New York Natural Heritage Program
Dwarf Wedgemussel
Alasmidonta heterodon (I. Lea, 1830)
Bivalves

Threats [-]
Water pollution and impoundments are the primary threats. This species requires a low silt environment with a slow to moderate current, a situation that dams alter both upstream and downstream of the impoundment. A wide array of industrial, agricultural and domestic pollutants have been responsible for the dwarf wedgemussel's disappearance from much of its historical range and continues to be a problem in most aquatic systems (USFWS 1993). The darter and sculpin glocidial host fish species are generally pollutant sensitive taxa and a healthy fish assemblage is critical to viable mussel populations (Pinkney et al. 1997). The low densities (< 0.5 per square meter) in which this species occurs is problematical since successful reproduction is density dependent. Females need to be in close proximity to a sperm- releasing male to be successfully fertilized (Strayer et al. 1996). Competition with exotic bivalves, both the Asian clam (Corbicula fluminea) and zebra mussel (Dreissena polymorpha) could pose a threat because they are expected to eventually invade all of New York's watersheds, although neither has yet invaded the upper Delaware system (Strayer and Ralley 1991). The majority of 45 individuals collected from the Neversink population were 4 and 5 year old specimens, and none were older than 8 years (Michaelson and Neves 1995). This indicates recent successful reproduction. However, since this species can live at least 20 years, and perhaps much longer, the lack of older individuals in the popualtion could indicate high rates of mortality of older animals, possibly from pollution. Ageing Unionids by counting growth rings has recently been shown to vastly underestimate the true age of individuals, so the ages cited above may be too low by up to an order of magnitude (Strayer 2004).

Conservation Strategies and Management Practices [-]
When feasible, the removal of impoundments in order to restore rivers to their natural flow would be benefical. The Nature Conservancy negotiated with the Army Corps of Engineers for the removal of the Cuddebackville Dam in the summer of 2003 in order to restore natural flow patterns to the lower Neversink and is working to reduce alterations to the natural flow caused by the upstream Neversink Reservoir Dam. The outcome of this management strategy on the mussel populations has not been evaluated. The U.S. Fish and Wildlife Service has suggested that reintroductions may need to be undertaken to bring low- density populations back up to viable levels and re-establish populations extirpated from certain rivers (USFWS 1993).

Research Needs [-]
There is still much to learn about this species, including confirmation of host fish(es) in the Delaware and the Neversink rivers, diet, age and growth, and mortality factors. Details about habitat requirements (current speed, water depth, substrate grain size, substrate stability, water temperature, and water quality factors) also need work. However, Strayer and Ralley (1993) found that the distribution of this species was not related to these typical physical habitat qualities, but instead to long term stability of the substrate (i.e., flow refuges). Both large and smaller scale forces promoting the patchy occurrence of Unionid mussel beds is an active area of research (Strayer 2004).