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.

  Description

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.

  Habitat

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).

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 ellipse is bradytictic, with females brooding their young long-term, and individuals can be found in an active state of development for most of the year before they are released as glochidia in mid summer (van der Shalie and van der Shalie 1963). 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. Known fish hosts for the glochidia of the ellipse mussel include the sunfish (Lepomis spp.), sauger (Stizostedion canadense), and drum (Aplodinotus grunnieus) (Riusech and Barnhart 2000; Allen et al. 2007).

  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.