Coral Spawning in the Maldives 2023 Q3-Q4 Updates

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

July 2023 Observations – Gametogenesis

Tracking Gametogenesis

At Landaa during July, we surveyed both wild and frame colonies of mature Acropora latistella in preparation for the upcoming full moon on 1 August. White and pale gametes were observed.

Our observations at Kuda Huraa this month included:

  • White eggs were found in six Acropora species across the Starfish, Blue Hole, and Water Villas sites
  • White, pale, and pigmented eggs were found in Acropora latistella colonies located in the Starfish and Blue Hole sites
Reefscapers coral gametogenesis Acropora plantagenea

Coral gametogenesis – white eggs in Acropora

August Observations – Coral Spawning (presumed)

Our observations at Kuda Huraa this month included:

  • During August, white coral gametes were observed in 41 coral colonies across eight different Acropora species
  • In July, several colonies of A.latistella had been identified with white and pigmented eggs. In August, when these colonies were revisited, most had since lost their eggs after a presumed spawning event.

At Landaa, with spawning season approaching, we were checking colonies of various Acropora species for the development of gametes. Colonies of A.latistella have shown varying degrees of pigmentation since July; in mid-August, two colonies were observed with pale-pigmented gametes, while most others remained white/pale. In total, white eggs were recorded in nine different Acropora species, including one specific colony of A.secale that had spawned in April 2023.

Presumed Coral Spawning Event

  • 1 August full moon – spawning checks were carried out over the period, but we did not observe any spawning, or loss of gametes.
  • 16 August new moon – no spawning events or loss of gametes.
  • 31 August full moon – from our periodic surveys, we noted that four wild colonies of A.latistella had already lost their gametes, but we did not observe the actual spawning event.

September Observations

  • 31 August (full moon) – we held nightly snorkels around this period, August into September, but no spawning was observed, and no eggs were collected.

With spawning season approaching, we are checking mature coral colonies (mainly on our frames) for the development of gametes. White immature eggs have been now been recorded in 12 different species of Acropora over August to September.

  • Pale pigmented gametes observed in species of Acropora and Sarcophytia
  • White gametes observed in several Acropora species
Reefscapers coral gametes Maldives

White and pale eggs in various Acropora species were observed and documented at Kuda Huraa in September

Reefscapers white coral eggs (Acropora sp)

Top: A.humulis / A.secale
Bottom: A.vermiculata / A.rosaria

October Observations

At Kuda Huraa this month, we conducted 13 hours of spawning monitoring, over six nights across two locations. We recorded mature pigmented gametes in various Acropora species, indicating which colonies would be more likely to spawn in the coming weeks.

At Landaa, we have been checking colonies of a variety of Acropora species for the development of gametes. Across several sites around the island, gradual pigmentation has been observed in various colonies of Acropora species, with colours ranging from white eggs to deeply pigmented eggs. We anticipate spawning will be split between the October full moon and the November full moon.

October Coral Spawning

Reefscapers coral spawning Maldives
Reefscapers coral bundling & spawning Maldives
  • 27 October – coral spawning nets were placed over mature colonies of secale. Following the nights when these colonies did not spawn, the nets were removed at 08:00 to reduce coral stress, and reattached the same evening at 16:00. Our intern, Damaris, has been designing variations on our gamete collection devices and experimenting with fishing weights and drawstrings.
  • 29 October (full moon) – we performed nightly snorkels around this period, looking for signs of spawning. Our checks start before sunset for some species (A.tenuis, A.valida, A.vermiculata) and continue for more than one hour after sunset for all other species.
  • 31 October – 13 colonies were observed to spawn, including: secale, A.humilis, A.plantaginea.

November Observations

Reefscapers large coral gamete collection system

It was all hands on deck at Kuda Huraa this month, when our entire team pitched in to help conduct 68 hours of spawning monitoring, over 17 nights across three locations. We were also pleased to observe spawning for the first time in the mature coral colonies at our Coral Pyramids rescue site. We successfully recorded pigmented gametes in seven Acropora species and one Sarcophyton species, and collected spawning data for six Acropora species (A. tenuis, A. secale, A. plantaginea, A. plantaginea cf., A. humilis, A. globiceps).

At Landaa during October, we had observed gradual gamete pigmentation in seven different Acropora species, alongside the presence of white (immature) gametes; hence, we anticipated split spawning between the months of October and November. All colonies with pigmented eggs and a few colonies with white eggs went on to spawn in early November. Gamete checks in late November revealed that white eggs had subsequently matured.

