Three specimens of blue mussel (Mytilus edulis), a medium-sized edible marine bivalve mollusc. Blue mussels live in intertidal areas around the world, attached to rocks and other hard substrates by strong (and somewhat elastic) thread-like structures called byssal threads. They are commonly harvested as food in many different cuisines. In the upper left, the mussel is closed. The upper right shows the mussel slightly open, with the white posterior adductor muscle visible. Lastly, in the bottom individual, the adductor muscle has been cut to allow the valves to open fully.Photo: Rainer Zenz