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a family of bivalve mollusks. The adult forms attach themselves to underwater objects and the bottoms of ships with the aid of the byssus that they secrete. Their development includes a free-swimming larval stage. The mollusks live mainly in fresh water, but also in the Caspian and Aral seas, the Sea of Azov, and in the freshened sections of the Black Sea. The species D. polymorpha is the one most widely found in fresh water; it readily spreads to river systems and quickly settles in new bodies of water (reservoirs). It can settle in large masses in hydraulic structures and interfere with their normal functioning. The mollusks are controlled by chlorinating the water and painting the hydraulic structures with paints toxic to the larvae; cathode protection of hydraulic structures is now a tactic, and methods are being devised for fighting the larvae with ultrasound.