Coral spawning Maldives Acropora rosaria eggs

Coral readying to spawn – presence of small white eggs in Acropora rosaria

Coral Spawning

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

Monthly Progress

At Kuda Huraa this month, we’ve been training our newly appointed coral biologist, and we transplanted 3 new coral frames sponsored by Resort guests. We monitored 33 frames at our Water Villas site, and relocated 42 frames from the deep Blue Hole refuge site (where sand movement is an issue) to firm substrate at the House Reef site. We also assessed the health of our special Christmas tree frames from the previous 4 years; rather than ordering a new frame for 2020, we decided to replenish the smallest tree, as the corals are the least healthy.

 At Landaa this month, a total of 409 coral frames were monitored (cleaned, maintained, photographed) mainly at the Blu and Coral Trail sites. We also transplanted 23 new coral frames, thanks mainly to a generous online donation that enabled us to build a B-shaped pattern as a personalised commemoration. Video footage of the whole process – from collection of coral fragments through to transplantation and arranging the completed frames – was sent to the sponsor.

Reefscapers Xmas tree frames
Reefscapers Bissie's reef

Coral Plates in Aquarium One (plates KH01, KH02, KH05) and Aquarium Two (plates KH03, KH04)

  • KH01 – the newly replenished fragments of Acropora digitifera and Acropora millepora have regained a healthy colouration and have now started to encrust onto the plate.
  • KH02 – the Galaxea fascicularis fragments remain healthy overall, and the fragments are fusing with one another (although seemingly at a slower rate).
  • KH03 – thoroughly cleaned of algae. The calcified areas are growing and about to fuse. The fragments of Acropora valida remain healthy and continue to grow, although they haven’t yet calcified onto the plate.
  • KH04 – also cleaned of algae. Only two original fragments remain (due to the loss from algal overgrowth) and of the 11 newly transplanted fragments, 4 have started to calcify onto the plate (photos, below).
  • KH05 – 1 fragment died after bleaching, but the others have now regained a healthy colouration and continue to grow (2 fragments are encrusting).

Time Lapse Experiment to Record Coral Bleaching

A new heater was installed this month to stabilise the water temperature, thus removing the need to add hot water manually (which causes temperature swings). We replicated our coral bleaching experiment twice this month (#4 and #5).

  • Replicate #4 (at 34°C) successfully recorded the bleaching process, but no gas release was observed. Initially, air could be seen being expelled from the colony, however this was attributed to the coral being out of the water during the preparation process and the polyps removing any absorbed air.
  • Replicate #5 (at 33°C) encountered a recording problem, and as a result only captured the initial 3 days of the experiment. Despite this, a small gas release was observed from one radial polyp (photo). From the footage, it does appear that the outer edge of the calyx has sustained some minor damage, which would be in keeping with the findings from the other replicates.
  • Replicate #6 has been started.
Reefscapers coral bleaching timelapse experiment R5

Artificial Intelligence and Coral Identification

During October, we ran our computer model on all the small-sized coral frames in our Four Seasons database, totalling more than 2,300 frames (with 75,000+ coral fragments). With such a large set of data, we can now generate more reliable results, and analyse the program’s success. (These numeric results will change as we improve the accuracy of our models).

The statistical analysis reveals survival rates of the coral fragments within our programs of 31% (after 2 years) reducing to 21% after 4 years. Despite the devastating global coral bleaching events of recent years, the surviving coral colonies keep on growing, producing ever-increasing volumes of live healthy corals.

This data also enables us to improve our positive-feedback loop, and detect the areas where we have the most mistakes, so we can retrain the models for better performance in these specific contexts. To continue to improve the data modelling, we have annotated a further 153 new monitoring photographs, drawing boundary boxes around the coral colonies, plus a further 24 pictures outlining the contours of the metal frame. Both coral detection and frame detection models were retrained with this new data, and we are now in the process of rerunning these updated models on the database.

We are currently writing a scientific paper, which will likely be the first coral restoration study to utilise Convolutional Neural Networks on a colony scale. It describes the main results of our Reefscapers coral propagation program, as well as the novel methods we have used to regenerate our artificial reefs. Our program has been made possible thanks to the thousands of kind donations by our guest sponsors, and the continuing support of Four Seasons Resorts Maldives. Thanks to such loyal backing, our dedicated teams of coral biologists have been able to transplant many thousands of coral frames into the ocean, followed by consistent monitoring and hard work over many years.

Read the full project history of our autonomous catamaran and AI coral identification project over at Reefscapers AI-4Corals.

Reefscapers volumes of corals Acropora (top) Pocillopora (bottom)

Select Analysis of Total Volumes of Living and Dead Corals, from 2016-2020: Acropora (top) Pocillopora (bottom)

Coral Reproduction Spawning Experiment

In our on-going coral reproduction experiments, no fatalities were recorded during October.

It has now been one year since we started raising corals from tiny polyps, and our colonies continue to grow steadily.

The largest of our lab-grown Pocillopora colonies has now reached 2cm in height! (right)

Flashback to our Intagram posts at the start of the experiments, below.

Reefscapers Maldives coral reproduction 1 year growth Pocillopora
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