New York Natural Heritage Program
Brook Floater
Alasmidonta varicosa (Lamarck, 1819)
Bivalves

General Description [-]
The thin shell is subtrapezoidal to subovate and small (<70mm), with a prominent, rounded posterior ridge. The posterior slope has fine, well-defined ridges running across the growth lines. The beak sculpture is coarse and the shape is variable. The periostracum is greenish with dark green color rays that are often continuous. The pseudocardinals are thin and lamellar, with smooth surfaces. The laterals are absent or represented as low, rounded ridges along the hinge line. The nacre bluish white (Strayer and Jirka 1997).

Identifying Characteristics [-]
This species can be recognized by its unique shape, a prominent posterior ridge, and the fine corrugations on the posterior slope (Strayer and Jirka 1997).

Best Life Stage for Proper Identification [-]
Adult.

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 fish hosts. Being ectothermic, activity levels of mussels are thought to be greatly reduced during colder months of the year.

Diet [-]
Larvae (glochidia) of this species are parasitic on the gills of several fish species including Blacknose and Longnose Dace, Golden Shiners, Pumpkinseeds, Yellow Perch, Slimy Sculpins, and the Margined Madtom. Adult Pearly Mussels are filter feeders, able to ingest a wide range of particle sizes; Algae, detritus and Bacteria are all important food sources. Mussels in turn are eaten by Muskrats, Raccoons, Fish, and Birds (Strayer and Jirka 1997).
Brook Floater 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 easiest to locate during late summer when water levels are lowest. This species is a long-term breeder (August to April) (Clarke 1981) and larvae (glochidia) are released into the water from mid-April to May (Wicklow 2004).
J F M A M J J A S O N D
Active Reproducing Larvae present and active
The time of year you would expect to find Brook Floater active (blue shading), reproducing (green shading) and larvae present and active (orange shading) in New York.
Similar Species
  • Triangle Floater (Alasmidonta undulata)
    Very young animals of A. undulata may have ridges on the posterior slope and can be misidentified as A. varicosa.
  • Elktoe (Alasmidonta marginata)
    A. varicosa is smaller and less truncate than A. marginata, and has a more rounded posterior ridge.