What characteristic do adult mussels have?

Adult mussels typically possess two smooth, striped shells, which are a defining characteristic of their physical appearance. These shells, known as valves, serve to protect the soft body inside the mussel and play a crucial role in its survival. The smooth texture and distinct striping can help in identifying different species of mussels. Many species exhibit this feature, contributing to their classification and understanding in the context of aquatic environments.

In comparison to the other choices, while mussels can exhibit some color variations, the most consistent and recognized characteristic across species is the smoothness and striping of their shells. A rough, spiky shell is not typical of mussels, which tend to have more streamlined shapes for efficient movement through water. Similarly, a soft, flexible body describes the internal anatomy of the mussel rather than a characteristic of the shell itself, making it less relevant to the question being asked. Colorful patterns can occur but are not as universally applicable as the smooth, striped shell characteristic, which is more definitive of adult mussels.