Ligumia subrostrata (Say, 1831)
Basis for Listing
The Pondmussel (Ligumia subrostrata) is broadly distributed across the central and east central United States in the Mississippi River and Ohio River systems (Williams et al. 2008), and Minnesota is the northernmost extent of its range. This species was first discovered in Minnesota from the Rock River system in 1999 during a statewide survey initiated by the Minnesota Department of Natural Resources (Sietman et al. 2003). Based on extant populations and relic shells, we know the Pondmussel historically occurred in the Missouri (Big Sioux River) and Minnesota River drainages in southern Minnesota. However, extensive surveys show that its range in the state has contracted considerably, and it is now quite rare. It is apparently extirpated from the Minnesota River system, and its range is reduced in the Missouri River system. Less than 40 live individuals have been collected from relatively short reaches of the Rock River, Split Rock Creek, Mound Creek, and Poplar Creek. The remnant population in the Big Sioux River is of considerable conservation importance as it appears to be a stronghold for this species in the upper Midwest (Sietman et al. 2003). Given its limited and diminishing geographic range in Minnesota, as well as the small number and size of documented populations, the Pondmussel was listed as a threatened species in Minnesota in 2013.
The Pondmussel has a pair of relatively thin elongate or elliptical shells, up to 8.5 cm (3.3 in.) long. This species is sexually dimorphic; adult females have a more inflated and truncated posterior, whereas males are bluntly pointed. The outside surface of the shell is smooth (lacks ridges or pustules) and has a cloth-like texture. Color is yellowish tan (especially when young), greenish, brown, or black, often with green rays that are most pronounced on the shell’s posterior half. Beak sculpture (see photo) consists of several fine and broad ?-shaped lines. The pseudocardinal and lateral teeth are well developed but thin, and the shell interior (nacre) is white. The Pondmussel is distinguished from the Fatmucket (Lampsilis siliquoidea) and Black Sandshell (Ligumia recta) by its cloth-like periostracum (outermost layer of shell), smaller size, and ?-shaped beak sculpture.
In Minnesota, the Pondmussel occurs in creeks and small rivers in mud or sand substrate and is typically found in areas with little or no current, such as pools and stream margins. In other parts of its range, it is often introduced into artificial ponds through stocking of Bluegill (Lepomis macrochirus) and Largemouth Bass (Micropterus salmoides) carrying larvae (Williams et al. 2008).
Biology / Life History
For most of their lives mussels are sedentary filter feeders that live buried in the sediments of permanent rivers or lakes. They frequently live in dense aggregations called “mussel beds” that contain 10-30 different species. In Minnesota however, the Pondmussel usually occurs in sparsely populated streams near stable stream margins. Mussels burrow into the substrate and can move around slowly with the use of their foot, a hatchet shaped muscle that is extended out between the valves. A mussel will insert its foot into the sediment and then contract it to pull itself along the bottom of its aquatic habitat. The lifespan of mussels varies widely among species, with some attaining advanced ages of many decades to over a century (Haag 2012). The lifespan of a Pondmussel is probably less than 10 years in most cases.
Mussels eat by filtering out small organic particles, including bacteria, algae, and fungus from the water. They draw in water through an incurrent siphon, remove food and oxygen with their gills, and expel the water and waste through their excurrent siphon. Food particles are carried to the mussel's mouth by tiny hair-like cilia on the gills (Cummings and Graf 2014).
Mussels employ a variety of strategies to attract and transfer their glochidia to a suitable host, many of which involve elaborate instances of food mimicry. Female Pondmussels display a mantle lure composed of a small flap with fine papillae and an eyespot. The lure resembles a larval mayfly or small fish and has a fluttering motion (Corey et al. 2006). Glochidia transfer occurs when the fish attacks the lure, rupturing the mussel's gills and releasing the glochidia. Pondmussels also release elongate cream-colored packages of larvae (conglutinates) that the host fish eat, mistaking them for food (Corey et al. 2006).
Sunfishes are likely the primary hosts for the Pondmussel in nature, and several species have been identified as hosts in laboratory trials: Bluegill (Lepomis macrochirus), Green Sunfish (Lepomis cyanellus), Orangespotted Sunfish (Lepomis humilis), Pumpkinseed (Lepomis gibbosus), Warmouth (Lepomis gulosus), Rock Bass (Ambloplites rupestris), Black Crappie (Pomoxis nigromaculatus), Largemouth Bass (Micropterus salmoides), Smallmouth Bass (Micropterus dolomieu), Central Mudminnow (Umbra limi), Banded Killifish (Fundulus diaphanus), Bigmouth Buffalo (Ictiobus cyprinellus), and Bowfin (Amia calva) (Lefevre and Curtis 1912; Hove et al. 2016; Minnesota DNR, unpublished data). Bluegill, Warmouth, and Green Sunfish have been found infested with Pondmussel glochidia in the wild (Stern and Felder 1978). In a pond experiment, Haag and Stoeckel (2015) reported successful natural recruitment of Pondmussel using stocked Bluegill.
Conservation / Management
Because mussels are sedentary animals with a complex life cycle, they are sensitive to a variety of environmental disturbances. Glochidia and newly transformed juveniles are especially vulnerable. Pollution, habitat degradation, and fragmentation by dams throughout the Pondmussel’s range are ongoing threats. Altered river flow regimes from reduced water storage in watersheds cause streambank erosion, streambed instability, and increased suspended sediment. High levels of suspended solids have been shown to disrupt Pondmussel reproduction (Gascho Landis et al. 2013; Gascho Landis and Stoeckle 2015). Dams fragment mussel populations by blocking host fish movements and creating areas of unsuitable habitat, which increases the likelihood of local population declines and extinctions (Haag 2012). Livestock that have direct access to streams can also disturb the streambed and cause mortality through trampling.
Conservation Efforts in Minnesota
The MN DNR Mussel Program continues to survey streams in the Missouri River drainage, particularly creeks, to locate new populations and monitor known populations of this species. A critical conservation need is to identify suitable host fishes of this mussel in Minnesota. Host trials are planned for 2016 and will facilitate plans to propagate and reintroduce the Pondmussel into streams or stream reaches where it is extirpated.
The MN DNR has field guides available to help identify freshwater mussels occurring in Minnesota (Sietman 2003) and the Mississippi River (Tiemann et al. 2015). Additional information on the MN DNR’s mussel program can be found here (http://dnr.state.mn.us/mussels/index.htmlhttp://dnr.state.mn.us/mussels/index.html).
Bernard E. Sietman (MNDNR), 2018
(Note: all content ©MNDNR)