The utilization of polychaetes in descriptive ecology is reviewed in the light of recent research especially concerning the biota hard bottom environments. Polychaetes, often linked in the past to the concept of opportunistic species able to proliferate after an increase in organic matter, have played an important role especially with regard to impacted soft-bottom habitats. Increased knowledge of the group, suggests that not only opportunistic species can be utilised as indicators, so that these organisms can be disengaged from the old concept of opportunistic taxa. Moreover, recent researches conducted on this group allowed demonstrating as surrogacy is not always applicable. Among polychaetes inhabiting hard bottom environment, the analysis of family Syllidae appears particularly promising. Studied conducted in our laboratory demonstrated as syllid species decrease in abundance or completely disappear under varying sources of negative impact. The distribution of species also appeared indicative in underlying effects of marine protected areas (MPA) functioning, or in describing different climatic areas within biogeographical sectors. It is obvious that good results can only be obtained on the basis of good taxonomic resolution. We suggested that, in monitoring studies, operational time could be optimized not only by working at a higher-level on the whole invertebrate data set, but by also selecting a particularly indicative group and working at fine level.