Rapa (27°28′S, 144°20′W), in the southern Austral Archipelago, is a small (~40 km2) volcanic island. Mean SSTs are 20–23°C, and this accounts for a reduced process of reef accretion. A gently sloping submarine platform (2.0–4.5 km width) surrounds the island and its steep, rugged coastline has several bays (Fig. 1). Coral distribution is sparse on the volcanic platform to 20 m depth, where dense macroalgae dominated by Sargassum occur; however, more diversified and abundant coral assemblages occur below this to 55 m, and in shallower habitats inside bays.

Fig. 1
figure 1

Satellite image of Rapa. The large inlet on the eastern side is Haurei Bay (NASA Data Buy Purchase Program)

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The BIODIV Expedition (November 2002) assessed the diversity of marine organisms in Rapa. Scleractinian corals were collected during 42 dives to a maximum 58 m depth, covering all biotopes. Identification of this collection has revealed an unexpected high coral diversity, of 112 species from 32 genera, including five species new to science (previously, 61 species were recorded from 17 dives to a maximum of 40 m depth; Faure 1985). Most diverse is Acropora, with 37 species (Fig. 2), followed by Montipora (13 species), Pavona (seven species), and Leptoseris, Fungia, Favia, and Goniastrea (four species each). This was unexpected not only from Rapa’s geographic isolation in the depauperate South Central Pacific (with ca. 170 species, French Polynesia is a low diversity area), but also in comparison with other subtropical islands, such as Kermadec (17 species; Brook 1999) and Lord Howe (83 species; Harriott et al. 1995). The coral fauna of Rapa has strong similarities with that of the Cook Islands (Chevalier 1982). Goniastrea, Hydnophora, Merulina, Scolymia, and Turbinaria are recorded at Rapa and at other islands in the Australs but nowhere else in French Polynesia (Chevalier 1982). Colonies of Alveopora, Echinophyllia, and Leptoria are particularly abundant at Rapa, whereas Pocillopora and Porites, which are among the dominant corals in the Society and the Tuamotus, are rare.

Fig. 2
figure 2

A diversified coral assemblage of Haurei, with several Acropora species (photo by Yves Lefèvre, BIOSPHOTO)

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