Coral Spawning (2010) [IMG_0031] 1080

Maldives Coral Spawning 2013

Report from March 2013

We’re approaching that time of year again here in the Maldives for the annual coral mass spawning event – an incredible natural phenomena that will see millions of corals simultaneously release their sperm and eggs into the water.

There are a few indicators as to when these incredible spawning events will happen, however, predicting them is very tricky. First, the coral polyps begin to produce pink or white gametes (small bundles of eggs and sperm, which get released into the water for external fertilisation). We initially spotted the presence of these gametes on 15 March – in both frame fragments and wild colonies of several Acropora species – but there has been no sign of spawning yet, perhaps due to this year’s weather being windier than usual.

collecting Coral Spawn samples (2009)Spawning always happens at night, often around the full moon and after an increase in the sea temperature has stimulated the maturation of the gametes. But predicting precisely which night and at what time is another thing altogether … After hearing reports of some coral spawning in the Maldives over the weekend of the 23-24 March, we have been closely monitoring our lagoons looking for local signs of spawning. Having spotted gametes visible deep inside the skeleton of a few colonies, we know we’re not going to have long to wait! By way of an experiment, this year we have deployed different materials (terracotta tile, ceramic tile and a metal frame) at a depth of 5m in the lagoon to collect any settling coral larvae. We will retrieve these tiles after 2 weeks, in order to observe what species (if any) have settled on them.

Coral Spawning (2010) [IMG_0033] 1080
Updates April 2013

Whilst working on our coral frames on the House Reef at Kuda Huraa, we started to observe the development of gametes in some of them. Over the next couple of days we spotted even more, and on 20 April (five days before the full moon) we went down to the Water Villa area to look for signs of gametes being released into the water. When we first arrived at 21:30h there was nothing out of the ordinary, but just as we were about to leave we spotted a few gametes floating on the surface current. Over the next couple of hours the number of gametes increased and became much easier to see – they often show as a white or pinky-red ‘slick’ on the water surface, and when present in large amounts can be accompanied by a characteristic aroma likened to “sweet fish oil”.

Coral Gametes visible in coral AcroporaSitting on the edge of the jetty like this in the dark did attract some questions from curious guests – so we explained what we were doing, and how unique this one-night annual (sometimes bi-annual) spawning event actually is. Soon we had a small interested group of guests sitting with us to enjoy the rare spectacle.
The following morning on the House Reef we observed that the corals were now missing their gametes – a good sign that the corals had all successfully spawned simultaneously overnight.

Things were slightly different in Landaa Giraavaru however – here, eggs were observed in coral colonies on various days, ranging from 22 April (Favites species) to 26 April (Acropora tenuis) but no single mass spawning was witnessed. In addition, on 24 April a group of night-divers observed the release of a “white cloudy substance” (assumed to be male gametes) from branching corals at a depth of 9m.

Read more about coral spawning here at NOAA.

Maldives Coral Spawning 2014

On the night of 7 November, which was a full moon, we were very lucky to witness and capture on film the coral spawning event at our house reef here at Four Seasons Resort Landaa Giraavaru !
20:30 – we noticed light pink eggs just under coral’s corallites (cup-like skeleton).
20:45 – many corals were seen to be spawning, and we successfully filmed Acropora humilis species as it released spawn on the natural house reef.
We were also excited to witness spawning from our coral frames, and captured this on film for the very first time. Watch the video to see coral frame #LG714 (transplanted on October 2010) successfully spawning, thus proving the reproductive success of our coral transplantation project.

Maldives Coral Spawning 2016

A huge coral spawning event was witnessed on the southern beaches of Landaa Giraavaru in the morning of the 16 April 2016. Although the team went back in the water on the same night until 22h, no direct coral spawning was further observed.

Coral spawning, beach Landaa Giraavaru, Maldives Marine Savers

Coral Spawning in the Maldives 2020

October 2020

Towards the middle of October, we noticed eggs within one of the coral colonies on our frames during a collection event. As the days progressed, we noticed more eggs within different species, again during collection events.

The eggs were white in colour and located deep within the skeleton; just before spawning they should turn pink, indicating maturity.

‘Massive’ coral species are generally hermaphrodites, meaning they produce sperm and eggs simultaneously. Other species produce single sex colonies and are classified as gonochoric. In these single sex colonies, all the polyps in one colony produce only sperm, with the polyps in another colony producing eggs. Fertilisation therefore occurs inside the body of a polyp (hermaphroditic) or fertilised in the water column (gonochoric). Gonochoric activity is classed as a spawning event and relies on environmental cues to stimulate the colonies into mass spawning events.

In the Maldives it has been reported that these spawning events occur twice a year, in March and October, around the full moon.

We have been monitoring the colonies on our house reef every night and have yet to see a spawning event. Full moon occurs on 31 October, so monitoring will continue until the eggs are no longer present. A collection device was constructed and deployed over the colony to harvest a spawn sample.

Coral spawning Maldives Acropora egg collection
November 2020

On 5 November, we were fortunate to witness frame coral colonies as they spawned. After 11 days of careful monitoring every evening, the mature colonies released their eggs and sperm into the water column at approximately 21:30.

We have compiled a short video (below) that shows colonies (Acropora secale, A.rosaria, A.plantaginea) on our frames in the act of synchronised spawning (only previously recorded in 2014).

Various triggers are known to stimulate coral spawning:

  • lunar cycle (coordinates the exact moment);
  • sunset time (corals spawn under the cover of darkness);
  • gradual rise in sea temperature (causing the eggs to mature);
  • tides provide the final cue (Prof. Bette Willis, James Cook).

Sakai et al. (2020) found that Acropora sp. (in Australia and Japan) have the capacity to adjust their development and physiology in response to environmental factors to alter the timing of synchronous spawning. This enables the corals to maximise post-fertilisation survival and therefore reproductive success.

During full moon there is a large difference between high and low tide, meaning greater water movement. Around 5 days after full moon, a neap tide occurs. During neap tides, the difference between high and low water levels is less, resulting in reduced water movement. The corals take advantage of this neap tide, as the reduced water movement increases the chances of the eggs and sperm mixing and fertilising.

This information is corroborated by our observations this year, as coral spawning occurred at low tide, 5 days after full moon. The three tides post-spawning were 14-19cm lower, compared to the tides immediately after full moon on 31 October. Whilst we had expected spawning to occur on an outgoing tide, in fact we observed the opposite. We had theorised that an outgoing tide would increase the distribution of larvae to the surrounding regions, however, it seems that more emphasis is placed on immediate fertilisation as opposed to increased distribution.

Sea surface temperature (SST) gradually increased over the three weeks prior to the spawning event. This has been strongly linked to the maturation of the eggs within the colonies. Spawning occurred at an average SST of 29.3°C.

Coral spawning event Maldives sea temperatures
Sea Surface Temperatures (SST) Oct-Nov 2020
Coral spawning Maldives tidal range 5-6 Nov
Tidal Range (5th – 6th November 2020)

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