Healthy Acropora gemmifera and A. digitifera growing on our coral frames (Aug-Oct)
Coral Propagation – Monthly Progress
At Landaa during October, we transplanted 1043 coral fragments onto 21 new coral frames (10 guest-sponsored, 10 hotel-sponsored, one online order). We also monitored (photographed, maintained) a total of 410 frames at various sites around Landaa Giraavaru, plus 30 old frames were recycled back onto the reef.
healthy Galaxea fascicularis growing on a Reefscapers coral frame
At Kuda Huraa this month, we transplanted 6 new coral frames and recycled a further 12 frames, using a total of 732 coral fragments (from 13 different coral species).
Water Villas (WV) – this site continues to be badly affected by the seasonally shifting sands in the lagoon, with one mid-October storm burying 17 frames overnight. We have been busy dismantling our aerial patterns, relocating 124 frames this month, bringing the yearly relocation total to 710 frames (including 30 recycled old frames).
The Dolphin-shape is at risk of the shifting sands, totalling 183 healthy frames that harbour a good diversity of mature coral colonies, with abundant resident populations of fish and invertebrates.
On 21 October, we spotted coral eggs on many mature colonies of Acropora tenuis. We hastily constructed several spawn-collection devices to harvest the eggs in-situ, but the spawning did not occur when predicted so we continued to monitor the corals closely. Changes in egg pigmentation were observed in Acropora tenuis, but the other Acropora hermaphroditic broadcasters were not yet close to spawning.
Blue Hole (BH) – we continue to remove the accumulated marine debris and drifting sea grass from this site, and the coral colonies remain healthy. On 27 October, mature coral eggs were spotted on colonies of Acropora secale (3m depth) and we observed coral spawning from 20:00 to 20:15 (Maldives time, GMT+5). We were able to harvest some eggs and placed them into our aquarium for observation, along with a dead coral skeleton as a settlement substrate. Some eggs were observed attached by 20:45 (30 mins after spawning) but we did not see coral larvae.
From our notes, we observed this spawning event occurred:
– 1.5 hours before the lowest tide (which was at 21:42);
– Seven days after the full moon (which was on 20 October);
– One day later than in Landaa Giraavaru (Baa Atoll).
Channel (CH) – 14 of the new monthly frames were located here, plus two mini coral frames, each with 19 fragments of Blue Coral (Heliopora coerulea, a reef-building octocoral with high resistance to thermo-stress and bleaching). A total of 54 frames were monitored and retransplanted (using fragments of Montipora digitata, Porites cylindrica and species of Acropora).
Sea Star (SS) – Aerial photographs of this site were taken to correct the QGIS location mapping, and we plan to construct a new aerial crab-shape.
House Reef (HR) – the temperature logger was redeployed on 8 October, and we removed two corallivorous predators (one Cushion starfish, one Crown of Thorns). The two new monthly coral frames at this site were transplanted with species of Acropora and Pocillopora verrucosa.
Reefscapers frame transplanted with fragments of blue coral (Heliopora coerulea)
Coral Plates in Aquarium One (plates KH01, KH02, KH05) and Aquarium Two (plates KH03, KH04, KH06)
• KH01 (Galaxea fascicularis; Acropora species replenished Sep-Oct 2021) – added six new fragments of Heliopora coerulea.
• KH02 (G. fascicularis) – algal overgrowth has caused some coral tissue death.
• KH03 (Acropora species, replenished July 2021) – healthy and encrusting.
• KH04 (A.digitifera, A.millepora) – healthy and growing well.
• KH05 (G.fascicularis) – fragments continue to grow and fuse; cyanobacterial growth regularly removed.
• KH06 (G.fascicularis) – steady health, growing, fusing.
Preparation for Coral Spawning
This month, we took part in the second series of a two-day online coral larvae workshop in partnership with CSIRO, the MMRI and MUI (among others) to further understand and identify coral spawning events across the Maldives. This informative workshop detailed the use of coral larvae in reef restoration, to upscale future efforts. It is evident that sexual propagation will result in higher genetic diversity amongst species in comparison to asexual propagation. Moreover, corals are extremely fecund, meaning a large number of juvenile corals could be produced if mortality at early life stages of fertilisation and settlement are reduced. This technique could enhance coral propagation programs here in the Maldives, if the asexual technique is coupled with sexual broadcast spawning and rearing.
At Marine Savers, we have transplanted and grown many different species of corals over the years which are large enough to produce mature spawning colonies. Our large database allows us to pinpoint transplantation dates, identify age of maturity, track mature colonies, and record coral spawning across multiple years. This data can be used to ensure genetic diversity, as we can confirm gamete collections from different frames based on these dates.
To expand our knowledge of coral spawning around Landaa Giraavaru, we reviewed the data from our previously documented spawning events (2013 – 2020). It is incredibly difficult to predict when coral might spawn, therefore it is important to take note of tidal cycles, lunar cycles, and water temperatures from previous years, to be utilised as indicators for predicting spawning this year.
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).
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.”
- 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.)
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
- 29 October (67 to 70hrs post-fertilisation) – 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.
- 31 October (70hrs post-fertilisation) – 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.
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.
Harvested coral spawn, photographed in our Fish Lab (October 2021)