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.
Coral Spawning Event in the Maldives, October 2021
Coral eggs in Acropora tenuis
Identifying Mature Coral Colonies
An initial sign that a coral colony will spawn is the presence of gametes within the coral tissue. It is possible to visually see ‘white’ eggs, a sign that the colony is sexually mature. Medium eggs with no colouration can take between 8 to 12 weeks to mature. Large, pigmented eggs (typically pink/orange) will likely spawn around the next full moon.
- 19 October – we identified bundles of both white and pale orange eggs within Acropora secale and A. aspera colonies growing on our coral frames (we did not observe eggs or witness spawning on the natural reef). Colonies with eggs were tagged with red/white tape for later identification. HOBO temperature loggers were placed near the frames (4m depth at low tide).
- 20 October – more eggs (pigmented) were observed.
- 25 October – more eggs (rounded and pigmented) were identified in our sample coral colonies.
‘Bundling’ observed in Acropora secale
Collecting Coral Eggs In-situ
- 20-27 October (at 23:00) – we conducted nightly hour-long snorkels to check for coral eggs and ‘bundling’.
- 21 October – Acroporidae coral gametes are buoyant, and they float to the ocean surface so they can be relatively easily collected. We made several coral spawn collection nets from basic aquarium materials. 80-micron mesh ‘cones’ were cut into semi-circles and stitched together, supported by a metal ring base to maintain the shape. A drainage bulkhead was attached to the top of the net, with a threaded seal. To the collection bottle, we inserted a PVC pipe, and attached four anchor points (made from metal wire and crimps) to secure to our frames.
- 22-23 October – we trialled the nets in the lagoon, with various amounts of flotation ensure the bottle sat upright. The simple mechanism allowed us to remove the water bottle from the net and attach a cap to contain all the gametes.
Pigmented round coral eggs in Acropora secale (left) and Acropora aspera (right)
During each of the expected spawning dates, between 17:00 and 19:00, we dived to place our nets (with labelled collection bottles) in the water on our selected tagged frames. Attachment of each net took 10 to 15mins; four nets were used in various locations at our Dive Site.
Frames with healthy mature Acropora colonies were chosen from different years, to ensure genetic diversity:
LG1031 (June 2011), LG1953 (November 2013), LG3517 (April 2018), LG3859 (May 2019), LG4096 (September 2019).
- 25 October – one colony of Acropora secale (frame LG1031) spawned at 20:00, 16 minutes prior to the lowest point of the outgoing tide (at 19:44).
- 26 October – a synchronised spawning event took place. Bundles were observed on all of our selected tagged frames (plus others) at 19:35 (33 minutes prior to lowest point of outgoing tide).
At 19:57, coral spawning began (11 minutes prior to the lowest point of the incoming tide at 20:08).
The spawning event lasted 20 to 30 minutes, but one frame (LG3517) did not produce adequate amounts of gametes so has been excluded from the experiment. The remaining three coral collection devices containing plenty gametes were sealed using bottle caps and transported quickly and safely to ex-situ fertilisation tanks to be mixed with gametes from other colonies. (It is reported that mixing between 3 to 6 colonies will increase fertilisation success as well as increase genetic diversity of offspring.)
We used tidal charts to predict the ‘Neap Tide’, which is typically five days after the Full Moon, and specifically defined as:
“the time at which the line from the Earth to the Moon is at right angles to the line from the Earth to the Sun. Thus, the tide-producing effects of the Sun and the Moon cancel each other out, causing tidal ranges that are usually 10-30% less than the mean tidal range.”
Coral Fertilisation & Embryogenesis
Once gametes had been placed into ex-situ fertilisation tanks, they were topped up with fresh seawater to ensure oxygenation. Seawater salinity was also monitored using a refractometer, to ensure the water quality matched that of the ambient ocean (34 ppt).
