Freshwater mollusks are a large, diverse group, which includes snails (class Gastropoda) and clams and mussels (class Bivalvia). Mollusks have a body plan that includes a calcium carbonate shell, mantle tissue, and a muscular foot. Gastropods have a well-developed head with tentacles, a foot modified for creeping, and a univalve shell that is usually coiled. Snails are primarily herbivores and detritivores, and grazing by snails can control algal quantity and quality. They display a wide range of life-history strategies from separate sexes to hermaphroditism and parthenogenesis. They are common intermediate hosts for digenetic trematodes. Freshwater bivalves include fingernail clams and larger freshwater mussels that burrow in the sediment, as well as attached forms such as zebra mussels. They have a shell composed of two valves, a reduced head, and a foot modified for digging. Bivalves are suspension feeders that use their enlarged gills to filter material from the water column; through this activity they link the water column with the benthic sediments by cycling nutrients and other materials. Bivalves in the order Unionoida have a complex life cycle in which the larvae (glochidia) are ectoparasites on fish. Invasive mollusks causing ecological changes and economic damage include Bithynia, Dreissena, and Corbicula.