/sperm bundles that would allow fertilisation.
  • We placed the egg/sperm mixture into two open flow tanks systems (with base drainage to reduce loss of gametes).
  • The first stages of embryogenesis were tracked to ensure successful fertilisation. We watched the first stages of cellular divisions (from two-cell blastomere to the ‘bowl’ stage).
  • A few days later, we observed the ‘tear drop’ stage and finally planulae developed.
  • Rubble was collected and placed in the tank systems, along with glass slides and tiles, to study the preferential settlement substrates of our coral planulae.

We will check for settlement and begin experiments on rearing juvenile coral polyps at varying temperatures, to determine if we can produce offspring with a higher thermo-tolerance (to increase resilience to elevated ocean temperatures).

Key Findings

  • Six species spawned over four consecutive days:
    A. millepora, A. nasuta, A. gemmifera, A. digitifera, A. humilis, Goniastrea spp).
  • A. millepora gametes collected in-situ and fertilised ex-situ.
  • Fecund mature fragments of Acropora able to spawn, day after adult counterpart (A. gemmifera, A. humilis, A. millepora).
  • To increase awareness about our coral restoration success and spawning events, our Assistant Coral Biologist created a compilation video of our best Reefscapers footage:
    March coral spawning.

Can Fragmented Coral Spawn?

As we fragment mature colonies to check for the presence of gametes, we also utilise these as part of the coral restoration program to create sponsored frames and thus minimise damage. To track competency of sexual reproduction and survival post-fragmentation, we collected Acropora fragments (A. gemmifera, A. millepora, A. humilis – photos below) of varying sizes (2.2cm to 8.8cm) from fecund mature colonies, and placed them in our outdoor open-flow tank system. Interestingly, we observed that each fragment bundled and spawned a day later compared to their adult donor counterpart. A more robust study to determine fragment size and spawning would need to be carried out to confirm these observations at different developmental stages.

Okubo (2007), demonstrated that fragment size does affect oocyte development and the gametogenesis developmental stage:

‘Oocytes in the early vitellogenic stage at the time of fragmentation were resorbed, whereas those in the late stage continued developing. Smaller fragments showed a lower survival rate and histological observations of their gonads revealed resorption of oocytes, suggesting that there was a trade-off of energy between reproduction and survival.’
Okubo, N., Motokawa, T., & Omori, M. (2007). When fragmented coral spawn? Effect of size and timing on survivorship and fecundity of fragmentation in Acropora formosa. Marine Biology, 151(1).

Measuring Coral Colonies (that spawned in March)

Increasing our knowledge on regional coral populations and their reproductive biology is important for assessing coral connectivity. By measuring spawning colonies (callipers and a ruler), we can identify their size to determine when they become ‘mature’ by calculating ecological volume (Height, Length and Width). We measured the colonies that spawned in March, and will measure the April spawned colonies next.

Reefscapers Matthew measuring spawned colonies Maldives
Reefscapers Matthew measuring spawned colonies Maldives
Reefscapers Margaux measuring spawned colonies Maldives

Coral Spawning April 2022

Reefscapers Margaux witnessing coral spawning Maldives

Our coral biologists are studying mass coral spawning events in the Maldives

During April 2022, a total of four coral species were observed to spawn over two consecutive days, with a multi-specific synchronous spawning event five days after full moon around the lowest point of the tide. Lunar cycles were closely monitored. Corals spawned 4-5 days after full moon at the ‘Wanning Gibbous’ stage. Average sea surface temperature was 30°C; data for wind data and precipitation were also compiled.

At the start of April 2022, it seems the average sea surface temperature (SST) increased as the wind speed decreased. As SST decreases the wind speed begins to increase again before dropping. A few days before coral spawning, there appears to be a slight increase in SST, then we observe coral spawning as the SST decreases again. Overall, there does not seem to be a clear trend with wind speed or precipitation.

  • 1 April (new moon) – no signs of coral spawning.
  • 16 April (full moon) – spawning nets were placed over mature colonies of digitifera at our dive site.
  • 16-21 April – tidal charts were examined. We carried out nightly snorkels around low tide (19:00-23:00) to determine the time of bundling and coral spawning. In addition, we placed three wild colonies of digitifera into our aquaria to witness spawning within a controlled environment. Unfortunately, the corals did not fare well, and only spawned sporadically (as seen last month in A. millepora; however, A. humilis spawned in sync with wild colonies on the reef).
  • 20 April (four days after full moon) – we witnessed bundling and spawning in a small section of a single colony of digitifera [LG3922]. (We think the other sections of this colony had spawned last month.)
  • 21 April (five days after full moon) – we witnessed spawning in multiple frame and wild colonies (A. millepora, A. digitifera, A. nasuta, A. samoensis).

