Coral Spawning in the Maldives 2023 Q1-Q2 Updates
Please also see our main Coral Spawning reports page, from 2013 onwards, including our pioneering lab work in Oct-Nov 2021.
January 2023 Observations
Coral spawning at Kuda Huraa (2022-23)
Coral gamete monitoring (Acropora digitifera) [RIGHT]
Tracking Gametogenesis
At Landaa during January, we recorded pale pigmented eggs in Acropora secale, and white immature eggs in five species of Acroporidae (A. gemmifera, A. millepora, A. digitifera, A samoensis, A. nasuta).
At Kuda Huraa this month, we conducted 14 night-snorkels to monitor for the presence of coral gametes, around the periods of the full moon and the new moon. Immature gametes of various species were recorded, which are predicted to mature in time for the next mass coral spawning event in April-May.
To allow for easier gamete collection, we relocated corals with pigmented gametes to the Water Villas – a total of 16 colonies of A. gemmifera and five colonies of A. humilis.
Daily gamete checks were carried out at the Starfish site, and spawning was presumed (due to gamete loss) in both A. humilis and A. tenuis (no evidence of spawning in A. gemmifera).
Mature coral gametes in Acropora tenuis
Coral polyp releasing gamete bundle (close-up)
Coral Settlement Update
Over the past months, our coral biology teams have worked tirelessly on our Coral Reproduction Program, and at Kuda Huraa we have successfully fertilised and settled three coral species (Acropora secale, A. tenuis, A. muricata).
This month, the A. muricata polyps exhibit stable health, and symbiodinium (zooxanthellae) were observed within their tissues for the first time (symbiont uptake occurs one month after settlement for all three species).
Being the oldest, the survival rate for A. secale has decreased over time as expected, mainly down to algal overgrowth (despite regular cleaning of the aquaria and the coral settlement units).
Coral settlement – polyp growth over time
Acropora tenuis polyps, at 2-months post-settlement
Coral polyps – settled, growing, healthy (Acropora secale)
Coral polyps (Acropora muricata) update of symbionts
February Observations
Coral Settlement
Acropora tenuis settlers continue to display growth and steady health, more than three months post-settlement.
Monitoring for Coral Gametes and Spawning
During February, immature gametes of different species were found at Kuda Huraa’s Water Villas and Starfish sites, in preparation for the next mass spawning wave expected in April-May. One specific colony of Acropora retusa (on a frame at the Water Villas site) has presented white gametes since way back in August 2022!
Temperature Loggers
This month, we added two extra temperature loggers at Landaa’s House Reef (at depths of 2m and 5m) and two loggers at the ‘watersports bommie’, where we witnessed wild coral colonies spawning in November 2022. We also transplanted an experimental frame with fragments that spawned in November 2022 (A. humilis, A. secale, A. globiceps). We want to see if these corals will spawn next season, or whether they put all their energy into growth.
March Observations
Coral Settlement
We are improving our experimental settlement pools in time for the upcoming spawning season.
Monitoring for Coral Gametes and Spawning
Immature gametes of different species are being found in the Water villas and Starfish sites, in readiness for the next spawning wave in April-May. (Refer to the gamete photos and pigmentation state.)
Our teams are getting ready to restart intensive monitoring observations, around the full moon period and our predicted coral spawning dates. Once again, we plan to collect gametes in situ, for fertilising and settling ex situ in our Lab.
Feeding Experiment: ex-situ coral larvae
It has been shown for various Acroporidae that feeding the ex-situ coral larvae can increase growth rate and then increase survivorship when out-planted onto a reef (Schutter et al. 2023; Toh et al. 2013).
To replicate this experiment with our A.digitifera recruits, two of our open-flow tanks were each fed daily with 50ml of Artemia (with the flow turned off for one hour). In addition, we added several mature fragments of A.digitifera to the tanks to allow uptake of zooxanthellae by the recruits.
References
- Schutter, M., et al (2023). Enhancing survival of ex-situ reared sexual recruits of Acropora palmata for reef rehabilitation. Ecological Engineering, 191, p.106962
- Toh, T.C., et al (2013). Heterotrophy in recruits of the scleractinian coral Pocillopora damicornis. Marine and Freshwater Behaviour and Physiology, 46(5), pp.313-320
April Coral Spawning Event
At Kuda Huraa, we conducted 22 hours of coral monitoring across three sites around the island Huraa (thanks to our colleagues at Tropicsurf for the valuable assistance).
- spawning was observed in more than 130 coral colonies, at all three sites
- data was collected on 139 colonies representing nine different coral species
- coral ‘slick’ (floating pink/orange gametes) was observed at the Channel site
At Landaa, our team started nightly snorkels around both the full moon and new moon periods, and recorded environmental parameters such as tides and local weather conditions. Pigmented eggs had already been observed in colonies of various species of Acropora, so in advance of a predicted coral spawning event, we started the nightly placing of four collection nets over mature colonies of A. digitifera.
Over several nights, we observed multi-specific spawning in over 100 coral colonies (from eight different species) at various times after sunset, and we successfully collected gametes for ex-situ fertilisation and settlement.
The bottles of gametes were divided into four seawater buckets to increase genetic diversity and enhance fertilisation success. Excess sperm was removed, the buckets were gently mixed to dissociate the bundles, and then the surface layer of eggs was divided into five open-flow tank systems. All the tanks contained rubble coated with crustose coralline algae (CCA), and two tanks also contained nubbin trays that had been pre-conditioned for settlement. Tanks were gently aerated and stirred to prevent coagulation of the larvae.
Without disturbing the main tanks, we periodically withdrew microscopic samples from a side-sample contained in a glass beaker. We observed gametogenesis from the two-blastomere stage (23:00 on day #1), the ‘bowl stage’ (09:00 day #2), and the ‘tear-drop stage’ (day #3), all the way through to free-swimming planulae on day #4.
