Spawn, Coral, Spawn!

Time sure flies by! It is now a decade since I first became a certified SCUBA diver, and to think that it all started with research projects with the Marine Bio Lab in NUS. (Didn’t even realise that they revamped the website!) One of the most interesting phenomena that I have had the good fortune to observe is the annual mass spawning of hard corals in Singapore waters. (Yes! We have coral reefs in Singapore!) This year, the expected coral spawning period is this week (25-28 April), and I was able to tag along for one of the nights. 🙂

Coral reproduction
Unlike terrestrial animals, once hard coral larvae have settled down onto a substrate (i.e. rock or dead coral) they are fixed in place for the rest of their lives. So how do they go about reproducing?

Corals can reproduce both asexually and sexually. As each coral is made up of a colony of individuals, the colony’s growth is due to asexual reproduction of the individual(s) of the colony, through the process known as budding.

Hard corals can reproduce sexually through brooding (fertilization occurs internally) or broadcast spawning (fertilization occurs externally). Broadcast spawning or mass spawning is known to occur in around 75% of coral species, where large numbers of sperm and/or eggs are released (some times seemingly ejected!) into the water column. These sperm and eggs would float to the water surface where fertilization would take place, forming free-floating larvae (aka planulae). Ocean currents would help in the dispersal of these planulae, just like how winds aid in the dispersal of the light seeds of big rainforest trees.

Synchronous mass coral spawning
Synchronized mass coral spawning increases the chance of cross-fertilization the sperm and eggs of the same species that may be located far from each other. Additionally, when many different species release their sperm and eggs all at the same time, they would also increase the survival rate of the planulae from predation by supplying more gametes than the predators can eat.

Here in Singapore, synchronous mass coral spawning was first observed in 2002, and NParks has been monitoring this event on an annual basis. Seasonal cues that affect that are suspected to affect the long-term maturation of the gonads are seasonal changes in sea temperature and day length, and the more immediate cues for the coral to prepare to spawn would be from the lunar cycle. The final cue to the actual release of spawn is believed to be the time of sunset. I do wonder if the other sea critters also use similar cues to decide when there is a likely annual nocturnal Michelin delicacy in the form of coral spawn…… There always seems to be an increase in feeding activity during spawning week. Every time I turn away from a colony that has just spawned, I would spook away a huge school of fish with my torch!

Just to give you an idea of how the reef looks like during coral spawning, here’s a video of one of the most impressive coral spawning events I have had the honour of observing first hand.

How would one describe this spectacle? The most common description that we love to use is that it looks like an underwater snow storm, or even like popping a pimple for those species that take their time in releasing the egg-sperm bundles. (Yes, we biologists can be a little be “eeewwwww” in our descriptions ;P) One species that I observed for the first time this year (Moon coral, Diploastrea heliopora) actually spawned THREE times to the delight of my dive buddy and myself, and we agreed that it looked as if the colony was sneezing out the spawn! Here’s the video that she managed to take:

Here are more close-ups photos of coral spawning, making the corals almost alien looking… Not to forget, here are some photos that I took during the dives on Tuesday:

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From what I observed, and hearing from the rest of the divers about the other nights, it seems that this year’s spawning was not as extensive as previous years. Could this be (partly) due to the increased temperatures the past few months?

Additional Reading:
(1) Confessions of a Serial Coral Spawning Enthusiast:
(2) Coral Reproduction:
(3) Coral reef mass spawning:
(4) Sexual Reproduction:


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