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.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Dwarf wedgemussels are usually found in waters with slow to moderate current that have muddy sand to sand and gravel substrate.

• Habitat Regions
• freshwater freshwater

• 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.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes shaped differently

Development

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 .

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

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.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

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.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal

Females brood fertilized eggs in the marsupia from seven to 10 months. The larvae (glochidia) are released in the spring.

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

Lifespan/Longevity

The average lifespan of a dwarf wedgemussel is approximately 15 years.

Behavior

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.

• Key Behaviors
• parasite parasite
• motile motile
• sedentary sedentary

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.

• Communication Channels
• chemical chemical

• Perception Channels
• tactile tactile
• vibrations vibrations
• chemical chemical

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.

• Primary Diet
• planktivore planktivore
• detritivore detritivore

• Plant Foods
• phytoplankton phytoplankton

• Other Foods
• detritus detritus

• Foraging Behavior
• filter-feeding filter-feeding

Predation

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.

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.

• Ecosystem Impact
• parasite parasite

Economic Importance for Humans: Positive

Freshwater mussels in general are important water quality indicator for streams and rivers.

• 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.

Additional Links

Encyclopedia of Life

Contributors

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

References

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 http://www.nj.gov/dep/fgw/ensp/somaug.htm .

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 http://fishwild.vt.edu/mussel/PDFfiles/wedgemussel.pdf .

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 http://www.fws.gov/northeast/njfieldoffice/Endangered/dwarfwedge.html .

NatureServe. 2012. "Nature Serve Explorer: An Online Encyclopedia of Life" (On-line). Alasmidonta heterodon . Accessed July 06, 2012 at http://www.natureserve.org/explorer/servlet/NatureServe?searchName=Alasmidonta+heterodon .