Mussel, Dwarf Wedge

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Mussel, dwarf wedge

Alasmidonta heterodon

phylum: Mollusca

class: Bivalvia

order: Unionoida

family: Unionidae

status: Endangered, ESA

range: USA (Maryland, New Hampshire, North Carolina, Vermont)

Description and biology

The dwarf wedge mussel is a very small mussel species. It does not grow any longer than 1.5 inches (3.8 centimeters). The outer side of its shell is dark in color; the inner part is much lighter. The mussel feeds on plankton (microscopic plants and small animals) and other plant matter it removes from the water through a tube called a siphon.

Like other mussels, the dwarf wedge mussel reproduces in a unique way. Males release sperm, which is carried by currents downstream. As they feed, females take in the sperm, which fertilizes the eggs stored in their gills. When the eggs hatch, the glochidia (pronounced glow-KID-ee-a; larval forms of the mussel) continue to develop in the gills.

After a certain time, the glochidia are released and attach themselves to the gills or fins of a particular fish species (those glochidia unable to attach themselves to a fish sink to the

river bottom and die). In a few weeks, after having developed a shell, the young mussels detach from the fish and sink to the riverbed. Here, they bury themselves, leaving only their shell margins (edges) and siphons exposed.

Habitat and current distribution

Dwarf wedge mussels inhabit sandy and muddy bottoms of rivers where there is not much current and very little silt (mineral particles). Too much silt or sediment (sand and stones) in the water can clog a mussel's siphon and kill it. Major populations of this mussel are found in only four states: Maryland (in the McIntosh Run and Tuckahoe Creek), New Hampshire (Ashuelot and Connecticut Rivers), North Carolina (Little and Tar Rivers), and Vermont (Connecticut River).

History and conservation measures

The dwarf wedge mussel's total population size is currently unknown, but it is obviously in decline. Previously, it was found as far north as New Brunswick, Canada. Its range extended from there south through North Carolina. The populations that remain are isolated and contained in a relatively small region that is growing smaller.

As with other North American freshwater mussels, dams and water pollution have destroyed much of the dwarf wedge mussel's habitat. When dams are built, the water upstream becomes filled with silt. Downstream, water levels, currents, and temperature change often. Industrial wastes and pesticide runoff from farms are the main pollutants of the mussel's habitat. If these pollutants do not kill the mussel immediately, they accumulate in its tissues and will eventually kill it.

Saving the dwarf wedge mussel habitat is the only way to ensure the survival of this species.