Many studies carried out in environments polluted mainly by organic matter, point out the gradual change from biologically adapted communities to communities predominantly physically controlled where only a few well-represented species survive. However, of these factors the one concerning the nature of the bottom loses more and more of its importance in the distribution of Polychaetes and no longer has that fundamental importance which is ascribed to it as a limiting factor in biologically accomodated communities. This is an extreme example of the adaptive strategy of species with respect to the difficulties of the environment.