Atlantic ribbed mussels (Modiolus demissus)—Walter Dawn
Any of numerous bivalve species of either the marine family Mytilidae, found worldwide, or the freshwater superfamily Unionacea, called naiads, found mostly in the U.S. and Southeast Asia. Marine mussels are usually wedge-shaped or pear-shaped and 2–6 in. (5–15 cm) long. They may be smooth or ribbed and often have a hairy covering. The shells of many species are dark blue or greenish brown on the outside and pearly on the inside. Mussels attach themselves to solid objects or to one another, often forming dense clusters. Some burrow into soft mud or wood. They are eaten by birds and starfishes, and some species are raised commercially for food.