Eurynia dilatata (Rafinesque, 1820)
Basis for Listing
Historically widespread and locally abundant in Minnesota (Dawley 1947), the spike, also known as the lady finger, is now common only in the St. Croix River and its tributaries (Doolittle 1988; Havlik 1993), Rose Creek, and at the outlet of Lake Pepin on the Mississippi River (Hart 1999). Occasional specimens are collected from tributaries to the Mississippi River (Davis 1987), but it is apparently extirpated in the mainstem of the Minnesota River and its tributaries, despite ample habitat (Bright et al. 1990; Minnesota DNR, unpublished data). It has recently been found alive in only a small number of drainages, and degradation of its stream habitat is a continuing threat. For these reasons, the spike was listed as a special concern species in Minnesota in 1996.
The shell of the spike is elongate, moderately thick to heavy, compressed or slightly inflated, and up to 15 cm (6 in.) long. The outside of the shell is brown to black, occasionally with green rays. The beak sculpture consists of coarse loops raised slightly in the center. The pseudocardinal and lateral teeth are well-developed, and the inner shell is usually purple, sometimes white or orange, or a combination. The black sandshell (Ligumia recta) is similar, but is often heavily rayed and lacks beak sculpture.
Spike mussels are usually found in small to large rivers, but they are also known to inhabit reservoirs and lakes. Whether in rivers or lakes, they are most often found in sand and gravel substrates in depths ranging from 0.6-7.3 m (2-24 ft.) (Parmalee and Bogan 1998). When spike mussels do inhabit lakes or reservoirs, they are usually associated with outlet habitats dominated by swift currents.
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. Survival rates for populations of spike mussels, much like other long-lived organisms, are high. Hart et al. (2001) reported that spike mussels have mean annual survival rates greater than 90%. These survival rates were measured in habitats colonized by zebra mussels (Dreissena polymorpha), and the survival rates in habitats without zebra mussels would most likely be even higher.
Conservation / Management
Degradation of mussel habitat in streams throughout the spike's known range is a continuing threat. Spike populations in Minnesota are vulnerable to further decline because of hydrologic alteration of streams and their watersheds; the continuing decline in habitat conditions on the Mississippi River associated with its management as a navigation canal; and non-point and point source water pollution and sedimentation. Dams, channelization, and dredging increase siltation, physically alter habitat conditions, and block the movement of fish hosts. The spike mussel is also being impacted by the infestation of non-native zebra mussels in the Mississippi River and its tributaries. Zebra mussels can attach themselves in large numbers to the shells of native mussels, eventually causing death by suffocation. If current trends cannot be reversed, the spike may become threatened in the future. Further survey work in rivers where the spike mussel was formerly documented is needed to verify its status in that former range.
Conservation Efforts in Minnesota
A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the spike mussel's ecology and current status in Minnesota. A research project designed to measure the survival rate of a population of spike mussels is also ongoing in the Mississippi River (Hart 1999; Hart et al. 2001). Additionally, a small number of spikes were collected from zebra mussel infested habitats in the Mississippi River in 2000 and translocated into areas of the Mississippi River south of the Twin Cities, where habitats were devoid of zebra mussels.