Dwarf Wedge Mussel

Dwarf Wedge Mussel Fact, (Alasmidonta heterodon)The dwarf wedge mussel is a small freshwater mussel that rarely exceeds 1.5 inches (38 mm) in length. It is brown or yellowish-brown in color. Adult mussels are filter-feeders, feeding on algae and other small suspended particles. They spend most of their time buried almost completely in the bottom of streams and rivers.

The shell is thin but does thicken somewhat with age, especially toward the anterior end. The anterior end is rounded while the posteriorend is angular forming a point near the posterio-ventral margin. The ventral margin is only slightly curved. The nacre is bluish-white, appearing whiter in the thicker anterior end. The most distinctive shell character of the dwarf wedgemussel is the arrangement of the lateral teeth.

There are two lateral teeth in the right valve and one in the left valve. The typical arrangement for most freshwater mussel species consists of two lateral teeth in the left valve and one in the right valve. The incurrent and excurrent apertures and their associated papillae are usually white. The foot and other organs are also white.

Population Status in Massachusetts: The dwarf wedgemussel is one of the most endangered mussels in all of northeastern North America. In Massachusetts, it was historically known from the mainstem Connecticut River, several of its tributaries, and four other rivers in the southeastern and northeastern parts of the state. Dwarf wedgemussels occur discontinuously within these river systems and usually at low population densities, raising concern about the viability of the populations.

Life Cycle: The dwarf wedge mussel is sexually dimorphic, with separate sexes, unlike some mussels which are hermaphroditic, with individuals having both male and female reproductive organs. Even so, the dimorphism is very subtle; routine determination of sex in dwarf wedge mussels is at best difficult. Male dwarf wedge mussels release sperm into the water column during the mid-summer or fall.

Females collect the sperm while siphoning water for food; the eggs are then fertilized and kept within the female until they are released the following spring. By then, each egg has developed into a parasitic larvae called a glochidium. After release from the female, the glochidium attaches itself to a fish with the aid of a small hook-like appendage. Mussel glochidia are generally species-specific and will only live if they find the correct host.

With dwarf wedge mussels, the right hosts are small bottom-dwelling fish, the tessellated darter (Etheostoma olmstedi) and the mottled sculpin (Cottus bairdi). It appears that the glochidium receives little nutrition from the fish, but uses it only as a means of dispersal. After several weeks, the glochidium detaches itself from the unharmed fish and drops to the river bottom. It is then a juvenile mussel.

Threats: Construction of impoundments and pollution from industrial, agricultural, and domestic sources has degraded the habitat and water quality.

Conservation and Management Recommendations: Discovery and protection of viable mussel populations is critical for the long-term conservation of freshwater mussels. Currently, much of the available mussel occurrence data are the result of limited presence/absence surveys at road crossings or other easily accessed points of entry. Regulatory protection under MESA only applies to rare species occurrences that are less than 25 years old.

Surveys are critically needed to monitor known populations, evaluate habitat, locate new populations, and assess population viability at various spatial scales (e.g., river, watershed, state) so that conservation and restoration efforts, as well as regulatory protection, can be effectively targeted. The NHESP has produced Freshwater Mussel Habitat Assessment and Survey Guidelines and has been working with qualified experts to conduct surveys.

Maintain naturally variable river flow and limit water withdrawals. Identify, mitigate, or eliminate sources of pollution to rivers. Identify dispersal barriers (e.g., dams, impassable culverts) for host fish, especially those that fragment the species range within a river or watershed, and seek options to improve fish passage or remove the barrier. Maintain adequate vegetated riparian buffer. Protect or acquire land at high priority sites

Controls:Preservation and restoration of high water quality and habitat. Vegetative buffer strips, conservation easements, and development of mussel sanctuaries are suggested recovery methods. Research of ecology and life history as well as identification of species of fish host(s) is needed.

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