Download The Reef Builders

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NINGALOO: Australia’s Untamed Reef
TOPIC BOX 3
The Reef Builders
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The organisms responsible for actual reef building
are a diverse bunch, but mainly constitute corals and
algae. The so-called ‘hard corals’ are probably the
best-known organisms to build reefs, although this
name refers to a group (i.e. order Scleractinia) that
also includes some species that do not participate in
this process. A better term for reef-building corals is
hermatypic corals. Hermatypic corals precipitate
(i.e. secrete) calcium carbonate (i.e. limestone),
which forms the framework for coral reefs. The
process relies, however, on an intimate relationship
between the hermatypic coral and tiny single-celled
algae called zooxanthellae. Up to 30,000
zooxanthellae may live in a cubic millimetre of coral
tissue, especially the lining of the gut and the
tentacles, and they are the reason why corals behave
like plants in many ways58.
Unlike the coral itself, the zooxanthellae are
photosynthetic, which means that they rely on
sunlight to produce carbohydrates and the byproduct oxygen, like green plants. Since the coral
host gains up to an incredible 95% of the amino
acids (for building proteins), carbohydrates (for
energy) and lipids (for structure), produced by the
algae, the growth of the coral increases with an
increase in sunlight (like a plant), even though the
coral itself would not be able to use the energy of the
sunlight without the algae58. Once it takes on
zooxanthellae the coral really becomes ‘solar
powered’, which explains why coral reefs are
restricted to shallow, clear and sunlit waters.
Besides the production of useful organic
compounds, the algae are responsible for producing
the calcium carbonate necessary for reef growth.
The excess of limestone from the algae in its tissues is
precipitated by the coral and builds the skeleton of
the colony. Each consecutive generation of coral
polyps adds a new layer of calcium carbonate to the
internal structure. Ultimately, these limestone
skeletons of the coral colonies form the foundation of
the reef.
In return for the favours the zooxanthellae
grant the coral, they gain nutrition from the coral’s
waste products and protection. Hermatypic corals
are easily recognised by a hard structure and, in
most cases, by a green or golden-brown colouration,
due to the chloroplasts present in the zooxanthellae.
Apart from most coral species in order
Scleractinia, the blue corals of genus Heliopora
(order Helioporacae) and the fire corals of the genus
Millepora (order Milleporina) also precipitate
calcium carbonate due to a relationship with
zooxanthellae141,53. Not all corals that have
zooxanthellae in their tissues are hermatypic, though.
By the same token, not all organisms that precipitate
limestone have a symbiotic relationship with
zooxanthellae. Coralline algae, for example, are
algae that build reefs the same way that hermatypic
corals do: by precipitating calcium carbonate, but
they accomplish this without the help of their smaller
single-celled algae relatives. Their role in producing
limestone is often underestimated and may rival that
of hermatypic corals in some places.
Many other organisms contribute indirectly
to the reef-building process by the cementation of
structural body parts after death141. Forams are
probably the most important of these. These tiny
protists (neither plant or animal) are very common
on coral reefs and many species depend on
zooxanthellae (or other algae) in their tissues for
nutrients and probably also for the formation of their
calcium carbonate shells. In some areas where
forams are especially abundant, the shells of dead
individuals may add several kilograms of calcium
carbonate per square meter annually. Sponges, soft
corals, black corals and gorgonians also add to the
structure of the reef as their bodies are strengthened
with hard spicules (known as sclerites), horn or
calcium carbonate, or a combination of these.
Others have calcified skeletons (e.g. bryozoans),
shells (e.g. snails) or internal skeletons (vertebrates)
that eventually are incorporated into the reef through
the process of cementation.
The growth of a coral reef is generally very
slow because it largely depends on the growth rates
of the stronger mound-shaped corals, roughly
between 1 and 10 mm per year. The branching
species, such as staghorn coral, grow much faster (in
the order of 100 mm per year) but they are relatively
brittle and more susceptible to destruction.
Coral reefs grow up and outward, and are
restricted by depth. Reef growth stops at a depth
where insufficient sunlight penetrates for the
zooxanthellae to produce an excess of calcium
carbonate. This critical depth (generally between
25-50 m) varies with the local conditions – more
turbid waters will cause a shallower critical depth,
for example. At the surface of the water column, on
the other hand, the critical depth depends mainly on
the local tides. Corals are sensitive to the ultraviolet
radiation of direct sunlight and do not usually
survive frequent or prolonged periods of emersion
above the water surface during a low tide.
Ningaloo Origins
A Pachyseris coral (Family Agariciidae). (Photo:
Craig Kitson)
A lobed brain coral (Lobophyllia sp., Family
Mussidae). (Photo: Blue Office / MIRG
Australia)
While Porites corals are characterised by tiny
corallites, a single colony may form a coral
bombie of several metres across. (Photo: Blue
Office / MIRG Australia)
Serpent coral (Pachyseris sp., Family
Agariciidae). (Photo: Blue Office / MIRG
Australia)
Most hermatypic corals are colonies of individual
coral polyps (or ‘corallites’), each of which
contributes to a shared skeleton for the whole
colony. Because consecutive generations settle
over each other, the skeleton and the colony
typically grows up- or outward over time,
increasing in size. (Photo: Blue Office / MIRG
Australia)
A brain coral (Goniastrea or Platygyra sp.,
Family Faviidae). (Photo: Blue Office / MIRG
Australia)
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