Molluscs on subtidal cliffs: patterns of spatial distribution

The distribution of molluscan assemblages across and along a vertical rocky cli¡ near Otranto (southern Adriatic Sea, Italy) was studied. Sampling was undertaken in May and November 2000 at three sites approximately 100m apart. Three depths (5, 15, 25m) were sampled at each site, by scraping o¡ three replicate 2020 cm quadrats. Samples yielded 6722 specimens, referred to as 133 species. Multivariate analyses showed that the structure of the assemblages signi¢cantly di¡ered among depths with a clear gradient of distribution from shallow to deep stands. However, a considerable variation in the structure of the assemblage was also observed among sites at each of the three depths. The species contributing most to characterize depths and/or di¡erentiating sites at each level of the shore were identi¢ed. Some potential causes of the observed di¡erences are discussed.