The coral

Mehdi Adjeroud , EPHE

The fauna inhabiting the coral reefs in French Polynesia belong to 170 species, and these belong in turn to 48 genus. However, this is quite a low  level of diversity in comparison with what occurs in the Indo-Malay archipelago and Australia (where more than 70 genus are to be found) : the latter are the peak regions on the species diversity map. The reasons for the relatively low diversity which exists in Polynesia are first, that these atolls are located far from the peak regions, and secondly, that the predominant currents flow from east to west, which prevents the species from spreading from the main centres in which they are concentrated. Most of the species found to occur in Polynesia are also to be found in the western Pacific. There exist very few coral species which are known to be endemic to Polynesia.  

Generally speaking, the fauna inhabiting the French Polynesian coral reefs are more numerous on the reefs surrounding the high volcanic islands than on the atolls.

On the parts of the barrier reefs which are in direct contact with the ocean waters (the inner reef flats and the outer slopes), the genus Pocillopora, Montastrea, Leptoseris and Fungia are also to be found.

Some genus, such as  Psammocora and Porites (Synarea), occur only rarely if at all on the atolls, but play an essential role in the construction of the reefs surrounding the high volcanic islands.

The coral communities inhabiting the Polynesian atolls mostly belong to  a large pool of commonly occurring species, in addition to a small number of occasional or rarely occurring species. On each atoll, communities of two main kinds are to be found: those established on the barrier reef, which come into direct contact with the ocean waters, and those inhabiting the lagoon

Among the communities inhabiting the barrier reefs, some are established on the outer reef flats, and others, on the outer slopes. The communities installed on the outer reef flats are generally rather sparsely populated because of the extremely severe hydro-dynamic conditions and the exundation. On the outer slopes, a conspicuous depth-dependent zoning of the communities can be observed, due to the light and hydro-dynamic gradients. It is on the outer slopes that the communities are the most densely populated and the most highly diversified.
the reef flats	the crest	the slopes

at the bottom of the lagoon	In the lagoon, it is possible to distinguish the communities inhabiting the inner shores, those inhabiting the bottom of the lagoon, and those inhabiting pinnacles and coral mounds.		a pinnacle
	the inner shores	a hoa

In the lagoon, the greatest diversity is usually to be found on the sides and at the tips of the pinnacles, as well as on the inner shores, near the passes and hoas receiving large inputs from the open sea. The diversity of the species decreases as one moves away from these points exposed to ocean water inputs. The coral structures to be found at the bottom of the lagoons are usually fairly sparse, especially when there are no coral mounds.

The coral communities present on the barrier reefs and the outer slopes generally show very few differences from one atoll to another, since the environmental conditions are very similar throughout the Tuamotu archipelago. The only differences focus on a few fairly occasional or rare species. However, the lagoon communities differ considerably from one atoll to another, mainly due to physical factors  (Click here for further information). These differences are to be found both in the range of species present and in the number of species and the large number of coral communities. For example, 22 different kinds of coral are to be found in the large, open lagoon on the atoll of Kauehi, whereas only one kind has been recorded in the completely closed, hyper-saline Taiaro lagoon and in the tiny Reka Reka lagoon. Apart from these presumably rather extreme cases, however, most of the Tuamotu atoll lagoons harbour between 10 and 18 kinds of coral. The quantitative data recorded are given in tables a1  and a2.

References

Adjeroud M, Andréfouët S, Payri C, Orempüller J, 2000. Physical factors of differentiation in macrobenthic communities between atoll lagoons in the Central Tuamotu Archipelago (French Polynesia). Marine Ecology Progress Series, 196 : 25-38.

Bouchon C, 1983. Les communautés de scléractiniaires de l'atoll de Takapoto (Polynésie française). Journal de la Société des Océanistes, 39 : 35-42.

Chevalier JP, 1979. La faune corallienne (Scléractiniaires et Hydrocoralliaires) de la Polynésie française. Cahiers de l'Indo-pacifique, 1 : 129-151.

Faure G, 1993. Les peuplements coralliens. In : Atlas de la Polynésie française, éditions de l'ORSTOM.