Two communities dominated by deposit-feeding bivalves are recognized in Quisset Harbor, Massachusetts. Interference with life position and sediment preconditioning restrict the distribution of some species. The Eel Grass Community, dominated by Nucula proxima, comprises primarily non-siphonate forms which live in fecal-pellet rich, high-water-content, oxygenated sediment. The sediment texture results from active burrowing. This soft, mobile sediment readily clogs the siphons of siphonate bivalves. The Channel Community, dominated by Yoldia limatula, comprises primarily siphonate forms which live in partly compacted, low-water-content muds. Each community is characterized by abundant surface, shallow, and deep feeding species. Competition for food has generated this distinct stratification in feeding depth of the dominant species with each community.
Similar competitive relationships, affecting both species occurrence and trophic stratification, are apparent in three Silurian age deposit-feeding bivalve communities from the McAdam Brook Formation of Nova Scotia. A community dominated by four non-siphonate species inhabited soupy, high-water-content muds. A community dominated by three siphonate species lived in cohesive muds. A mixed community, dominated by four species, two non-siphonate and two siphonate species (the most abundant of which are present, but not as the most abundant forms, in the other communities), characterized the mid-way point of the ecotone between the other two communities.
These deposit-feeding bivalve dominated communities have similar structural patterns although separated by 400 million years and despite environmental differences and evolutionary changes. Both response to physical factors, such as sediment character, and interspecific competition impose and maintain this type of community structure.