Mussel of the Month

Bivalves use their anterior and posterior adductor muscles to close (i.e., adduct) the valves of their double-valved shells. Open or closed, the dorsal margins of the valves are held together by a stretchy ligament. When the adductors contract, they pull the ventral margins of the valves together, typically sealing the soft-parts within the protection of the shell. Closing the valves loads the dorsal ligament like a spring, and relaxing the adductors allows the ligament to ease the tension by pulling the valves open.

Freshwater mussels have a variety of muscles that attach to the valves: the posterior adductor, the anterior adductor, the posterior pedal retractors, the anterior pedal retractors, the pedal protractors, the dorsal elevators... even the pallial line. Like Theliderma intermedia, all freshwater mussels have a posterior adductor muscle. However, two species that are monomyarian have only a single adductor. Acostaea rivoli and Pseudomulleria dalyi evolved to take on the anatomy of oysters. They lack their anterior adductor muscles.

Freshwater mussel anatomy from Cummings & Graf (2009).