Author: Andrew Baird

*This article was originally written for the Norfolk Island’s Reef website.

As adults, corals are sedentary or sessile animals, meaning that they are attached to the ocean floor. One consequence of being sessile is that they cannot move to find a mate with which to reproduce. When the time comes to breed they shed their eggs and sperm (gametes) into the water column (a phenomenon known as broadcast spawning) where these gametes basically drift until they meet the gametes of another individual of the same species and fertilization occurs. Once fertilized the embryos develop into planula larvae which can swim, but not very well, and these larvae stay in the plankton for days to month before settling on the ocean floor and growing into an adult.

An adult coral is made up of thousands of individual polyps. Each polyp only produces one batch of eggs and sperm per year and all the polyps in a colony generally release their gametes at much the same time once per year. In the laboratory some colonies releases gametes on more than one night, but this is rare. The eggs and sperm don’t last for more than a few hours in the water column. These features of coral biology, being sedentary, only breeding once per year and the limited life of the gametes make it imperative that different colonies spawn at the same time to ensure fertilization success. This explains why there are high levels of spawning synchrony within a given coral species at any one site.

Another interesting feature of the reproductive biology of corals is multi-species synchronous spawning commonly known as mass spawning. In these events many different species spawn at the same time, up to 30 different species within a few hours on the one reef. We still don’t know why this happens. Indeed, there is no obvious advantage to different species spawning at the same time. Our best guess is that the different species all use the same environmental cues, such as temperature or moon light, to decide when to spawn. So it could be just a coincidence that many different species spawn at the same time.

Coral mass spawning has captured the imagination of the public and some of the coverage in the media has cemented a number of myths surrounding the event. The most pervasive myth is that mass spawning only occurs on one night each year. As mention above, each colony generally only spawns once, but on any one reef, there are thousands of colonies from hundreds of species and they don’t all spawn at the same time. Indeed, coral mass spawning would be better described as the mass spawning period. For example, mass spawning on the Great Barrier Reef typically first occurs a night or two after the full moon in October on inshore reefs, such as Magnetic Island. On every following night for at least a week there will be similar mass spawning, with a different suite of species spawning each night. A similar mass spawning period can also occur following the full moon in the following month. Indeed, if you search hard enough you will find multiple species spawning following every full moon from October to March on the Great Barrier Reef. So, far from mass spawning only occurring on one night per year. There are probably dozens of nights at any one location in any one year where you can see mass spawning in action.

Another prominent and pervasive myth is that coral mass spawning only occurs at a small number of sites around the globe. For example, in the documentary “Vamizi” the narrator claimed mass spawning had only been observed at five other places globally. This is just not true. In 2009 my colleagues and I published a review that documented mass spawning at 28 sites around the globe (Baird et al. 2009), and we have since observed mass spawning at many more reefs. Mass spawning occurs throughout the Indo-Pacific Ocean, from sub-tropical reefs on Lord Howe Island to the heart of the tropics in Papua New Guinea, from the far western reaches of the Indian Ocean in the Red Sea to Tahiti in the eastern Pacific. Mass coral spawning has also been documented at multiple sites throughout the Caribbean. Indeed, we are almost certain it occurs everywhere you find species rich coral assemblages. This means that if you live near a coral reef you can almost certainly get to see mass coral spawning in the flesh on your doorstep.

A few tips for catch mass coral spawning in the ocean

Most broadcast spawning corals spawn at night on or around the full moon following rapid seasonal rises in sea surface temperatures (Keith et al. 2016). Throughout most of the tropics in the south-Pacific Ocean, this means the first mass spawning events of the season occur in October or November. On Lord Howe Island this means December or January. In the Red Sea this means March or April. In Japan this happens in June or July. Most of corals spawn within a narrow window between 2 and 4 hours after sunset. Broadcast spawning corals are also far more abundant in shallow water above 10 m depth. So, the best way to ensure you see the event is to go for a snorkel around 2 hours after sunset.

Literature Cited

Baird AH, Guest JR, Willis BL (2009) Systematic and biogeographical patterns in the reproductive biology of scleractinian corals. Annu Rev Ecol Evol Syst 40:551–571

Keith SA, Maynard JA, Edwards AJ, Guest JR, Bauman AG, van Hooidonk R, Heron SF, Berumen ML, Bouwmeester J, Piromvaragorn S, Rahbek C, Baird AH (2016) Coral mass spawning predicted by rapid seasonal rise in ocean temperature. Proc R Soc B Biol Sci 283