How do the two shell valves of a bivalve open and close?

The two shell valves of a bivalve open and close through the opposing actions of the adductor muscles and the hinge ligament.12

• Closing: The bivalve closes its shell by contracting its adductor muscles, which pull the two valves together. These muscles are often divided into "quick" (striated) components for rapid closure and "slow" (smooth) components for maintaining tension over long periods with minimal energy expenditure.13
• Opening: The shell opens via the spring action of the elastic ligament located middorsally. When the adductor muscles relax, the ligament creates an opening thrust that forces the valves apart.12

This mechanism allows the mollusk to open its shell to feed or extend its foot for movement, and to close it securely for protection. Interlocking hinge teeth also help ensure the valves remain properly aligned when closed.12