Relationship of pollution to rocky substratum polychaetes on the French Mediterranean coast

The distribution of Polychaetous Annelids (Serpulidae excluded) from infralittoral rocky shores of the French Mediterranean coast has been studied for 18 yr. Several hundred rock scrapings, each measuring 400 cm², have been made and the results analysed for species composition, Shannon-Weaver index of diversity and Sanders' similarity coefficient. These mathematical measures are well correlated with the degree of pollution. In the levels considered, the annelid settlements are structurally identical. No unique group of species was associated with the degree of pollution as is typical in the sediments. An index of biological detectors is proposed which is based on the premise of the sensitive modification of the polychaete population as a whole and is defined as the ratio of the amount of dominances of ‘polluted waters biological detectors’ (Platynereis dumerili, Theostoma oerstedi, Dorvillea rudolphi, …) to ‘pure water’ ones (Syllis spp., Amphiglena mediterranea). This ratio increases with the intensity of pollution and is always higher than 1 in a polluted environment.