The Fawnsfoot

Description
Habitat
Threats
Further Information
Scientific Information

Description

The Fawnsfoot (Truncilla donaciformis) is a small freshwater mussel in the family Unionidae. It has the following characteristics:

• Shell is oval to rectangular in shape, smooth and moderately thick;
• Posterior ridge is rounded and flattened dorsally;
• Beaks are full, central and slightly elevated above the hinge line;
• Beaks have three to eight fine bars. The first bar is concentric;
• Fully developed hinge teeth;
• Shell is yellow to greenish in colour with dark green V-shaped or chevron markings; and
• Average length is 35 mm; maximum length is 45 mm.

Truncilla donaciformis

Photo credit: Environment Canada

Habitat

The Fawnsfoot is widely distributed throughout central North America, occurring in 23 American states and one Canadian province. In Canada, the Fawnsfoot occurs only in the Great Lakes drainage of southern Ontario. Historically, this mussel was reported in lakes Huron, St. Clair and Erie and some of their tributaries. Currently, its distribution is restricted to the lower Thames River and to single sites in the St. Clair delta, Muskrat Creek (Saugeen River drainage), lower Sydenham River and lower Grand River. At two of these sites, only a single specimen has been found.

The Fawnsfoot is generally found in the lower portions of medium to large rivers, at depths ranging from less than one to over five metres. This mussel is usually associated with substrates of mud, soft sand or even gravel. Spawning occurs in the spring, when females use their gills to filter the sperm released by males. Eggs are fertilized and maintained in a specialized area of the gills (marsupia) until reaching the larval stage. Immature larvae (glochidia) are released into the water column where they must find a suitable fish host before metamorphosing into juveniles. After undergoing transformation, the juveniles release themselves from the fish host and burrow in the substrate where they remain for three to five years. Adults reside at the substrate surface in the summer but may burrow in the winter. They are long-lived, with lifespans up to 20 years.

Two potential fish hosts of the Fawnsfoot are the Freshwater Drum (Aplodinotus grunniens) and Sauger (Sander canadensis).

Juvenile Fawnsfoot feed on bacteria, detritus, and algae located in the sediment in which they are buried. Adults are filter-feeders and consume bacteria, plankton and algae from the water column.

The map indicates the distribution of the Fawnsfoot in southern Ontario.

Threats

The establishment of invasive zebra and quagga mussels (Dreissena polymorpha and D. bugensis) is the most important factor contributing to the decline of the Fawnsfoot. Available habitat is further limited by the fragmented distribution of the fish host and is impaired by declining water quality, due to increased water mixing, chemical contaminants and nutrient-loading resulting from agricultural sources and urban influences.

Further Information

For more information, visit the SARA Registry at www.SARAregistry.gc.ca.

Text Sources: COSEWIC Assessment & Status Report 2008.

Scientific Information:

Scientific Name: Truncilla donaciformis
SARA Status: Under consideration
COSEWIC Status: Endangered (April 2008)