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PART 2: INFORMATION ABOUT THE SPECIES
Last examined by COSEWIC: April 2008
The Fawnsfoot is a small freshwater mussel with a typical adult length of approximately 35 mm. The shell is oval to triangular in shape, smooth and moderately thick and yellow to greenish with numerous dark green rays often broken into v-shaped or chevron markings. The prominent posterior ridge is rounded and flattened dorsally. The beaks are full, central and slightly elevated above the hinge line. The beaks are marked with 3-8 fine bars.
The Fawnsfoot is a long-term brooder. Adults spawn in spring and females brood the young from egg to larval stage in their gills over the winter months. In spring the mature larvae are released into the water and when they encounter a suitable fish host will attach themselves onto the gills of the fish. The larvae then are encased in the gills and proceed to undergo a period of development into juveniles. Juveniles eventually drop off the fish host and grow to adulthood in habitats that are sheltered, with slow-moving water, and fine sand and mud bottom. Adults and juveniles have similar diets which consist of organic debris, algae and bacteria gathered from the water column and/or sediments.
Where are they found?
The Fawnsfoot is found only in the Great Lakes drainages of southern Ontario. Currently, its range 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. In two of these sites, only a single individual has been found.
How many mussels are there?
The Fawnsfoot has always been a small component of the overall mussel community (< 5%). Of the five existing populations, two are represented by single specimens (St. Clair delta and Muskrat Creek) while another two represent (Sydenham River and lower Grand River) multiple individuals but from only a single site. Only the Thames River occurrence represents multiple animals collected at more than one site. Populations in the Sydenham and Grand rivers still occupy the known historical range. No assessment can be made of the Muskrat Creek and Thames River populations as no historical information exists.
Threats to the population
The establishment of invasive zebra mussels 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 declining water quality due to increased turbidity, chemical contaminants and nutrient loading resulting from upstream non-point agricultural sources and urban influences.
COSEWIC Reason for Designation:
This freshwater mussel is widely distributed in central North America, with the northern portion of its range extending into the Lake Erie, Lake St. Clair and lower Lake Huron drainages of south western Ontario. It appears to have always been a rare species in Canada, representing < 5% of the freshwater mussel community in terms of abundance wherever it occurs. Approximately 86% of historical records are in waters that are now infested with zebra mussels and therefore uninhabitable. Zebra mussels, which were accidentally introduced into the Great Lakes, attach to the shells of native freshwater mussels, causing them to suffocate or die from lack of food. The species has declined dramatically and has been lost from four historical locations resulting in a 51% reduction in its range. It is now found in only five widely separated locations, two of which represent single specimens. In two locations, the species’ distribution may be limited by the presence of dams that restrict the movements of the freshwater drum, the presumed fish host of the juvenile mussels. Poor water quality resulting from rural and urban influences poses an additional continuing threat.
What will happen if this mussel is added to the SARA List?
A recovery strategy must be prepared within one year of the Fawnsfoot being added to the SARA List.
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