All bivalves are aquatic, requiring water for reproductive processes, respiration, and typically for feeding. They range in depth from the intertidal zone to the deep sea; one supratidal species (Enigmonia) lives in the tidal spray on mangrove leaves or seawalls in Australia, achieving the most "terrestrial" life-mode of any bivalve. Several independent lineages of bivalves have invaded freshwater habitats, where they have diversified to produce one of the most endemic faunas as well as some of the most important biofoulers among invertebrates. Most bivalves are free-living, either epibenthic (e.g., Pectinidae) or infaunal, burrowing into sand or mud with the muscular foot (e.g., Veneridae, Donacidae). Others are cemented by one valve (e.g., Ostreidae) or permanently attached by byssal threads (e.g., Mytilidae, Dreissenidae). Specialized members of the class burrow into rock or wood (e.g., Pholadidae, Teredinidae), using one or a combination of chemical and mechanical methods. Commensal and parasitic forms (e.g., Galeommatoidea) live associated with, attached to, or within the bodies of other invertebrates.
The habitat of a bivalve is often reflected in the form of its shell. Nestlers and cementing bivalves frequently take the shape of their substrates. Individuals in calm waters often have more delicate or leaflike shell sculpture than their counterparts in fast-flowing currents.
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