Adductor muscles allow clams to open and close their shells. The muscles also allow clams to close their valves tightly when they are exposed to air, low water levels or predators.
The adductor muscles are used by file clams to allow the mollusks to swim. The clams do this by alternately contracting and relaxing the adductor muscles rapidly to open and close their shells. The rapid movement also opens and closes the valves of the clams causing water to be ejected from either side of the hinged area of their shells. The flapping valves located in the front of the clams allow them to move.
Clams contract their adductor muscles to close their shells in order to protect themselves from predators. When clams relax their adductor muscles, their shells are automatically pulled open with the aid of an elastic-like ligament that joins their valves together. The ligament is typically located on the hinge line between the umbos of the shells.
The adductor muscles are the main muscular system in clams and other bivalve mollusks. The majority of bivalve species have two adductor muscles located on the anterior and posterior sides of their bodies. Some species of bivalves only have one adductor muscle; rare cases have three adductor muscles.