First documentation of coral spawning in Nusa Penida, Bali
Technical terms are labeled with * and explained in the glossary below.
On the 22nd of November 2024, the Indo Ocean Project marine biologists (me, Pascal, Qinthan, and Kristova) were treated with an auspicious sighting of corals spawning in the north coast of Nusa Penida MPA*. Rumored to occur annually around November and December, this rare phenomenon had never been documented until now.
A brief warning for you with trypophobia: the photos and videos I’m about to share could look unsettling to you, so proceed with caution…
A colony of Acropora cf. samoensis releasing eggs (small red balls) from their individual polyps.
It all began during the day when our friends from Nuansa Pulau tipped us that he saw some corals around their restoration site were close to releasing gametes*. I’ve heard stories that corals have spawned en masse around Christmas time a few years back. Then last month, we heard about eggs being spotted inside several Acropora colonies – but when we went snorkeling that night, we came out empty-handed. So, when we finally heard that the eggs were “literally bulging out of the polyps” we got really excited and dropped whatever plans we had made that night. We pretty much just locked our eyes at each other and said, “So… what time are we diving tonight?”
The marine biology team was excited to see corals spawning!
Many benthic organisms*, including corals, synchronize the release of their gametes into a brief period every year and saturate the reef with billions of sperm and egg. They do this in order to ensure fertilization success by maximizing chances of gametes to come into contact with each other, promoting inter-colony genetic exchange, and satiating predators and filter feeders with too much food to eat (Harrison et al. 1984). That spawning period usually happens in the evening and is synced to around one week after a full moon – one that occurs simultaneously with a sharp increase in sea surface temperature (Sakai et al. 2019). The mechanism underlying coral phenology* is still debated by scientists, but rising temperature, along with weakening wind and current speed, may influence gametogenesis and reproductive cycles.
Back to the story. We met up around 7 PM that night and geared up for one shore dive. After about half an hour circling the reef flat, we finally found a colony of Acropora with eggs on its polyps! However, strangely only half of its colony was releasing eggs, so we were worried that we may have missed the big event and we were only witnessing (half) a late bloomer in action.
Only half of this Acropora colony was spawning! (right side)
(Sound on) The moment we realized the corals were spawning. Video credits to Qinthan Aulia
But then suddenly we started seeing small red balls adrift in the water, and we realized that there were more colonies spawning! Blown away, we all huddled around the colonies to capture that rare moment when an egg is released from the tip of the polyp. After I was satisfied taking photos and videos of the Acropora colony, I started exploring the surrounding area and found a Galaxea colony also reproducing. This coral undergoes a slightly different spawning method: unlike Acropora that release individual fertilized eggs, called brooding spawners - Galaxea release bundles of eggs and sperms separately, called broadcast spawners.
In the end, we found two Acropora species and one Galaxea species spawning. Our friends from Nuansa Pulau also reported Merulina, Echinopora, and Lobophyllia spawning, which brings the total to at least six taxa spawning simultaneously. It’s a bit shy from a massive event we were hoping, but it was incredible nonetheless. The water was still covered in coral eggs, and to be honest, smelled a bit funky. After the dive, I was covered in coral gametes from head to toe and I could swear I still smelled rubbery and fishy for two whole days!
A colony of Galaxea fascicularis releasing egg bundles
As this is the first time data has been recorded from a spawning event in Nusa Penida, we hope that it can provide more insight about the timing and environmental cues to better predict future spawning events. We can confirm that the spawning did coincide with a spike in sea surface temperature (about 29-31°C vs the typical 25-27°C) but not with weakening current speed and wave strength. We are excited to record more coral spawning events in the future to elucidate this complex yet important matter!
Lastly, here’s a short video I took of one of the spawning Acropora colonies. It resembled a snowfall in the tropics!
* Glossary
Benthic organism: a marine organism that is associated with the seafloor; it could be sessile (non-moving) like a coral or motile (able to move on its own) like a starfish.
Gamete: an organism’s reproductive cell, which is either a sperm or an egg. Gametogenesis refers to the cellular process of producing gametes.
MPA: Marine Protected Area, or a defined area managed for the long-term conservation of marine resources. Nusa Penida was designated an MPA in 2010, in which commercial fishing and recreational activities are more restricted compared to surrounding areas.
Phenology: the study of periodic events in biological life cycles and how these are influenced by seasonal and inter-annual variations in climate, as well as habitat factors and other environmental cues.
Polyp: a single animal related to the phylum Cnidaria, which is the group that corals and jellyfish belong to. A coral colony consists of multiple polyps with identical genetic materials.
