Mollusc assemblages of three concrete artificial reefs (ARs) in the Gulf of Castellammare were compared with those of two nearby natural reefs (NRs). The reefs were located in areas characterized by different degrees of water transparency, with average annual Secchi disk visibility ranging from clear (>20 m) to turbid waters (<6 m). In spring 1995, 28 samples of 400 cm 2 were scraped off the reefs at depths of 16-22 m (20 from ARs; 8 from NRs), which yielded a total of 116 species and 1084 specimens of molluscs. The assemblage derived from a NR in clear water showed the highest diversity, species richness, and evenness, while the highest number of specimens and dominance values were observed at an AR in turbid water. The lowest values of diversity and species richness were found at ARs in turbid water and the lowest number of specimens at a NR in turbid water. Comparison of the five reefs showed significant differences in average number of species and in diversity values, while differences in the number of specimens were not significant. Factorial correspondence analysis showed a pattern that was strongly polarized along the first axis by the NR in clear water and by the AR in turbid water. The most distinctive feature was the dominance of Bittium latreillii at AR sites. Three years after deployment, mollusc assemblages of ARs in the Gulf remain entities that are distinct from those of nearby natural reefs. However, the potential of the assemblages in terms of density of individuals is comparable to that of natural reefs.

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