Mussels which live on bottom culture plots frequently visited by starfish have been shown to develop thicker shells, and stronger adductor muscles
However, both these changes are not immediate mechanisms of protection, as they are adaptive changes that develop over time.
A mussel does have an instinctual reaction to the presence of a starfish though. When it senses a starfish presence, due in some capacity to the excretions of digestive products from the starfish, the mussel begins to close its valves