Mussel of the

Page last updated
1 April 2021

Mussel of the Month

The April 2021 Muscle of the Month is the posterior adductor muscle of Theliderma intermedia. All adult freshwater mussels have posterior adductors, so there are a lot of them out there.


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).


Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Superfamily UNIONOIDEA Rafinesque, 1820
Family UNIONIDAE s.s.
Subfamily AMBLEMINAE Rafinesque, 1820
Tribe QUADRULINI Ihering, 1901

Genus Theliderma Swainson, 1840

Species Theliderma intermedia (Conrad, 1836)

To find out more about posterior adductor muscles and general mussel anatomy, check out:
  • Cummings, K.S. D.L. Graf. 2009.┬áMollusca: Bivalvia. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition. Academic Press-Elsevier, New York. pp. 309-384.
NSF icon MUSSEL icon
"Making the world a better place, one mollusk at a time."