B ivalves (clams and mussels) are among the most familiar of aquatic organisms. Many have been used by humans for centuries as important sources of food and ornament, and some species are economically important pests, fouling water intakes and other structures. It is only recently, however, that ecologists have begun to understand that bivalves also play many important roles in ecosystems (e.g., Dame 1996). The functional importance of bivalves, especially in fresh water, is still not fully appreciated. For example, recent fresh water ecology I textbooks (Wetzel 1983, Horne and Goldman 1994, Allan 1995, Petts and Calow 1996) scarcely mention the ecological roles of bivalves (the words “bivalve, ” “clam,” and “mussel” do not even appear in the index of any of these books). By contrast,