Home
The French Polynesian Atolls Fundamentals of Reef Ecology The Tuamotu atoll Communities
Home     Glossary  
  Fluxes of Matter in the Tuamotu atolls Types of atoll and the Ecosystems Natural Resources and their Management
The reefs

Life on the reefs

Destruction of the reefs

Bio-erosion of the reefs

Mireille Peyrot-Clausade

 

 

The 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.

The 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.

The 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 reef sedimentation.

shema1.jpg (22711 octets)

Two 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 grazers.

 

Drilling microorganisms
  • Bacteriea and cyanobacteria or cyanophytic drillers 

hyella.jpg (19484 octets)

These 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  eroded

Since 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, rhodophyta and chlorophyta

  • Chlorophyta

The 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

In addition to the predominant species Ostreobium quekettii, dead substrates also carry Pheophila dendroides, a pioneer algal species.

  • Rhodophyta

A Conchocelis stage of Porphyra and Bangia is known to inhabit the coral living in regions with very low light levels.

  • Fungi

Fungi 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 of Mayotte.

  • Mollusca 

 Lithophaga
 drilling into
 a living Porites

litho2.jpg (20757 octets)

 

The bivalve molluscan drillers inhabiting the reefs belong mainly to the following families: Mytilidae, Pectinidae, Arcidae, Gastrochaenidae, Petricolidae and Tridacnidae.

The passages  they 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 cavities.

Most 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 coral complexes.

Four molluscan species predominate in the Tikehau lagoon.

 

Gastrochaenia dentifera Gastrochaenia cuneiformis
litho3.jpg (7790 octets) botula2.jpg (15617 octets)
Lithophaga nigra Botula cinnamomea

 

  • Siponculoids 

Siponculoid drillers are among the most abundant bioerosive organisms living on the reefs

sip.jpg (30665 octets)

The 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 cross-section.

  • Polycheta 

eunice.jpg (29244 octets)

The drilling species include many families of Polycheta. These families consist mainly of Spionidae, Dorvilleidae, Cirratulidae, Sabellidae, Eunicidae and Lumbrinereidae.

The 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. 

  • Sponges 

eponges.jpg (27029 octets)

Some 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 Siphonodyction

 

These 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.

Most 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.

The organisms (madreporarians and corallinaceans) grazing on the algal foliage growing on dead substrates ingest CaCO3, which they subsequently expel in their faeces.

  • Fish 

Scaridae 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 carbonates.

Scaridae 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.

scar.jpg (30014 octets)

The 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

colobo.jpg (30797 octets)

heteromi.jpg (18413 octets)

echiobl.jpg (27057 octets)

Colobocentrotus  Heterocentrotus  Echinometra oblonga

Echinoidea of the algal ridge on Tikehau

Sea-urchins 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

Grazing 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

Synergy between grazers and drilling micro-organisms

The 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.

 

References:

M    Peyrot-Clausade  , Le Campion-Alsumard T , Harmelin -Vivien M , Romano JC , Chazottes V  Le Campion J  et Pari N 1995. La bioérosion dans le cycle des carbonates : essais de quantification de ses processus en Polynésie Française .Bull Soc geol. France  66/1 : 83-92

M Peyrot-Clausade et V. Chazottes 2001. La bioérosion récifale et son rôle dans la sédimentogénèse : Moorea (Polynésie Française) et Trou d'Eau (île de la Réunion). Oceanis.  

update : 07/10/08

Search

Atoll_site_webmaster