Abstract
We investigated the mollusk fauna of 53 closed lagoons (atolls without passes) among the 83 French Polynesian atolls. These confined lagoons of closed atolls reveal a limited diversity of mollusks but with wide distribution of some species. A few species occur in these lagoons but each lagoon is unique with a particular qualitative mollusk composition related to only 5 species: Tridacna maxima, Pinctada margaritifera, Pinctada maculata, Chama iostoma and Arca ventricosa. Due to the absence or poor data on 14 closed lagoons, our detailed study was limited to 39. We characterised the mollusk fauna of each closed lagoon and attempted to identify group of lagoons with the same dominant mollusk composition. T. maxima, the giant clam, appeared to be the most characteristic species of these confined lagoons with large densities and populations in 32 of the 39 lagoons. P. margaritifera, the mother-of-pearl oyster, also occurs as large populations in 18 lagoons; these two species are of economic importance now as they were in the past. The three other species mentioned above occurred respectively in 25, 28 and 20 lagoons. There are 11 lagoons with original and unique characteristics (their assemblages of dominant species do not occur in any other lagoon), 28 are grouped in 7 categories of assemblages. The most numerous groups contain all 5 species or at least 4 of them. Research topics are suggested in order to understand the ability of each mollusk species to colonize and maintain its dominance in a confined lagoon.
