Several types of corals on our reefs

Used with permission from J.E.N. Veron, Australian Inst. Marine Science.

The islands of American Samoa are blessed with an abundance of coral, so this article presents an introduction to these unusual organisms.

Corals are animals like ourselves, although that may not be readily apparent because many look like whitish rocks, especially those washed up on the beach. In a sense, corals are indeed partly rock, because only the outer thin layer of the coral is inhabited by the coral animal itself. In that way, corals are like large trees – the inner part is hard and provides structural support, the outer part is the living, growing organism. And, like trees, most coral animals are permanently attached to one spot on the reef.

The coral rubble that Samoans traditionally spread outside their houses, and the coral rocks along our beaches, are old, dead pieces that broke off the reef during a storm, got tumbled around and tossed up on the beach.

Living corals grow primarily on the outer reef flat and in deeper water. Although they take varied shapes, the coral animals inhabiting the surface of all these types are similar. They look somewhat like miniature sea anemones (matamalu, ulumane) or upside-down jellyfish (alualu) with short tentacles that give the coral a slightly fuzzy appearance when the tentacles are extended. Each single coral animal is called a polyp, but the coral branch or block we see on the reef is actually not a single animal but a colony of hundreds or thousands of tiny polyps living side by side, giving the appearance of being a single coral animal.

A polyp with tentacles extended	Diagram of a polyp

The coral's short tentacles can be pulled back into the hard part of the coral when the animal is disturbed or when the coral is exposed at low tide, so even a live coral can look like a rock at such times.

In the coral shown at the right, a single polyp lives in each hole. Together, they form a colony of polyps.

It seems almost inconceivable that these tiny coral polyps can build the hard coral ‘rocks’ that we see on the reef. They do this by secreting layers of a hard substance (calcium carbonate) beneath their living cells. It’s as if each tiny polyp built a rock-solid house for itself but then, as it grows taller, it decides to close-off the bottom rooms in its house. Then it grows some more and closes-off another layer of bottom rooms, and so on. In this way, the coral polyp always lives in the outer, top layer, which has been built upon layers and layers of rooms below.

Each polyp also cements its high-rise house to those of its adjacent neighbors which strengthens the whole structure, resembling a solidly built apartment complex. Adding on these new rooms is a slow process. Growth varies from about 0.5-3 inches per year depending on the species. The very largest corals on our reefs may be hundreds of years old. Over very long time periods, these corals grow into massively strong reef structures that can bear the brunt of powerful waves that crash upon them day after day.

Corals are one of the few organisms on earth that continually build on top of their old ‘houses’, forming such large solid structures. This is not like a bird that might build its nest on top of another nest, because both of these nests decay and disappear in a short time. In fact, most organisms on earth leave little trace after they die as their bones or shells disintegrate (dust to dust). Not corals. They build structures much larger and longer-lasting than the Pharaoh's pyramids. What other organism can do this (except modern man with his steel and cement)?

Consider Swains Island or Rose Atoll, for example. Both are the remnants of old volcanos that, after millions of years, finally sank back down beneath the ocean’s surface and disappeared altogether as volcanic islands usually do (see This volcano we live on). But as they slowly sank, the coral continued to grow on top of the submerged mountain tops, layer by layer, keeping pace with the sinking rate of the mountain. The thickness of the coral there now is probably hundreds or thousands of feet thick on top of the old mountain peak and it's all that’s left poking above the ocean surface. Were it not for this thick coral foundation on top of these old mountains, Swains Island and Rose Atoll would not exist today.

P. Craig, NPS