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Dreissenidae
Morphology. Mussels, enclosed by a hinged, bivalve shell.
Eyes absent (the animal with no head). Attaching to stones, etc., via a byssus.
The shell 25–40 mm in its maximum dimension; 25–40 mm wide (and about half as long across). The shell bent, with one margin quite sharply incurved and the other strongly curved outwardly. The shell opaque (and robust); yellowish, brownish or greyish, often with light-and-dark stripes.
General biology, ecology. Freshwater aquatic. Breathing via complex gills. In clean, well oxygenated, lowland rivers, canals and reservoirs, attaching to stones and other hard surfaces; sometimes in slightly brackish water.
Classification. Bivalvia.
British representation. Dreissena (1, “Zebra mussel”).