Off timing coral spawning spotted in South Sulawesi

How much do we really know about corals? The answer is: Not so much, actually. While the general rule for coral spawning is a few days after the full moon, 2 hours after sunset, just before the water temperature high peak, we got caught off guard by this huge colony of Montipora verrucosa in Bira, South Sulawesi, Indonesia.

I’m lucky enough to have witnessed other species of Montipora, like this Montipora hirsuta spawning in Bali, right on time, synchronizing with all other species. So a rule for Montipora cannot be asserted.

Spawning events are always a feast for the eye. Especially if they are unexpected.

While we dove in the morning and enjoyed the beautiful size and structure of that particular colony, there was no way to predict it would spawn in the evening.

2 Days after New Moon:

We can clearly see eggs bundles under polyp mouth and ready to go off.

Usually, coral spawning can be observed 2-3 days after the full moon. And if we know now that there are no fixed rules in this domain. Some corals like Porites rus spawn in the morning, some every month, and some have pre-spawned a month before. We were not expecting a full-blown spawn 2 days after the new moon. We were just very lucky to pass by in front of that colony at the right time. You never know what to expect on night dives!

Just a bit more than an hour after sunset:

The spawning probably lasted for 10-15 minutes only, and we only witnessed the last 5 minutes.

Not only the day was off, but also the timing, which was completely unexpected. Mass spawning usually starts 2 hours after sunset. But sunset time was 17.39, and the corals went by 18.50. While it should have been around 45 minutes later, at least.

Just after a slight rise in temperature:

The other very interesting parameter is that this year, the water in Indonesia has been particularly cold, while crazy hot, all around other places. Just a moment of rest given to corals just before El Nino reaches our shores.

When the coral is spawning, thousands of eggs are released in a few minutes within the same colony, in a synchronous event.

While in that particular area, they saw temperature dropping to chilling 21 C, it just started warming up, with temperature around 24-26 C, but still far from their usual average 26-28 C that they should reach within a couple of months.

Green water:

Another very interesting fact is that in the few last weeks, water visibility has been very poor. High thick phytoplankton and green water have been shading the area. Plenty of very rich food, to feed corals, so they can produce large quantities of eggs.

The tips of the colony are focused on growth, and thus rarely produce any eggs, only polyps at the base, the center of the colony release eggs.

How much luck do we need to understand properly coral sexuality? It’s one of the few domains where I see technology being able to help us, by creating spawning detection devices that we could attach to certain corals.

Every polyp within the base of the colony releases its own egg bundle, composed of a few eggs and a layer of spermatozoids.

I just see it as a very good sign, just 2 days before Jake’s celebration of life diving trip. A wonderful gift to celebrate him!

Vincent Chalias

Vincent is a coral passionate aquarist with extensive experience in the industry, travelling the world to learn about corals. His background is a master degree in Marine Aquaculture. He set up the first Indonesian Coral mariculture farms at the beginning of the century and spent over 20 years in Indonesia running them. He's been extensively diving and documenting corals all over the Indo-Pacific.