Based on the nature of substratum, the bottom of the Pulicat Lake has been divided into three zones- a zone characterised by predominance of sand in the substratum with little admixture of mud; a second zone having sand and mud in equal proportions with patches of weeds; and a third zone consisting entirely of mud. Of these, the second zone was observed to harbour a rich fauna, with amphipods as the dominant element. The first zone was characterised by dominance of polychaetes while the third zone was faunistically poor.