Creation of QGIS Spawning database

This month, we created a new QGIS spawning layer to track coral spawning on both wild colonies and frame colonies. This database will allow us to track how many times a colony has spawned over consecutive years by changing colour. This in turn will streamline research efforts and it can continuously be updated.

Bundling in A. nasuta Reefscapers Maldives

Acropora nasuta – bundling

Spawning A. nasuta Reefscapers Maldives

Acropora nasuta – spawning

Coral Gamete Collection and Fertilisation

gamete collection A. digitifera Reefscapers Maldives

Coral gametes are naturally buouyant, enabling us to harvest them as they are released (Acropora digitifera)

On 21 April, we collected coral gametes in situ from six colonies of A. digitifera from four frames at our dive site. After spawning, the collection devices were removed, and the spawn was mixed with fresh seawater. (Excess sperm was not added into this mixture). The concoction was mixed to dissociate gamete bundles and thus allow fertilisation to take place. The gametes were skimmed carefully from the surface and placed into three open flow tank systems with specialised drainage from the base. This was then left untouched, and embryogenesis was tracked. By month end, we witnessed successful settlement.
Coral Settlement

  • 23 March – Acropora millepora gamete bundles were collected during spawning.
  • 14 April – settlement occurred.
  • 29 April – we observed the uptake of symbiodinium.

May 2022 Observations

Reefscapers coral eggs in Acropora digitifera

Coral eggs in Acropora digitifera (May 2022) Kuda Huraa

During May, eggs were observed in three species on coral frames at the Kuda Huraa Water Villas. Interestingly, frames of A. digitifera that were relocated from the Water Villas to the Starfish site in January did not show signs of spawning. We also did not find any eggs in the Gulhifalhu rescue colonies.

  • 18 May – Acropora muricata colonies with pigmented orange eggs.
  • 26 May – Acropora tenuis colonies with pigmented red eggs (KH2404).
  • 23 & 26 May – Acropora digitifera white & mature eggs (KH2590, KH2617, KH2273, KH2054).
  • 30 or 31 May – Acropora digitifera spawning (New moon / Waxing crescent).

Measuring April Spawned Colonies

During May, we measured the 52 colonies that spawned in April. Increasing our knowledge on corals and their reproductive biology is important for assessing connectivity of regional coral populations. We hope to determine when corals mature by measuring the spawning colonies (ecological volume using calipers to measure height-length-width).

Coral Settlement Update

23 March – Acropora millepora gamete bundles were collected in-water using our spawn collection devices.
In our lab, the settled coral polyps continue to grow well, especially the Acropora plantaginea fertilised and settled in November 2021.

Reefscapers coral polyp growth A.plantaginea

Coral polyp growth (Acropora plantaginea)  – fertilised and settled in November 2021, photographed in May 2022 with healthy growth

Reefscapers coral polyp settlement A.Millepora

Coral polyp growth (A. millepora) – gametes were collected (March), settled in our Lab (April), and continue to grow (May)

Key 2022 Spawning Observations

So far this year:

  • At Landaa Giraavaru, we have tracked 41 coral colonies spawning from 7 different coral species (Acropora nasuta, A. samoensis, A. millepora, A. digitifera, A. humilis, A. gemmifera, Goniastrea spp).
  • At Kuda Huraa, we have recorded gametes in 11 coral species, and witnessed spawning events in 4 different species(A. secale, A. plantaginea, A. tenuis, Montipora digitata) since October 2021.
  • We have successfully settled 3 coral species: A. digitifera, A. millepora A.plantaginea.
  • Over the past few years, coral species have tended to spawn four to six days after the full moon (with a few exceptions) on days with zero precipitation.

June 2022 Observations

Tracking Gametogenesis

During June, we observed immature oocytes in A. squarrosa and A. plantaginea. As we near the second coral spawning season here in the Maldives (October/November), it is important we track gametogenesis. Understanding the reproductive biology at a local and regional scale is crucial to uncover how populations maintain fecundity.
Research shows that thermal stress reduces reproductive output of coral species, which in turn could hinder their ability to recover after disturbance. It is crucial we understand the reproductive biology of Acropora species in the Maldives, to better understand life history traits.

Coral Settlement Update

Our Acropora millepora and Acropora digitifera polyps from March/April 2022 are growing nicely. Regular maintenance includes removing any competition (algae) and keeping the tanks clean.

Reefscapers settled Acropora millepora

Coral polyp growth (Acropora millepora)

Reefscapers settled Acropora millepora

Coral polyp growth (Acropora millepora)