At Landaa, we experimented with a tarpaulin, suspended underwater to act as a large funnel. This would channel countless coral eggs into a large floating tank at the water’s surface. Here, the eggs are constrained within the tank so cannot disperse, thus encouraging fertilisation. The developing larvae would then settle back down within the confines of the tarpaulin, directly seeding a selected area of reef. This method of “forced settlement” can increase settlement success, as eggs and larvae are not swept away with currents into the open ocean. But this month, our experimentation was unsuccessful as the support pipes proved to be too weak to hold the weights in position. We plan to improve our methodology for next year. 🤞 

Reefscapers large coral gamete collection system
Reefscapers large coral gamete collection system

November Coral Spawning

Landaa Giraavaru

  • 28 October (full moon) – we continued our two nightly snorkels around this period, to check for live coral spawning at some sites, and to check for loss of gametes in tagged colonies at other sites. In total, spawning was recorded in 466 colonies of eight species over a five-day period. For all colonies that spawned (via observation or loss of gametes), measurements of height, width and length were taken to calculate ecological volume.
  • 13 November (new moon) – two nightly snorkels were carried out around this period. A total of 29 colonies of four different species spawned over a two-day period.
  • 27 November (full moon) – two nightly snorkels were carried out around this period. Spawning was recorded in a total of 200 colonies of eight species over a three-day period.

Kuda Huraa

  • Around 28 October (full moon) – 8 nights of spawning
  • Around 13 November (new moon) – 1 night of spawning
  • Around 27 November (full moon) – 1 night of spawning

November Embryogenesis

Kuda Huraa – Embryogenesis in Acropora secale

  • Day 1 – Colonies spawned; gametes successfully collected.
  • 20:30 – mixed eggs into 2 tubs of fresh seawater
  • 21:40 – eggs washed x3 with fresh seawater to remove excess sperm
  • 22:15 – fertilisation confirmed by observation of first cleavage
  • 23:15 – 4 cell blastomeres observed
  • Day 2 – 00:15 – 8 & 16 blastomeres observed
  • 01:00 – 03:00 – morula, prawn chip, bowl stages
  • 10:15 – round stage
  • 21:30 – eggs moved into coral pool and gentle flow introduced
  • 22:15 – tear drop stage
  • Day 3 – 19:30 – free swimming behaviour
  • Day 4 – 11:00 – experiment terminated; fertilisation success rate of 82%

At Landaa Giraavaru

🪸 1. Acropora secale – each day around the October full moon (into November), four coral spawning nets were placed over mature colonies of Acropora secale to collect gametes for ex-situ fertilisation and settlement in our Fish Lab.
Gamete collection bottles were inverted and left to settle, then the contents were divided into three large buckets of seawater to remove excess sperm. This ensured each bucket contained gametes from each of the colonies, to increase the genetic diversity of the mixture and increase rates of successful fertilisation. The buckets were mixed gently to dissociate the bundles before the surface layer of eggs were divided into two open-flow tank systems, with a small sample set aside in a glass beaker to track embryogenesis without disturbing the main tanks. All tanks contained rubble coated with crustose coralline algae (CCA); two tanks also contained pre-conditioned nubbin trays.
We took periodic samples from the beaker to observe embryogenesis stages under the microscope.
➡️ 2-cell (22:00), 4-cell (23:00), 8/16 (01:00), bowl (08:00), round (13:00).
The teardrop stage was observed on day 3, and larvae were visible in the tanks on day 4 morning.

🪸 2. Acropora humilis – two coral spawning nets were placed opportunistically over three colonies of bundling humilis. The colonies went on to spawn, and the gametes were processed as described earlier, allowing for close monitoring of the stages of embryogenesis.
➡️ 2-cell blastomere (22:00), 4-cell (23:00), 8 (01:00), 16 (02:00), round (day 2), larvae (day 4).
Instances of larval settlement are being photographed at intervals to record growth and survival. We have introduced grazers (snails) to one tank per species, to determine the extent to which grazing promotes growth of settlement by reducing competition with algae.

December Observations

Coral Gametogenesis

At Landaa during December, we observed small white eggs in colonies of Acropora humilis.

At Kuda Huraa, we conducted 23 hours of spawning monitoring across three locations (with assistance from the Four Seasons Dive Team), and recorded data for 130 colonies across six different coral species.
Results for Acropora include: pigmented gametes (one species), pale-coloured gametes (four species), white gametes (four species). We also sighted coral slick (floating spawn) at the Back Jetty on one night at the end of the month.

Coral Spawning

12 December (New Moon) – we checked for signs of coral spawning during this period. Several colonies of Montipora were observed spawning (releasing white gamete bundles).

November’s Coral Embryogenesis and Settlement

One month post-settlement, the average survivorship of our juveniles (A.secale and A.humilis) is 75%, with grazed tanks showing slightly increased survivorship.

Reefscapers coral gamete monitoring

Coral gamete monitoring (Kuda Huraa, December 2023)

Reefscapers coral spawning Maldives