CORAL REEFS
Coral reefs are the most diverse and beautiful of all marine habitats.
Large wave resistant structures have accumulated from the slow growth of
corals. The development of these structures is aided by algae that are symbiotic
with reef-building corals, known as zooxanthellae. Coralline algae, sponges,
and other organisms, combined with a number of cementation processes also
contribute to reef growth.
The dominant organisms are known as framework builders, because they
provide the matrix for the growing reef. Corals and coralline algae precipitate
calcium carbonate, whereas the framework- building sponges may also precipitate
silica. Most of these organisms are colonial, and the slow process of precipitation
moves the living surface layer of the reef upward and seaward.
The reef is topographically complex. Much like a rain forest, it has
many strata and areas of strong shade, cast by the overtowering coral colonies.
Because of the complexity, thousands of species of fish and invertebrates
live in association with reefs, which are by far our richest marine habitats.
In Caribbean reefs, for example, several hundred species of colonial invertebrates
can be found living on the undersides of platy corals. It is not unusual
for a reef to have several hundred species of snails, sixty species of corals,
and several hundred species of fish. Of all ocean habitats, reefs seem to
have the greatest development of complex symbiotic associations.
Branching and Mound-Shaped Corals
Reef-building corals may occur in a variety of growth forms, and there
often is strong variation in coral shape even within a species. In this
photograph, we can see the branching elkhorn coral Acropora palmata
(upper left), with arms showing strong orientation into an oscillatory current
between left and right. In the right foreground is the mound-shaped coral
Montastrea annularis. It is of great interest that such differently
shaped corals can occur side by side. What might that indicate to you?
Intraspecific Variation in Growth Form
Here is an intraspecific example of variation in growth. We see two
colonies of Montastrea annularis, but one (center) is platy in appearance,
whereas the other (right) is more mound-shaped.
Deeper Water form of "Monastrea annularis"
In deeper waters (ca. 30 m) in the reefs of the north coast of Jamaica,
one finds a greatly flattened species of Montastrea, which is a very
close relative of the mound-building shallow water Montastrea annularis.
The flattened mushroom-like shape may be an adaptation to capture light
efficiently.
Plate-like Growth of Agaricia sp.
By contrast, species of Agaricia tend to be plate-like in form and
do not display nearly as much variation as some other dominant corals on
the reef. A large fauna of sponges, bryozoans, articulate brachiopods, and
other sessile epibenthic invertebrates live on the undersides of these platy
corals.
Oriented Branches of the Elkhorn Coral
Here we see the occurrence of a group of colonies of
the elkhorn coral Acropora palmata, with arms conspicuously oriented
left to right. This probably reflects a predictably strong bidirectional
current. Because of the extensive growth plasticity of some corals, it is
often difficult to distinguish between distinct biological species and plasticity
of form within a species.
Platy Form of the Elkhorn Coral
Dancing Lady Relief, Jamaica
This shot of a reef on the north coast of Jamaica vividly captures
the large amount of shade cast by the dominant elkhorn coral Acropora
palmata. Because reef-building corals depend upon zooxanthellae for
colony growth, the shaded areas are not good areas for vigorous coral growth,
which depends upon light.
Hexacoral Polyps, Closeup
Hexacoral polyps (e.g., stony corals) have tentacles in groups of
six. In hermatypic - or reef-building - corals symbiotic zooxanthellae are
found within the endoderm and are usually concentrated in the tentacles,
which increases exposure to light. The tentacles are also the site of enormous
numbers of nematocysts, which can ensnare or puncture prey.
Polyps of Montastrea cavernosa
This is a closeup of the massive scleractinian coral Montastrea
cavernosa. The tentacles capture smaller zooplankton, by means of nematocysts.
Octocoral Polyps
Octocoral polyps (e.g., sea whips, sea fans) have 8 tentacles, instead
of 6. Particles such as smaller zooplankton are probably captured by direct
interception on the tentacles. In some cases, such as in sea fans, water
streams around the colony and polyps capture particles in the quiet eddies
that are formed on the downstream side of the colony.
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