Venustaconcha ellipsiformis (Conrad, 1836)
Basis for Listing
The ellipse went undetected in Minnesota until 1983, when it was discovered by the Minnesota DNR during a survey of the Straight River (Davis 1987). Little was known at the time about the species' distribution and status in the state, but the available data indicated it was uncommon and in need of protection. More recent sampling for this mussel in southern Minnesota has revealed populations in the Cannon (Davis 1987), Zumbro (Bright et al. 1988), Root, Cedar, and Upper Iowa river systems. The ellipse is the most common species in the upper tributaries of the Root River system, but populations in the other rivers do not appear to be as large. Relic shells are very rare from the Minnesota River Drainage, indicating it was historically present in that system in low numbers, but is likely extirpated there now. There are also a few historic records from the Mississippi River. Given its limited range and sparse population, the ellipse was listed as a threatened species in Minnesota in 1996.
The shell of the ellipse is generally small, reaching up to 10 cm (4 in.) long, and oblong with a bluntly pointed posterior. The shell is stout for its size. The outside of the shell is yellowish green to brown with green rays that become fine and wavy on the posterior half of the valve. The beak sculpture consists of fine double-looped lines, which are usually obscure. The pseudocardinal and lateral teeth are well developed and heavy for their size, and the inside of the shell is white with a shallow beak cavity. The ellipse resembles the mucket (Actinonaias ligamentina) and the spike (Elliptio dilatata) mussel.
The ellipse occurs primarily in the headwater reaches of rivers in gravel riffles (van der Schalie and van der Schalie 1963) and silty areas along stream banks.
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
Degradation of mussel habitat in streams throughout the ellipse's known range is a continuing threat to the species. Like most other freshwater mussels, the ellipse is vulnerable by virtue of its dependence on fish hosts for the metamorphism of its larvae (Neves 1993). Williams et al. (1993) considered the creation of dams and the ensuing siltation, dredging, and change in water flow, to be the most important threat leading to the extinction and extirpation of freshwater mussels. Impoundments change stream flows, which can in turn increase sedimentation rates and affect the physiological processes of mussels. Dams may also eliminate a mussel's requisite host fish species. Competition from other fish species may also locally reduce or eliminate the required host fish species from the environment. As a consequence, if the host species disappears from the river or stream, the mussel species will soon be extirpated from the habitat as well. To prevent the extirpation of mussel species, both the mussel and the host fish species must be preserved.
Conservation Efforts in Minnesota
A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the ellipse's ecology and current status in Minnesota.