Corals: Rainforests in the Sea

Three NAKED facts

Thoughtful question: What are these? Animal/plant/mineral?

The answer? You’re all correct!

NF#1: Corals are colonial animals - they clone themselves to form colonies that look like the boulders you see around you! The individual is called a 'polyp' and it looks like an (small) anemone! Animal characteristics include eating and breathing. In the water, especially at night, they extend their tentacles to feed, amongst other things... (like coral warfare - see below for Too Much Information)

NF#2: They are plants too! Like their other sting-ey cousins, corals also contain tiny algae within their tissues called zooxanthellae, that make food from light to share with their polyp hosts. The zooxanthellae are also what give them their pretty colours. If the water temperature gets too hot and the corals get stressed, the zooxanthellae will leave the corals (well it's not currently clear whether they leave or are kicked out or it's mutual), and the corals will 'bleach' - turn a white colour. Bleached corals may eventually recover, if the conditions become favourable and they can survive the stress.

Corny joke: Corals aren't the only things that get bleached. Blonde coral jokes, anyone?

An interactive activity: How do you tell a dead coral from a living one? (Tell them to be careful where they step!)

NF#3: So where’s the mineral part? Each polyp builds a hard skeleton for protection, where it lives in – just like people live in flats. That would make the colony a block of flats! At low tide, the polyps hide inside to keep from drying out. Actually only the thin layer on top remains ‘alive’, this layer can easily be damaged from careless contact, so be careful where you step! Underneath, the dead skeleton (white if freshly exposed, but soon gets covered over by silt and algae growing) left behind is what forms the coral reef structure. Like trees in the rainforest, this structure is where all the other reef animals live!

An interactive activity: How about a bleached coral from a (recently) dead one? (Think about it...a dead coral would just be the skeleton, so you wouldn't be able to see the tissue.)


Colony, and close ups of dead and living sections. The bits of colour in the centre of the dead section are the remains.

Thoughtful question: Why do you think there’s usually a ‘botak’ patch on the top of the coral? What is different about the top and the sides?

How NOT to hug a coral
As living corals are very easily damaged, careless contact by clueless humans can harm or kill them. It is best not to touch the corals at all – this means no poking, or kicking. And be careful where you step!

Helpful hints for Naked Hermit Crabs ...

Where seen?
Everywhere on the lower shore.

Too Much Information!

Only for Naked Hermit Crabs who want to know Don't force feed this info to visitors! They may fall asleep if you do.

What types are there? – a quick and dirty coral ID guide for the coral rookie
Lots of types, sorted visually by growth forms (branching, massive rock-like), the way the patterns on the coral look (brain, maze, round, little holes).


Anemone Coral (Goniopora sp.) has long fleshy polyps that extend out of the skeleton


Plate or Disc Coral (Turbinaria sp.) is plate-shaped, with large polyps sticking out.


Mushroom corals (Fungiidae) - Free-living species are not attached to the reef, unlike most other corals


Branching corals: Pocillopora damicornis (top) and Montipora sp. (bottom)



Stone Corals (Faviidae) are usually shaped like boulders – though there are other kinds shaped like boulders too…

What do they eat?
They get most of their nutrition from their zooxanthellae. Or they also eat plankton, caught from the water using their tentacles.

The secret world - sex and violence (RA!)
How do rooted things make luuuuurve? Like most marine creatures that don’t move around, corals simply release their eggs and sperm into the water! Tide and currents take care of the rest. This is a very coordinated event, in terms of timing, to ensure that spawn from different colonies meet and mix.

Mass spawning is an event happens within the time frame of 2-3h on a few nights every year, where many types of corals (and some other reef creatures too) release their eggs and sperm at the same time! On bigger reefs, this can form spawning slicks on the surface of the sea – like an oil slick but consisting of coral eggs and sperm bundles.

Scientists are still trying to figure out how this extraordinary coordination occurs. The baby corals (like little anemones, or jellyfish) drift on the currents, and eventually settle in a suitable place. The baby polyp will build its home there, and start building a whole new colony!

Colonies spawning (2 photos), and close up of Galaxea egg bundle.

Look for bands of silt-covered dead coral between neighbouring colonies, usu a few cm in width, and quite uniform. This is evidence of coral warfare. On the reef, competition for space can get quite violent, and many corals have special, extra long war tentacles that they use to zap other corals into submission. They then take over the space vacated.

How long do corals live?
This is an interesting question to answer, with regard to colonial animals. It depends on the definition of ‘life’ and ‘the individual’. Because coral polyps divide over and over again, producing essentially the same individual, external factors like predation, disease and environmental conditions aside, they can ostensibly live forever!

Human uses
Directly, coral skeletons (calcium carbonate) have long been used as building material in less-developed places. Coastal reefs are also thought to protect their coastlines from storms.

Indirectly, coral reefs are the habitat and nursery ground of many kinds of seafood. So no corals = no seafood.

Blue coral

Blue coral is a kind of "coral" that has an internal blue skeleton. It's not really a hard coral, as it belongs in the sub-class Octocorallia, along with the soft corals, whereas the hard corals lie within the subclass Zoantharia, which also includes the anemones.

Environmental issues
Corals are quite sensitive to environmental changes, and can only survive within a small range of temperatures and light levels. The recent increase in global temperatures have caused major coral bleaching problems throughout the world in the last decade. This is bad because bleached corals lose their helpful algae symbionts and are prone to die off. You can help the corals by doing your part to live sustainably!