The handful of mussel species we usually eat have become cosmopolitan: they have hitched rides or been intentionally introduced to various parts of the world, where they both grow naturally and are farmed and marketed at 2.5 in/6 cm in less than two years. The Mediterranean and Atlantic species of Mytilus have complementary habits; the Atlantic is in its prime in the spring and spawns in the summer; the Mediterranean is best in summer and spawns in winter.
Mussels anchor themselves in the intertidal zone by means of a thatch of tough proteinaceous fibers called the byssus, or “beard.” Where the clams have two similar adductor muscles to close and hold the shells tightly shut, the mussel has one large adductor at the wide end and a small one at the narrow end. The rest of the mussel body comprises the respiratory and digestive systems and the mantle. Sexual tissues develop throughout these systems. Coloration depends on sex, diet, and species; orange pigments from algae and crustaceans accumulate more in female and Atlantic mussels.