Living sculptures: our amazing hard corals!

How do these marvellous living sculptures come to be? And in such a fantastic variety of colours, patterns and shapes?

Here's a very quick and very unscientific introduction.

With examples of some of corals that can be seen even by non-divers (all these photos were taken above water).

Each hard coral is actually a group (colony) of many tiny animals called polyps.

Each polyp looks somewhat like a miniature sea anemone, with a body column topped with tentacles.
Each polyp produces a delicate tiny external skeleton made out of calcium carbonate, called a corallite.
The polyp can retract into its corallite to hide from predators or to avoid drying out when the colony is exposed at low tide.

Although each polyp is tiny, together, they may produce a stony structure that is several metres in diameter, weigh tons and be made up of hundreds of thousands of polyps!
Huge coral reefs are made up of the skeletons of these tiny polyps, living ones growing over the skeletons of dead ones.

How does a coral colony get bigger?

Each polyp has a fixed adult size. So as each polyp adds to its corallite, the corallite becomes deeper. The polyp periodically lifts its base and builds a new floor, sealing off a little space below. As the colony grows, there develops a 'condominium' of abandoned floors, with the living polyps only on the top floor! Thus the 'thickness' of the entire colony skeleton grows over time.

The colony also increases in size by increasing the number of polyps in the colony. New polyps budding off from existing polyps and as these new polyps produce their corallite, the entire colony gets that tiny bit bigger.

As the polyps are so tiny, hard corals tend to grow very slowly. Some colonies grow only 1cm a year. In these, a colony 1 metre wide can be 100 years old!

The various shapes and surface patterns of hard corals arise from the way the polyps are arranged. So you have to take a closer look at the coral to see what's really going on!

Here are some common shapes of hard corals often encountered on Singapore's shores.

Rounded boulders, with a rather smooth surface. While two colonies may appear similar in overall shape, they may be entirely different species of hard coral. Have a closer look to see the tiny polyps and the corallites that they create!

Probably Porites sp.

Possibly Psammocora sp.

A thin layer encrusting a hard surface.

Possibly Cyphastrea sp.?

Thin plates, sometimes in tiers or layers of plates.

Pachyseris sp.

Montipora sp.

Merulina sp.

Vertical plates, sometimes folded so that the colony resembles a cabbage or a rose.

Turbinaria sp.

Pavona sp.

A flat surface that is folded so that the colony looks like a cup or a vase.

Turbinaria sp.

Delicate branches so that the colony looks like a small bush.

Possibly Acropora sp.

Hydnophora sp.

Shorter branches or merely lumps.

Psammocora sp.

Pocillopora sp.

Here are some common textures of hard coral surfaces.

In some the corallites are quite obvious as large circles or rings.
In others, the corallites have 'shared' walls so that the resulting patter of 'valleys' resembles the surface of a brain.
Some corallites are long and trumpet-like, but with only the circular opening on the surface, we don't see the 'stem' that is usually hidden inside the colony.
And there are lots and lots more different kinds of corals that can be seen on our shores!

Links to more
Jeff has a fabulous series of diagrams and photos to help you ID corals on his catfish flickr site
More photos of corals on wildsingapore flickr and the IYOR flickr group