Congratulation to Hanaka Mera and colleagues for a paper just published in Coral Reefs that should be of interest to everyone working on coral spawning on the Great Barrier Reef. The paper is published with open access and can be viewed here.

The paper explored coral spawning patterns using over 30 years of direct coral spawning observations and 20 years of data on the stage of gamete maturity in colonies of Acropora (hereafter, colony reproductive condition) from the Palm Island group on the Great Barrier Reef to explore patterns within and among species. 

One significant finding was that the lunar month counted from the southern hemisphere winter solstice better predicts the first month of spawning in the Palm Islands than the Gregorian month. Specifically, spawning was never observed or predicted before the 5th lunar month on this mid-shelf reef. 

Most spawning occurred in the 5^th and 6^th lunar month and split spawning – i.e. colonies of a given species spawning in more than one month – also occurred more often than not in Acropora populations.

So, the bottom line is that there is going to be some spawning in some species in both the 5^th and 6^th lunar months every year. This information should make it easier to plan when to go in the field, depending on whether or not your research is species specific.

Another interesting result was that at least some Acropora colonies were mature in every lunar month from month 5 to 9. This means that if you miss the mass spawn there will be other opportunities to collect gametes later in the season. 

The data also suggest that some Acropora spp. spawned in four or five different lunar months. However, Mera et al. suggest that this pattern might be influenced by problems with the taxonomic framework. For example, some colonies that we identified following Veron 2000 as Acropora digitifera spawned in lunar month 5 and others in lunar month 9. Unpublished molecular and morphological data suggest that these two groups are likely to be different species. A similar pattern has previously been documented by Jackie Wolstenholme on Lizard Island (Wolstenholme 2004).

References

Mera H, Edwards AJ, Guest JR, Lachs L, Quigley KM, Willis BL, Baird AH (2025) Patterns of coral spawning in the Palm Islands, Great Barrier Reef. Coral Reefs

Wolstenholme JK (2004) Temporal reproductive isolation and gametic compatibility are evolutionary mechanisms in the Acropora humilis species group (Cnidaria; Scleractinia). Marine Biology 144:567–582