Dwarf wedgemussel

The dwarf wedgemussel (Alasmidonta heterodon) is an endangered species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.

This is a rare species found solely in North America's Atlantic coast streams and rivers of various sizes and moderate current.

The dwarf wedge mussel's current range extends from New Hampshire to North Carolina. The dwarf wedge mussel is federally listed as endangered and state-listed as endangered in Connecticut, Massachusetts, Maryland, North Carolina, New Hampshire, New Jersey, New York, Pennsylvania, Virginia, and Vermont.

It previously lived in New Brunswick, but it is locally extirpated in Canada since 1968. The Canadian Species at Risk Act listed the dwarf wedgemussel in the List of Wildlife Species at Risk as being extirpated in Canada.

This mussel may be found in small creeks to deep rivers in stable habitats with substrates ranging from mixed sand, pebble and gravel, to clay and silty sand. In the southern portion of its range, it is often found buried under logs or root mats in shallow water (USFWS 1993), where in the northern portion of its range, it may be found in firm substrates of mixed sand, gravel or cobble, or embedded in clay banks in water depths of a few inches to greater than 20 feet (Fichtel and Smith 1995; Gabriel 1995; Gabriel 1996; Nedeau and Werle 2003; Nedeau 2004a, 2004b, 2006).

The dwarf wedgemussel is possibly the only freshwater mussel from North America that has two lateral teeth on the right valve and only one on the left side. The dwarf wedgemussel is a small mussel, whose shell size rarely exceeds 45 millimetres (1.8 in) in length and 25 millimetres (0.98 in) high. The trapezoidal-shaped shells are colored brown or yellowish-olive, with reddish brown or greenish rays in young or pale specimens. The nacre is bluish or silvery white, and is iridescent on the posterior.

Its reproductive cycle is typical of other freshwater mussels, requiring a host fish on which its larvae (glochidia) parasitize and metamorphose into juvenile mussels. The dwarf wedgemussel is not a long-lived species as compared to other freshwater mussels; life expectancy is estimated at 10 to 12 years (Michaelson and Neves 1995).

A number of fish species have been positively identified as hosts for the dwarf wedgemussel. Michaelson and Neves (1995) confirmed the tessellated darter (Etheostoma olmstedi), Johnny darter (Etheostoma nigrum), and mottled sculpin (Cottus bairdi) as host fish for dwarf wedgemussels in the southern part of its range. Wicklow (in New Hampshire Wildlife Action Plan 2005) confirmed the slimy sculpin (Cottus congatus) and juvenile and parr of the Atlantic salmon (Salmo salar) as host fish for dwarf wedgemussels in New Hampshire.