We conducted a study on the taxonomic composition and diversity of thanatocoenoses of marine mollusks sampled at five sites on a circum-Sardinian island (San Pietro Island, South-Western Sardinia). The aim was to measure the differences in terms of biodiversity by analyzing shells >2 mm. We collected 71 taxa (level of diversity γ) at five different sites. Four species represented more than half of the sample: Bittium reticulatum, Glans trapezia, Lucinella divaricata and Truncatella subcylindrica. We observed a high difference in the number of taxa (level of diversity α) between sites (range: 18-41; after normalization using the Margalef index: 9.45-15.84). The turnover of β-diversity between low-energy sites was lower than that among high-wave energy sites; low-energy wave sites located near shallow and extensive Posidonia oceanica beds showed the highest values of diversity metrics. Climatic factors related to wave energy (exposure to prevailing West winds) and the presence of Posidonia oceanica beds could explain the diversity patterns. Although our diversity analysis conducted across mollusk thanatocoenoses may show some methodological weaknesses (the storage and accumulation time of the shells; the uncertainty regarding the taxonomy based exclusively on shells and the relative importance of the living populations in front of the sampling sites), we suggest that this "proxy" approach could allow for a rapid first assessment of the conservation status of coastal and marine ecosystems (i.e. Posidonion oceanicae habitat) when sampling time is short and there are several areas to explore.