Montana Field Guide

This light-shelled (somewhat fragile) species was previously known as Anodonta grandis ("no tooth"-indicating it's lack of a tooth on it's hinge). Shell smooth, light yellow or yellowish green with rays visible in younger specimens, becoming dark green to brown in older shells and the nacre is white. Length to 10 inches, usually size about 5-6 inches (100 to 150 mm). Unlike the other native MT mussels, it can be found in smaller prairie rivers and in intermittent streams, if permanent pools are present, and prefers softer benthic sediments. Many fish species are known to serve as hosts for this species. All of these are warm water species and most of them were introduced into Montana. The known native hosts are the freshwater drum, Iowa darter, brook stickleback and possibly channel catfish. This species is likely expanding it's range in Montana with the introduction and range expansion of more of its host fish. These include carp, bullheads, golden shiner, centrarchids and yellow perch.

Thin, fragile, very much swollen and having a slight wing along the posterior hinge line; yellow to brown and nearly black. Beak sculpture consists of loops or folds that are raised into more or less conspicuous nodes or pustules. Young shells smooth and shining, old ones usually roughened and dark, frequently eroded. Length 165, height 92, breadth 60 mm. (La Rocque 1967)

Adult freshwater mussels are mostly sedentary and spend their lives near the area where they first settled as juveniles. They will move short distances (a few meters) during spawning or if prompted by a stimulus such as a water disturbance nearby, exposure due to low water, or seasonal temperature changes. They may burrow into sediments (often doing so in the fall and emerging in the spring) or move away from the area horizontally.

May have dispersal through it's host fish, especially the carp.

The species is generally found in quiet backwaters of slow streams, rivers or lakes and prefers soft bottoms consisting of sand, silt and mud. Although, we do have records of this species occurring in a gravel/cobble mix in some small streams. It is usually found in water from a few centimeters to 2 m deep. A large number of shell types exist which exhibit various modifications in form as a result of adaption to varying environmental conditions (La Rocque 1967). (Clarke 1981) reports the subspecies Anodonta grandis grandis [now Pyganodon grandis grandis] occurring in permanent ponds, rivers, and lakes of various sizes. It is most abundant on mud, but is found on all types of substrates.

Freshwater mussels are mostly filter-feeders, siphoning in floating particulate organic materials (small plant or animal) from the water column and straining out the particles and expel the strained water. Pedal feeding with the foot muscle has also been observed, mostly in juveniles and younger age-classes.

(Clarke 1981) the subspecies Anodonta grandis grandis [now Pyganodon grandis grandis]: "The breeding season is reported to last from early August to the following April or May. Both dioecious (separate sexes) and monoecious (hermaphroditic) specimens occur. The glochidia are triangular-ovate, with spines, and measure from 0.31 to 0.36 mm long and 0.28 to 0.33 mm high. Native host fish species: Iowa darter, brook stickleback, channel catfish. Introduced host fish: carp, bullheads, centrarchids and yellow perch, green sunfish (Fuller 1974).

Threats to freshwater mussels and their habitats include: Pollution, Diversion of rivers for irrigation, hydroelectric and water supply projects, Elimination of natural fish hosts, Eutropification due to agricultural runoff and urbanization. The giant floater is the most tolerant mussel in Montana to any of the above mentioned threats.