Alasmidonta heterodon

Geographic Range

The dwarf wedgemussel, Alasmidonta heterodon, has a discontinuous range on the Atlantic coast of the United States. Twenty-four populations of dwarf wedgemussels are found within 12 states from Maine to North Carolina. This species may have occurred in Canada but is likely extirpated. ("Nature Serve Explorer: An Online Encyclopedia of Life", 2012)


Dwarf wedgemussels are usually found in waters with slow to moderate current that have muddy sand to sand and gravel substrate. (Shaw, et al., 2006; Windsor, 2012)

  • Aquatic Biomes
  • rivers and streams

Physical Description

This freshwater mussel has a trapezoidal shell usually less than 45 mm long and 25 mm high. The anterior end of the shell is thick and the posterior end is usually thinner. The periostracum, or the outer layer of the shell, is a brown color. In juveniles reddish brown colored rays of differing widths are visible. The most distinguishing characteristic of this species is the hinge teeth, with the right valve having two and the left having only one. These mussels are slightly sexually dimorphic, with the female shell swollen posteriorly and more trapezoidal than the male, and the male shell being more compressed, ovate and elongated. (Michaelson and Neves, 1995; "Nature Serve Explorer: An Online Encyclopedia of Life", 2012)

  • Sexual Dimorphism
  • sexes shaped differently
  • Average length
    45 mm
    1.77 in


Dwarf wedgemussel females carry eggs in their gills and receive sperm (released from males) through the gills. After eggs have been fertilized, they develop into parasitic bivalved larvae called glochidia. The time needed to develop from fertilized eggs to glochia is unknown. The newly formed glochidia are released from the female and into the water where they need to attach to a host fish to survive. The glochidia develop into juveniles while attached to a host. After metamorphosis, a juvenile mussel will be sloughed from its host, where it further develops on the stream/river bottom.

The parasitic larvae have been found to metamorphose on the following host species: mottled sculpin, tessellated darter, and johnny darter. (Windsor, 2012)


This species is a long term brooder that spawns in late summer. The male releases sperm out into the water, which float down stream and enter the females gills for fertilization. Females can receive sperm from multiple males. The resulting glochidia are then released by the female into the water. (Michaelson and Neves, 1995; "Nature Serve Explorer: An Online Encyclopedia of Life", 2012; Shaw, et al., 2006; Windsor, 2012)

The dwarf wedgemussel is a long term brooder that spawns in late summer and then becomes gravid in the fall and releases the glochidia anywhere from late March to early June. Starting around August, the males release their sperm into the water which is then carried towards the females. The females take the sperm in through their gills and they fertilize the eggs. Once fertilized, the eggs are held in the marsupia of the female which becomes swollen and dark when occupied. The eggs are held in the marsupia and recieve parental care through the winter until they are ready to be released as glochida anywhere from March to June. (Michaelson and Neves, 1995; "Nature Serve Explorer: An Online Encyclopedia of Life", 2012)

  • Breeding interval
    The dwarf wedgemussel spawns once a year.
  • Breeding season
    Males release sperm into the water around August.
  • Average gestation period
    8.5 months

Females brood fertilized eggs in the marsupia from seven to 10 months. The larvae (glochidia) are released in the spring. ("Nature Serve Explorer: An Online Encyclopedia of Life", 2012)

  • Parental Investment
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female


The average lifespan of a dwarf wedgemussel is approximately 15 years. (Div. of Wildlife, NJ Dept. of Env. Protection, 2004)

  • Average lifespan
    Status: wild
    15 years


Mussels in general are fairly sedentary. Slight movement downstream might occur but is not usually noted. Dispersal of the species occurs when the glochidia attach to their mobile fish hosts. ("Nature Serve Explorer: An Online Encyclopedia of Life", 2012)

Communication and Perception

Mussels in general are responsive to tactile and chemical stimulation. Many sensory organs are on the middle lobe of the mantle edge. In the foot, mussels have paired statocysts, fluid filled chambers with a solid granule or pellet. The mussels use the statocysts to orient themselves.

Unionids in general may have some form of chemical reception to recognize fish hosts.

Glochidia respond to both touch, light and some chemical cues. In general, when touched or a fluid is introduced, they will respond by clamping shut. (Arey, 1921; Brusca and Brusca, 2003)

Food Habits

The glochidia of the species are parasitic on its fish host. Once an adult, the dwarf wedgemussel is a filter feeder, feeding on phytoplankton and detritous. ("Nature Serve Explorer: An Online Encyclopedia of Life", 2012)


Young dwarf wedgemussels are often consumed by birds, such as ducks and herons. Young mussels are also eaten by fish. Mature dwarf wedgemussels can be consumed by some mammals, such as raccoons and muskrats. (Div. of Wildlife, NJ Dept. of Env. Protection, 2004)

Ecosystem Roles

Freshwater mussels in general occupy many tropic guilds by feeding on detritous. The mussels may also aid in the biodegradation of plant matter. ("Nature Serve Explorer: An Online Encyclopedia of Life", 2012)

Species Used as Host

Economic Importance for Humans: Positive

Freshwater mussels in general are important water quality indicator for streams and rivers. (Windsor, 2012)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are currently no known adverse effects of the dwarf wedgemussel on humans.

Conservation Status

The dwarf wedgemussel, Alasmidonta heterodon, was list as federally endangered on March 14, 1990. The dwarf wedgemussel was added to the federally endangered list primarily because of human impacts on its habitat and water quality. Some factors include: agriculture, construction, pollution, silt deposits, low oxygen levels, water level fluctuation, temperature fluctuations, flooding, erosion, and siltation. (Windsor, 2012)


jill Bowne (author), Rutgers University, pat dziamba (author), Rutgers University, David V. Howe (editor), Rutgers University, Renee Mulcrone (editor), Special Projects.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


an animal that mainly eats decomposed plants and/or animals


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.


mainly lives in water that is not salty.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


an animal that mainly eats plankton


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


movements of a hard surface that are produced by animals as signals to others


NatureServe. 2012. "Nature Serve Explorer: An Online Encyclopedia of Life" (On-line). Alasmidonta heterodon. Accessed July 06, 2012 at

Arey, L. 1921. An experimental study on glochidia and the factors underlying encystment. J. Exp. Zool., 33: 463-499.

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc.

Div. of Wildlife, NJ Dept. of Env. Protection, 2004. "Dwarf wedgemussel - August species of the month" (On-line). New Jersey Department of Environmental Protection, Division of Wildlife. Accessed July 06, 2012 at

Michaelson, D., R. Neves. 1995. Life history and habitat of the endangered dwarf wedgemussel, Alasmidonta heterodon. Journal of the North American Benthological Society, 14 (2): 324-240. Accessed September 01, 2012 at

Shaw, K., T. King, W. Lellis, M. Eackles. 2006. Isolation and characterization of microsatellite loci in Alasmidonta heterodon (Bivalvia: Unionidae). Molecular Ecology Notes, 1: 365-367.

Windsor, B. 2012. "Dwarf wedgemussel" (On-line). U.S. Fish & Wildlife Service. Accessed July 06, 2012 at