Ellipsaria lineolata (Rafinesque, 1820)
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Basis for Listing
Based on the presence of old shells, the butterfly was once present in the Minnesota, St. Croix, and Mississippi rivers, but was comparatively rare, even in the 1900s (Coker 1919; Bright et al. 1990). The butterfly mussel is now uncommon in the St. Croix River, where it was found by Doolittle (1988) at only 2 sampling points. Hornbach et al. (1995) quantified it as less than 1% of all the mussel fauna in the St. Croix River. The butterfly is also rare in the lower reaches of the Mississippi River in southeastern Minnesota (Fuller 1985). In a 1995 survey of Mississippi River Pool 7, only 1 live butterfly mussel was found out of the more than 2,000 specimens examined (Davis and Hart 1995). The butterfly has apparently been extirpated from the Minnesota River (Bright et al. 1990). Given its low numbers and the vulnerability of its habitat, the butterfly mussel was listed as a threatened species in Minnesota in 1996.
The shell of the butterfly mussel is somewhat triangular, with rounded dorsal, anterior, and ventral margins. The shells of male butterfly mussels are compressed, while those of females are modestly inflated. The shell of both sexes is thick and up to 12.7 cm (5 in.) long. The outside of the shell has numerous interrupted brown rays and is generally yellowish, but can be brown in old specimens. The hinge ligament is sometimes dark green. The beak is turned forward and the beak sculpture consists of a few fine, double-looped lines, which are usually obscure. The pseudocardinal and lateral teeth are well developed and the inside of the shell is white. The butterfly does not closely resemble any other Minnesota mussel species.
The butterfly mussel usually inhabits areas of large rivers with swift currents in sand or gravel substrates. However, it appears that the butterfly has adapted to life in reservoirs in some southern states, where it is found in water depths up to 6 m (20 ft). (Parmalee and Bogan 1998).
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).
Conservation / Management
The viability of remaining butterfly mussel populations in the Mississippi River is jeopardized by the continuing decline in habitat conditions associated with the river's management as a navigation canal, and with non-point and point source water and sediment pollution. Dams, channelization, and dredging increase siltation, physically alter habitat conditions, and block the movement of fish hosts. The butterfly is also being impacted by the infestation of non-native zebra mussels (Dreissena polymorpha) in the Mississippi River and its tributaries. Zebra mussels can attach in large numbers to the shells of native mussels, eventually causing death by suffocation. Historically the butterfly mussel was harvested for use in the pearl button industry (Baker 1928), and today it is harvested in some portions of its North American range for use in the cultured pearl industry (Oesch 1984). If observed trends cannot be reversed, the butterfly mussel may become endangered within Minnesota in the future.
Conservation Efforts in Minnesota
A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the butterfly mussel's ecology and current status in Minnesota. Additionally, over 500 butterfly mussels were collected from zebra mussel infested habitats in the Mississippi River in 2000 and translocated into areas devoid of zebra mussels south of the Twin Cities.