Molluscs on subtidal cliffs: patterns of spatial distribution

The distribution of molluscan assemblages across and along a vertical rocky cliff near Otranto (southern Adriatic Sea, Italy) was studied. Sampling was undertaken in May and November 2000 at three sites approximately 100 m apart. Three depths (5, 15, 25 m) were sampled at each site, by scraping off three replicate 20×20 cm quadrats. Samples yielded 6722 specimens, referred to as 133 species. Multivariate analyses showed that the structure of the assemblages significantly differed 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 differentiating sites at each level of the shore were identified. Some potential causes of the observed differences are discussed.