http://en.wikipedia.org/wiki/Dwarf_wedge_musselThe dwarf wedgemussel, Alasmidonta heterodon, is an endangered species of freshwater mussel. It is a rare species found solely in North America's Atlantic coast streams and rivers of various sizes and moderate current. The dwarf wedgemussel's current range extends from New Hampshire to North Carolina. In May 2000, the Canadian Species at Risk Act listed the Dwarf Wedgemussel as being extirpated in Canada. This mussel may be found in small creeks to deep rivers in stable habitat 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), whereas 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 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 mm in length and 25 mm 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 (E. 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 (C. congatus) and juvenile and parr of the Atlantic salmon (Salmo salar) as host fish for dwarf wedgemussels in New Hampshire.
The dwarf wedgemussel is considered to be a long-term brooder. In general, dwarf wedgemussel glochidia may be released between March and June, with peak release times varying from south to north. Michaelson and Neves (1995) documented the reproductive cycle of the dwarf wedgemussel from North Carolina and observed that this species spawns in late summer, becomes gravid in September, and releases glochidia in April. Wicklow (in New Hampshire Wildlife Action Plan 2005) observed glochidia release beginning in March and continuing through June in the Ashuelot River in New Hampshire. In a study of dwarf wedgemussel reproduction in the Mill River, Massachusetts, McLain and Ross (2005) observed that most glochidia were released in April and May.
Reproductive output appears to be correlated with local population abundance. McLain and Ross (2005) documented that sites with the highest abundance of adult dwarf wedgemussels also demonstrated the highest proportion of gravid females, glochidial density, host infection, and density of juvenile mussels.
At one time, this species was recorded from 70 localities in 15 major drainages ranging from North Carolina to New Brunswick, Canada. Since the species was first listed as endangered by the U.S. Fish and Wildlife Service in 1990 (55 FR 9447 9451), a number of new locations have been discovered and a number of known locations are possibly no longer extant. Based on preliminary information, the dwarf wedgemussel is currently found in 15 major drainages, comprising approximately 70 “sites” (one site may have multiple occurrences). At least 45 of these sites are based on less than five individuals or solely on spent shells. The only known occurrence in New Brunswick, Canada (Petticodiac River) appears to be historic; no live mussels or spent shells were found during a 1997 survey (Hanson 1998).
The mainstem of the Connecticut River in New Hampshire and Vermont is considered to have the largest remaining dwarf wedgemussel population, consisting of three distinct stretches of sporadically occupied habitat segmented by hydroelectric dams. It is estimated that there are hundreds of thousands of dwarf wedgemussels scattered within an approximate 75-mile stretch of the Connecticut River. The Ashuelot River in New Hampshire, the Farmington River in Connecticut, and the Neversink River in New York harbor large populations, but these number in the thousands only. The remaining populations from New Jersey south to North Carolina are estimated at a few individuals to a few hundred individuals.
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