Aquatic Species at Risk - Dwarf Wedgemussel

Species - Details

Scientific name:
Alasmidonta heterodon
Extirpated, listed under SARA
Region: Maritimes

Did You Know?

Can I hitch a ride?
Dwarf wedgemussels produce parasitic larvae called glochidium. Their larvae ‘hitch rides’ with host fish, attaching themselves for several weeks while developing. While exact host species are not known, controlled laboratory tests show five different species of fish could serve as hosts, including Atlantic salmon. However, scientists believe that the most likely fish host was the American shad. At early maturity the mussel drops off the unharmed fish and wedges itself into the bottom of the river, where it remains half-buried for the remainder of its up to 18 year lifespan.

At a glance

Freshwater mussels are one of the most endangered groups of species on earth. Of the 297 native freshwater mussel species in North America, 12 per cent went extinct in the last 100 years and almost 75 per cent are now in danger. The dwarf wedgemussel used to live in abundance in the Petitcodiac River of New Brunswick. But scientists believe the mussel disappeared from Canada, sometime after a causeway road was built dissecting this estuary river in 1968.

About the dwarf wedgemussel

A century ago the dwarf wedgemussel lived in at least 70 locations in 15 major watersheds along the Atlantic, from New Brunswick to North Carolina. Now this small mussel is extirpated from Canada and found in only nine American watersheds.

When it lived in Canada, it was limited to the Petitcodiac River system in southeast New Brunswick. But a kilometer long causeway road built across this river in 1968, connecting Moncton to Riverview, likely led to its demise.

Since 1968, no dwarf wedgemussels have been found in Canada, despite several attempts to locate them. In May 2000, their Canadian status was confirmed as extirpated (extinct in Canada).

How to recognize a dwarf wedgemussel

A typical dwarf wedgemussel is less than 5.5 centimetres long. Like all mussels, it has two hinged shells enclosing a soft inside. Its brown shell is oblong circular and may include yellowish-brown hues. The inside of its shell ranges from bluish to silvery-white, with an iridescent sheen in the back section. It is easy to overlook, as it tends to be half-buried in the river-bottom.

Where the dwarf wedgemussel lives

Dwarf wedgemussels continue to live in small freshwater streams and medium-sized rivers in the eastern United States. They prefer bodies of water that have slow to moderate currents, with underlying sand, fine sediment or gravel substrates and streamside vegetation. Waters clouded with suspended silt are inhabitable for dwarf wedgemussels.

Why it’s at risk

Freshwater mussels have proved extremely susceptible to human-caused disturbance, especially damming of rivers and channelling streams. In Canada, a kilometer long causeway road likely led to the dwarf wedgemussel’s demise by impeding the fish passage of its fish host. Without a fish host, the mussels were unable to complete their lifecycle.

In addition, these freshwater mussels are sensitive to pesticide and metal contamination, as well as low oxygen levels, all more likely to occur in dammed rivers. Declining populations of mussels in rivers and lakes is now considered an early indicator that pollution is interfering with the ecosystem.

What’s being done

Several federal and provincial studies have recommended replacing parts of the causeway road on the Petitcodiac River with a bridge. As of October, 2006, this was still being considered.

Dwarf wedgemussels are protected under the federal Species at Risk Act (SARA) and the federal Fisheries Act which prohibits destruction of fish habitat. However, the species is extirpated in Canada and given the current Petitcodiac River conditions, recovery is likely not feasible at this time.

What can you do?

Find out more about the dwarf wedgemussel and be aware of human-induced threats. Get involved with the Habitat Stewardship Program for Species at Risk (HSP) or another conservation organization.

For more information, visit the SARA Public Registry

Background information provided by Environment Canada, March 2006.