Epioblasma triquetra    (Rafinesque, 1820)

Snuffbox 


MN Status:
endangered
Federal Status:
endangered
CITES:
none
USFS:
none

Group:
mussel
Class:
Bivalvia
Order:
Unionoida
Family:
Unionidae
Habitats:

(Mouse over a habitat for definition)


Epioblasma triquetra Epioblasma triquetra

Click to enlarge

Epioblasma triquetra - male
Minnesota range map
Map Interpretation
North American range map
Map Interpretation

  Synonyms

Plagiola triquetra

  Basis for Listing

The snuffbox is the most widespread member of the genus and the only one not presumed extinct (Johnson 1978). It historically inhabited the Mississippi River in Minnesota (Grier 1922), but the only recent collections of this species have come from the St. Croix River, where it is confined to a small reach between Taylors Falls and Franconia. This isolated snuffbox population is possibly the largest in the entire upper Mississippi River drainage, and therefore in need of protection. The snuffbox was listed as a threatened species in Minnesota in 1996 and was proposed for federal endangered status in November 2010.

  Description

The shell of the snuffbox is up to 6.4 cm (2.5 in.) long and thick for its size, and it has a sharply angled posterior ridge, and a flat, finely grooved posterior slope. The beak sculpture consists of a few fine, double-looped lines, but it is usually obscure. The snuffbox is sexually dimorphic, with the females having a more elongate shell with a swollen posterior and serrated posterior shell margin. The males are somewhat triangular. The shell is yellowish with greenish-blue rays, which sometimes appear blotchy. The pseudocardinal and lateral teeth are well developed, and the inside of the shell is white. Male snuffbox mussels resemble deertoe mussels (Truncilla truncata), but deertoe mussels are more triangular and have broken V-shaped or zigzag markings that turn into distinctive rays with age. The female snuffbox resembles the elktoe (Alasmidonta marginata), but the snuffbox has a thicker shell and more well-developed teeth than the elktoe.

  Habitat

The snuffbox prefers rivers with steady current and sand and gravel substrates (Cummings and Mayer 1992).

  Biology / Life History

Mussels are long-lived animals. Members of many species may live for several decades and in some instances, a century or more. They spend most of their lives buried in the bottom sediments of permanent water bodies, and often live in multi-species communities called mussel beds (Sietman 2003).

Mussels are primarily sedentary, but they can move around with the use of their foot, which is a hatchet shaped muscle that can be extended out between the valves (shells). A mussel will burrow its foot into the sediment and then contract it to pull itself slowly along the bottom of its aquatic habitat (Sietman 2003).

Mussels eat by filtering bacteria, protozoans, algae, and other organic matter out of the water. They draw water into their body through their incurrent siphon, remove food and oxygen with their gills, and then expel the filtered water through their excurrent siphon. Food particles are carried to the mussel's mouth by tiny hairlike cilia located on the gills. Waste is expelled through the excurrent siphon (Sietman 2003).

Mussels have a complex and distinctive reproductive cycle. Males release sperm into the water, which are drawn in by females through their incurrent siphon. Fertilized eggs are brooded in the female's gills, where they develop into tiny larvae called glochidia. The snuffbox is bradytictic, with females brooding their young long-term, from September to May, before they are released as glochidia (Ortman 1919). Once the glochidia are expelled from the female's gills, they attach to fish gills or fins by clamping onto them with their valves. The glochidia live as parasites on the host fish until they develop into juvenile mussels, at which point they detach from the fish and fall to the streambed as free-living mussels. The only fish host documented for the glochidia of the snuffbox to date is the logperch (Percina caprodes) (Yeager and Saylor 1995).

  Conservation / Management

The continued existence of the snuffbox is threatened by non-point and point source water and sediment pollution. Recovery efforts are threatened by the infestation of non-native zebra mussels (Dreissena polymorpha) in the Mississippi River and its tributaries, areas that would otherwise be targeted for reintroductions. Zebra mussels can attach themselves in large numbers to the shells of native mussels, eventually causing death by suffocation. One area has been identified as an exception and is discussed in Davis' (2007) plan for mussel reintroductions between St Anthony Falls and Lake Pepin.

  Conservation Efforts in Minnesota

A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the snuffbox mussel's ecology and current status in Minnesota. Propagation efforts funded by the National Park Service are now underway and juveniles are being released into the gorge area of Mississippi River Pool 2 in downtown St. Paul, where habitat conditions have dramatically improved over the past decade.

  References and Additional Information

Cummings, K. S., and C. A. Mayer. 1992. Field guide to freshwater mussels of the Midwest. Illinois Natural History Survey Manual No. 5. 194 pp.

Davis, M. 2007. Site reintroduction plan for Actinonaias ligamentina (mucket) and Epioblasma triquetra (snuffbox) at St. Paul, MN (Mississippi River at Hidden Falls). 7 pp.

Doolittle, T. C. J. 1988. Distribution and relative abundance of freshwater mussels in the Saint Croix National Scenic Riverway. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resouces. Unpaged.

Grier, N. M. 1922. Final report on the study and appraisal of mussel resources in selected areas of the Upper Mississippi River. American Midland Naturalist 8:1-33.

Johnson, R. I. 1978. Systematics and zoogeography of Plagiola (Dysnomia Epioblasma), an almost extinct genus of freshwater mussels (Bivalvia: Unionidae) from Middle North America. Bulletin of the Museum of Comparative Zoology 148:239-321.

Ortmann, A. E. 1919. A monograph of the naiades of Pennsylvania. Part III: Systematic account of the genera and species. Memoirs of the Carnegie Museum 8:1-384.

Sietman, B. E. 2003. Field guide to the freshwater mussels of Minnesota. Minnesota Department of Natural Resources, St. Paul, Minnesota. 144 pp.

Yeager, B. L., and C. F. Saylor. 1995. Fish hosts of four species of freshwater mussels (Pelecypoda: Unionidae) in the Upper Tennessee River drainage. American Midland Naturalist 133:1-6.