Used
with permission from J.E.N. Veron, Australian Inst. Marine Science.
A
coral polyp with tentacles extended
Diagram
of a coral polyp
Adding
on these new rooms is a slow process. Growth varies from about 0.5-3 inches per
year depending on the species. The photo on the right shows this nicely (it was
part of a science project).
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.
The very largest corals on our reefs may be hundreds of years old. 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 Egyptian 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
volcanoes that, after millions of years, finally sank back down beneath the oceans
surface and disappeared altogether as volcanic islands usually do (see Chapter
4). 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 thats left poking above
the ocean surface. Were it not for this thick coral formation on top of these
old mountains, Swains Island and Rose Atoll would not exist today.
Its
as if each tiny polyp built a rock-solid house for itself but then, as it grows
bigger, it decides to closeoff 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 house to those of its adjacent
neighbors which strengthens the whole structure, resembling a solidly built high-rise
apartment complex.
When this particular coral was living on the reef, it was stained with a harmless
red die that was absorbed only by the outside layer of living tissue. Then, as
the coral grew, it added the red stain to the new skeleton it was making, thus
marking the size of the coral at the time it was stained. After the stain was
used up, the newly laid skeleton was again white. After a one-year period, the
coral was sawed in half and we can clearly see the pink band inside the coral.
The distance from the pink band to the new outer edge of the coral is the amount
that this coral grew in one year (about 0.3 inches in this case, as measured on
the actual sample).
Seven
general kinds of coral growth forms.
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 their surfaces 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. 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 left, a single polyp lives in each hole, but all the polyps
have withdrawn into their skeleton (the polyps of many coral species emerge only
at night). In the coral at right, many fuzzy-looking polyps have emerged to feed.
It
seems 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.
7. What is a coral?
The islands of American
Samoa are blessed with an abundance of coral (about 200 species), 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.