Coral reef mass spawning

Coral reefs have evolved a spectacular solution to the problem of having sex while being rooted firmly to the seabed.
11 February 2011

Interview with 

James guest, National University of Singapore


Coral spawning


James - It's a pretty spectacular event, particularly you only see if at night time. And just before the spawning , you know, nothing is really happening it's very quiet. There's no activity, and then suddenly you have within a period of about half an hour, you'll start to see bundles of eggs and sperm, they're small bundles, they're a few millimetres Brain coral spawningacross, they're often pink or red or orange. And they start to be released from many colonies at the same time and also often many species at the same time. And the effect is something like a sort of a snow storm in reverse, upside-down. So you can imagine at night-time, with all these brightly coloured bundles all being released the effect is very spectacular. It's something like perhaps being in one of those snow globes, those little toys that children have.

Sarah - So, how many times in a year do we see the spawning?

James - Well, typically, there is often one major event. So there's a kind of a peak in spawning activity. In some parts of the world it is around spring time. However, there are other spawning events, and not all the corals go off at one time. Some individual species and some colonies will go off more than once in a year.

Sarah - And how on earth to the corals manage to coordinate to make sure they all spawn at the same time?

James - Now there's still a bit of a mystery exactly how they do it. But basically it must be at two levels. So at one level, the corals must have some internal clocks that certainly run on a daily rhythm - so what we call circadian clocks. But they would also run on a lunar rhythm, so a circa- lunar clock. They may even run on an annual cycle so a circa-annual clock.

And we know quite a lot about circadian rhythms for a lot of other organisms but we don't have much information about that for corals yet, although there is some research being done now.

But then on another level there's also the environmental cycles. So the internal clocks that the corals have must be, sort of, entrained by the environmental rhythms. So the environmental rhythms kind of keep the clocks in check.

Sarah - And once they've all spawned and their eggs have been fertilised and started toSetting up a coral spawning trap develop, I suppose they face challenges like avoiding predation, and finding a suitable surface to settle on, but what other potential difficulties do the coral larvae and the young polyps face?

James - On top of all the natural challenges that they face, there's the addition of the, sort of, human impacts that almost all coral reefs around the world now are feeling. Aside from climate change, many reefs are experiencing very high sediment levels because of land reclamation and coastal development and so on. So, excess sediment, sort of, falling on top of the small coral polyps can very, very quickly smother it.

Another problem is overfishing. So many reefs around the world have lost the big herbivorous fish which do the job of cleaning a lot of the algae off the reef, macro algae, seaweeds and so on. And again when coral spat have to, sort of, compete for space with algae often the coral spat won't do as well, so that's a big challenge for them.

And then added on top of that there's the bigger problem of climate change.

There's not really much known about how climate change is affecting the coral reproduction. We know quite a lot about how it affects adult corals. And of course it's having a big affect because we're seeing lots of corals that are bleaching - that's when they loose their symbiotic algae and if they don't recover them relatively quickly they tend to die.

The few corals that are left, they're not going to have so many individuals that the can mate with. And so if you, if we go back to what we talked about earlier, these mass spawning events, I suppose the critical thing for those mass spawning events to be successful is to have lots and lots of individuals all spawning at once. But if the sort of critical mass of corals has suddenly dropped drastically, as you can imagine the chance of successful reproduction is going to be much, much lower.

Sarah - Because of the over production of gametes by the corals to buffer against things like predation, we could use the mass spawning to seed or repair damage to reefs. So either moving the collected spawn to new places or taking them back to the lab to rear them and then releasing them out into the field. How would that work?

Coral spawningJames - There's a couple of ways that this could be done. One way is to, sort of, harvest mass spawned slicks, just take billions and billions of embryos and exactly as you said if just to pump them down onto a bit of reef, maybe within some sort of enclosure, and try to just massively boost the amount of recruitment.

The other way is, again as you say, we could rear them in the lab so basically we try to sort of overcome that mortality bottleneck that you experience in the sea. And then settle then in a tank on land, and let the corals grow up to a size where they've got a good chance of surviving and then place them back on the reef.

So the first method, we've done some experiments with that and although, it works in the sense that we can boost recruitment, what we found is that if you look 6 months or 1 year down the line, the reefs that we have artificially seeded don't look any different from the reefs that have not had any seeding.

So at the moment that particular method doesn't look like it would be very successful.

The other method of rearing the corals in the lab for a period of time, potentially has some promise. And we've had a bit of success with that in designing some little substrates that are cheap and easy to make and we can settle corals onto them, we can rear them, and eventually we can plant them onto the reef.

And we've had some success with the technique. The main problem with it is that it's actually very expensive to do. So really, you know, if you have that kind of money to spend then you'd think that it might be much better to spend it on management initiatives, on protecting an area, methods that we know are more likely to be effective. At least at this stage. I think we need to do more research on these restoration techniques and it may in the future that they can have some really good uses and that there's some pretty exciting stuff that can be done.

I think whatever happens restoration is never going to be the only solution. We have to manage reef ecosystems effectively from the point of view of managing fisheries sensibly, coastal development, aquaculture development, and ultimately at the big scale managing climate change.

But I think we have to remain optimistic and we have to keep, sort of, pushing the message that reefs are really important, that they're threatened, that there have been some really good examples of reefs that when they are well managed they recover very well from disturbances.


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