New York Natural Heritage Program
Dwarf Wedgemussel
Alasmidonta heterodon (I. Lea, 1830)
Bivalves

General Description [-]
Alasmidonta heterodon is a small freshwater mussel, usually less than 55 mm long. Its shell is subrhomboidal to suntriangular, with a prominent posterior ridge. The beak sculpture consists of two concentric ridges surrounded by two to three trapezoidal ridges along the posterior slope. The periostracum is greenish to brownish, usually with many fine green color rays. Pseudocardinal and lateral teeth are both present and the nacre is white (Strayer and Jirka 1997).

Identifying Characteristics [-]
The small size, roundly pointed posterio-basal margin, and reversed lateral hinge teeth readily distinguish this species (Strayer and Jirka 1997).

Best Life Stage for Proper Identification [-]
Adults

Behavior [-]
Adults of this species are sessile with only limited movement in the substrate. Passive downstream movement may occur when they are displaced from the substrate during floods. More major dispersal occurs while glochidia are encysted on their darter and sculpin hosts, although McLain and Ross (2005) showed that tesselated darters move very little which promotes a patchy distribution of mussel beds. Being ectothermic, activity levels are reduced greatly during colder months of the year. This is a long- term brooder that spawns in late summer, becomes gravid in the fall and the larvae become active the following spring (Michaelson and Neves, 1995).
Dwarf Wedgemussel Images
click to enlarge
The Best Time to See
Little is known about the activity periods of Unionid mussels but they are presumed to be greatly reduced during cold times of the year. Freshwater mussels are most often easiest to locate during late summer when water levels are lowest. This species is a long-term breeder (winter- early spring) (Clarke 1981) and larvae (glochidia) are released into the water from early March to June (Wicklow 2004).
J F M A M J J A S O N D
Present Active Reproducing Larvae present and active
The time of year you would expect to find Dwarf Wedgemussel present (red shading), active (blue shading), reproducing (green shading) and larvae present and active (orange shading) in New York.
Similar Species
  • Brook Floater (Alasmidonta varicosa)
    Alasmidonta heterodon can be distinguished from A. varicosa by its reversed lateral teeth. Two occur in the right valve and one in the left, opposite of what is normally found in other unionoids.