At 21:52, mixing began to break up gamete bundles. Fertilisation can begin a few minutes after disturbance, and continue for up to two hours post-mixing. The post-fertilised mixture was stirred gently for 30 minutes to prevent oxygen depletion, before gently rinsing off the sperm. Water quality is extremely important.
The mixture was examined for signs of embryogenesis (Craggs et al. 2019) and not disturbed for three hours. At 23:40, we observed cell division and then four-blastomere division. The mixture was rinsed again to ensure sperm removal, and separated into two different tanks at 01.40, where water quality was regulated. Our team were fascinated by the process, and continued to observe multiple coral cell divisions for several hours, well into the night.
Spawn from Acropora secale – rinsing stage
- 28 October – the characteristic ‘tear drop’ stage was observed under the microscope.
Watch our video embedded below [or on YouTube] displaying coral gamete collection, fertilisation, ‘tear drop’ stage, and free-swimming planulae.
- 29 October – free swimming planulae with cilia were observed.
To optimise settlement, we collected pre-conditioned rock containing large amounts of crustose coralline algae (CCA) by freediving on the House Reef. (CCA has been shown to act as a settlement cue for planulae). We cleaned the rock pieces to eliminate competition, added them to the tanks, and introduced low light and gentle aeration. We observed the substrate for signs of planulae settlement and metamorphosis into single polyps.
- 30 October – successful settlement.
Acropora millepora embryogenesis (Craggs et al 2019) slightly enhanced for detail. Scale=500µm. [click for larger version]
(A) Newly released egg sperm bundles; (B) Bundle dissociation occurring, 30–40mins following release;
(C) Zygotes following in-vitro fertilisation; (D) First cell division forming 2 blastomeres, 1hr post-fertilisation (hpf);
(E) 4 & 8 blastomeres (2–3hpf); (F) 16 blastomeres (4hpf); (G) Morula stage (5–6hpf);
(H,I) ‘Prawn chip’ stage (6–8hpf); (J) ‘Bowl’ stage (10hpf); (K,L) ‘Round’ stage (18–21hpf);
(M,N) ‘Tear drop’ stage (67–70hpf); (O) Planula larval stage (75hpf).
Ex situ co-culturing of … Acropora millepora enhances early post-settlement survivorship.
Craggs, Guest, Bulling, et al. Sci Rep 9, 12984 (2019). Image reproduced under ‘fair use’ Creative Commons.
Coral Colony Sizes
Colony sizes were measured (height, length and width) using callipers to determine ecological volume of mature colonies that spawned. Next year, it will be interesting to note percentage growth rates of colonies and look at coral egg density to identify redundancies in size and maturity.
Acropora secale transplantation dates, spawning dates and ecological volume of colonies that spawned
Coral Genetics and Taxonomy
In order to further study coral genetics and thermo-tolerance, a sub-sample of Acropora secale eggs were taken on 26 October (post-spawning). Genetic and taxonomic methods will help to identify species-specific genes of the samples that we cross-fertilised, especially if we observe settlement success in the future.
- Firstly, all equipment was carefully sterilised.
- Two 500ml sub-samples of gametes/seawater were taken from the fertilisation mixture, put through a 50um micron mesh to filter off the excess seawater, and each 500ml was placed into separate test tubes.
- The taxonomy sample was placed in formalin.
- The genetics sample was placed in 95% lab-grade ethanol; fresh ethanol was used 24-48hrs later (excess seawater in the tube would dilute the 95% ethanol solution).
- Both samples were kept in the refrigerator.
Photographed in our Fish Lab (October 2021) – commencement of coral embryogenesis (first cell division, two-blastomere stage)
Settlement of Coral Larvae (November 2021)
Photos (below) showing tiny coral polyps, successfully settled on substrate in our laboratory.
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!
More updates in our November report! Keep an eye on our social media (links at bottom of page) to follow the growth of our new corals, and see settlements from a multitude of species via subsequent spawning events.