Dr Glen Burns is a Marine Biologist with three decades of experience in tropical marine ecosystems. Glen is one of the Great Barrier Reef’s first Master Reef Guides, professional reef story tellers who help visitors appreciate just how amazing the world’s most diverse reef is and to understand the challenges it faces. “I go to the reef nearly every day and have seen negative impacts, but I also see the regrowth that gives me hope for the future. We need to give reefs time to regenerate and adapt to a changing climate.”
Guests at The Reef House are invited to join Glen’s Reef Conservation talk to learn interesting facts about our oceans, like how Clownfish can change their sex and how long it takes for corals to regenerate. We’ve included a little taster for you below.
Q: How long does it take for a coral reef to regenerate after a coral bleaching event?
A: Regeneration can be surprisingly quick. Corals have two modes of reproduction…sexual and asexual. Asexual reproduction is going on constantly throughout the year. One coral polyp can divide into two, two into four etc. Each new polyp is a clone of the original and each one is able to extract calcium carbonate from the sea water and add it to the existing skeleton. In this way corals are growing constantly throughout the year. The Branching Staghorn Coral (Acropora sp.) which is one of the most common corals in the Cairns/Port Douglas area, can grow 15cm per year. So, in the past six years since the mass bleaching event in 2017 we have seen up to 90cm of coral growth across the reef in some areas! BUT….some areas are regenerating much more slowly. Each year we see some extremely low tides and the entire top of the reef (the Reef Flat) can be exposed for several hours. When this happens in summer, as it does every year, much of the new coral growth dies off and any new coral colonies can be killed from exposure to sun and wind. So, the regeneration of the reef flats has been much slower than the back reef lagoon where most people dive and snorkel. Some parts of the reef have already regained the coral they lost in the 2016 and 2017. In other areas it may take many more years to fully regenerate.
Q: Do corals have sex?
A: Yes. But they generally only do it once a year! Corals grow by asexual reproduction throughout the year. For corals to click into a sexual reproductive mode the conditions have to be just right. Here in the Cairns/Port Douglas area it usually occurs in Spring (November), when the water temperature reaches 26C, about 5 nights after the full moon, just after dark (and Barry White is playing gently in the background) all the hard corals spawn simultaneously! Each tiny coral polyp produces a bundle of eggs or sperm which it releases into the ocean. The eggs and sperm float upwards to produce huge slicks of coral spawn on the surface, which is where fertilization occurs. The right species of sperm reaching the right species of egg is then just a matter of luck. What hatches from the fertilized egg is a tiny planktonic larva called a planula and these float away in the ocean currents. Eventually they need to settle to the bottom. They can’t settle onto living corals because the coral polyps will eat them. They can’t settle onto mud or sand, because it’s too soft and mobile. They need to find hard substrate, so an area of old, dead coral is perfect. Once they’ve settled in an area like that, they cement themselves to the bottom and the start to divide. Each new polyp extracts calcium carbonate from the sea water to build a new skeleton and this is how we get new coral colonies forming each year. It’s an amazing event to witness and there are local boats which will take guests out to the reef on those nights to see it. The mass, synchronous annual spawning of the reef. It’s been described as “the world’s biggest orgasm from world’s biggest organism!”
Q: The Reef wasn’t as brightly coloured as I expected it to be, where are all the bright colours?
A: The colours of the corals now are the same colours they’ve always been. Unfortunately, some people come to the reef without understanding what a typical, healthy reef looks like. They’ve seen the documentaries and the beautiful photographs in the coffee table books and they don’t realize that a lot of those images are made using artificial light. The camera has a flash, the video camera has a floodlight and that allows us to see the red, pink and orange colours that you may not see under natural daylight conditions. Sea water is a natural colour filter. It filters out the reds and oranges and what we see then are the blues and greens (which is why oceans look bluey-green).
To see the reds, pinks and orange colours of the reef you need to be in shallow water (less than one metre deep), up close to the coral (less than one metre away), in very clear water, on a bright sunny day. Many people get exactly those conditions when the visit an outer barrier reef on a fine day. Or… you need artificial light.
Another thing to remember is that most of our corals are naturally brown, ranging from pale beige to a dark tan. Look at Prof. John Veron’s book “Corals of the World” and you’ll see what I mean. The few corals that are red, purple or orange are, of course, the favourites of underwater photographers so we tend to get a biased view of what the “typical” coral colours are.
And finally we also need to realize that our individual experience can vary from day to day especially if we happen to visit the reef on a day when the visibility is poor (due to sediment), the water is deep (as it can be at high tide) and the day is overcast and cloudy…. Then everything just looks bluey-grey. I’ve had Reef House guests rave to me about the beauty and colour of a particular reef and then two weeks later, another couple who told me the reef was dead and grey and they’d gone to exactly the same reef! Unfortunately the second couple had gone on a day when the tide was high, the water was merky, and the day was grey and overcast. Such a shame because they were convinced the reef was dead.
The Reef is beautiful, the colours are there and if you’re wondering about “coral bleaching” then perhaps refer back to one of our previous Q and A’s on the subject, or better still, join me in the Brigadiers Lounge for a glass of punch, a canape and a conversation on Mondays and Thursdays. Enjoy!
Q: I’ve heard that “Nemo” the Clownfish can change sex, is that true!?
A: Yes it is. As a matter of fact there are many reef fish that have the ability to change sex at some stage in their life cycle. Clownfish often live as a family group. There is usually one male, one female and then one or more juveniles living in amongst the tentacles of a particular sea anemone which is their home. In the case of the Clownfish (Nemo) the largest, dominant individual in the family is the female. She’s territorial and aggressive. She keeps with her a small submissive male, he’s reproductive but submissive. (I’m sure there’s a joke there somewhere). The juveniles are known as neuters, they haven’t decided what sex they’re going to be yet.
Now, the female patrols her territorial boundaries and occasionally she rushes in and bites her male. She bumps him, chases him around and basically she keeps him in a chronic state of stress (this may sound familiar to many of my male readers!). By stressing her male she is stimulating the gland that secretes the male hormone that keeps him male. If she stops harassing her male, he starts to change sex! Within four weeks he can become a fully functional female, that female then starts to harass the largest neuter, which then develops into a functional male and so the system goes on. So perhaps that’s the trick ladies “keep ‘em stressed!”
In the beautiful, herbivorous Parrotfish, the most flamboyantly coloured individuals are the males. But if there aren’t enough males in a school to do the job of fertilizing the eggs during the spawning season then one or more of the females can change sex, change colour and take over the role. This sex changing behaviour is known as sequential hermaphroditism…from the term hermaphrodite, which means having both male and female reproductive organs.
Recently, biologists have found some species which are able to switch backwards and forwards. They’re known as bidirectional sex changers! For example the Cleaner Wrasse can be male one season, switch to a female next season and then back to male again depending on the availability of suitable mates! It’s a curious world in which we live.
Want to learn more? Take a look at our weekly activity schedule here and join Glen for his Reef Conservation talk.