The Mya arenaria biocoenosis can be found on sandy and silty-sandy sediments, in habitats which mainly disperse into fresh waters (Gomoiou 1981, Petran and Gomoiou 1972, Zaitsev and Alexandrov 1998). This allochtonous species, recorded for the first time in the Black Sea in 1966, is tolerant to pollution and as such has substituted Lentidium mediterraneum, where this biocenosis has disappeared as a result of habitat degradation. This biocenosis is now present in the estuarine regions of the northwestern Black Sea. The dominant species (Mya) accounts for an average of 80% of the total density and biomass of the biocenosis itself. In areas with conspicuous river run-off such as the Danube-Dnester this value can rise to as much as 96%. The accompanying species are the polichaetes Nereis succinea and Polydora ciliata limicola. (Zaitsev and Alexandrov 1998).