of the reefs
construction of the coral reefs is accompanied by a concomitant
process of destruction by physical, chemical and biological
agents. The structure and the architecture of the reefs have
been found to result from interactions between the processes of
growth and erosion. The presence of eroded flats, erratic blocks
and bioclastic sediments on the present-day reefs shows that the
destructive processes are constantly at work, preventing the
further development of the reefs.
term bioerosion was first used by Neumann (1966) to denote the
action of various organisms degrading the calcareous substrates.
Bioerosion therefore means the degradation caused by the
activities of plant and animal organisms which either drill into
the substrate or erode the surface.
agents responsible for the erosion of the reefs often have
synergistic effects. Cyclones are all the more destructive when
the substrates have been previously weakened by the activities
of erosive organisms; and the cyclones are also responsible for
the death of some coral formations, which are then invaded by
the eroders. The main consequence of this erosion is that it
releases dissolved CaCO3 into the environment, and
this substance is then re-used by the productive organisms both
in this form and in the particulate form. This recycling process
favours the production of the bioclasts which are part of the
processes involved in
bioerosion are worth mentioning: the chemical
dissolution of the substrate resulting from the action
of acid substances, and the mechanical abrasion of the
substrate. The agents responsible
for bioerosion can be sub-divided into drillers and
micro-organisms develop in the first few millimeters
below the surface of dead coral substrates. They drill
so many passages (340000/cm2 of coral) that
although they are very small (the diameter of their
filaments ranges between 1 and 6 mm), they contribute
considerably to the biological process whereby the coral
substrate is dissolved and
cyanobacteria are able to adapt to a wide range of
environments, they are very widespread: they are to be
found from the upper limits reached by the sea-spray
down to great depths.
algal drillers Ostreobium quekettii can be easily seen
in the skeleton of live Porites, where they form a
green band stretching 1 to 5mm below the region inhabited by
the polyps. These algae colonise live coral, starting from a
necrosing part of the coral, and develops along with the coral
so as to always receive the minimum vital light requirements
(0.1% of the light reaching the surface of the coral). In the
green band, more than 25% of the substrate is sometimes
dissolved due to the action of these algae
addition to the predominant species Ostreobium quekettii,
dead substrates also carry Pheophila dendroides, a
pioneer algal species.
Conchocelis stage of Porphyra and Bangia is
known to inhabit the coral living in regions with very low
can be seen forming black streaks on the coral skeletons, in
association with Ostreobium quekettii. They enter the
pores of the skeleton via the tips of aragonite needles. The
fact that fungi can use the skeletal organic carbonate in the
absence of light gives them an advantage over the algae, and
enables them to develop at greater depths on the reefs and
farther into the skeleton. Since the mycelia of various
species are often very similar, it is necessary to observe the
reproductive organs of cell cultures in order to identify them.
It is difficult to quantify the bioerosion for which fungi are
responsible; but as an indication, it has been estimated that
between 167 and 195 fungal filaments per mm2 drill
into the Porites skeletons on the reefs off the island
a living Porites
bivalve molluscan drillers inhabiting the reefs belong mainly
to the following families: Mytilidae, Pectinidae, Arcidae,
Gastrochaenidae, Petricolidae and Tridacnidae.
drill into the coral result mainly from chemical processes and
from the mechanical effects of the rotational and vertical
movements made by the mollusca moving back and forth in their
of these molluscan species work their way into dead coral
after attaching their pelagic larvae to hollows where they are
safe from predators. However, the larvae of some species are
not digested by the enzymes present in the polyps' gastric
cavities; these larvae are therefore able to develop in living
molluscan species predominate in the Tikehau lagoon.
drillers are among the most abundant bioerosive
organisms living on the reefs.
main siponculoid species involved in the erosion of the coral
substrates are Phascolosoma, Aspidosiphon,
Paraspidosiphon, Cloeosiphon, Lithacrosiphon and
Themiste. These siponculoids act both physically and
chemically on their surroundings. The bodies and probes of
many of these species are equipped with hooks, spines and
papillae, which they use to grind down the walls of their
burrows. These cavities have a characteristically round
drilling species include many families of Polycheta.
These families consist mainly of Spionidae, Dorvilleidae,
Cirratulidae, Sabellidae, Eunicidae and Lumbrinereidae.
drilling mechanism used by the polycheta has not been
described in detail. Polydora
(which are members of the Spionidae family) seem to drill into
the coral after first dissolving it via a chemical process;
Cirratulidae and Sabellidae probably use a similar process.
Eunicidae, Lumbriconereidae and Dorvilleidae seem to have a
more mechanical or abrasive action involving the highly
developed teeth with which they are endowed.
of the other drillers are sponges belonging to the
Clionidae family, the most common genus of which is Cliona,
and the Phloeodictyidae family, represented by the genus
drillers work their way into calcareous substrates of all
kinds using a combination of chemical and mechanical processes,
and releasing chalk particles and dissolved mineral elements.
The only visible sign of these organisms is the presence of
the papillae through which they inhale and exhale.
of the Cliona hollow out cavities several millimetres deep,
which tend to have a characteristic shape. These cavities form
chambers which are separated from each other by narrow
calcareous partitions. The galleries built by Siphonodyction
are easily recognisable, due to their rather unusual
structure. They form spherical cavities measuring up to
several centimetres in diameter, which are linked together at
the surface by a series of tunnels.
organisms (madreporarians and corallinaceans) grazing on the
algal foliage growing on dead substrates ingest CaCO3,
which they subsequently expel in their faeces.
and Acanthuridae are the fish which contribute most actively
to the external bioerosion of the substrates. Their digestive
apparatus is particularly well adapted to the ingestion of
have teeth resembling those of parrot-fish, which they
use to scrape the coral, as well as pharyngeal teeth
which they use to grind down chalk and algae; and yet
these fish have no stomach.
teeth of Acanthuridae are less impressive but equally well
adapted; they have no pharyngeal teeth, but they do have a
stomach and a thickly walled gizzard.
Echinoidea of the
algal ridge on Tikehau
graze on the algal foliage using their special mandibular
appendages; some species of Echinometra dig out deep
hollows, in which they take refuge all day long. The algal
ridge on Tikehau has been particularly severely eroded by the
urchins Colobocentrotus, Heterocentrotus and Echinometra
mathaei var oblonga).
gastropods also contribute to the erosion of the reef
surfaces. They damage the surface with their radulae: these
raspy tongues are lined with calcite hooks which are harder
than the aragonite or the magnesium calcite of which the coral
substrates are composed.
between grazers and drilling micro-organisms
grazing organisms and the algal and cyanophytic drillers
contribute synergistically to the process of erosion.
The micro-drillers work their way into the coral up to
what has been called a "compensatory depth",
i.e., one at which there are still sufficiently strong
light levels for photosynthesis to be possible. As the
grazers feeding on the algal foliage erode the surface
of the substrate away,
the micro-organisms go on dissolving the coral
and burrowing to constantly receding compensatory depths.
The erosion of the surface, which is often called "external
erosion", therefore results from the two-fold
activities of the micro-drillers and the grazers.
Le Campion-Alsumard T , Harmelin -Vivien M , Romano JC ,
Chazottes V Le
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