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Montana Field Guides

Ubiquitous Peaclam - Pisidium casertanum

Global Rank: G5
State Rank: SNR

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General Description
Fingernail clams are small "mostly about the size of a finger or thumbnail" bottom-dwelling, filter-feeders found in ponds, lakes and streams throughout Montana. They are native and can be quite abundant, providing food for a variety of animals and producing large accumulations of empty shells. These shells can be quite fragile compared to introduced Asian clams of the family Corbiculidae, which have not been reported in Montana, yet.

Diagnostic Characteristics
Anterior cusp of left valve not twisted, but parallel to the dorsal margin; shell tapering ventrally in end view; hinge long (more than 3/4 shell length); anterior end rounded; shell without heavy ridges or with ridges only on the beaks; cardinals near anterior cusps; cusps of A II with steeply inclined sides, but not toothpick-like. Some specimens in which the hinge is short (less than 3/4 shell length) may be distinguished by the following features: cusp of P II distal or on distal side of center; anterior end curves gently into the dorsal margin; cusp of P II central or on distal side of center; beaks never ridged; dorsal margin almost straight or only slightly curved. Dimensions from various authors and collection sites in the following ranges: L. 8.2-1.7, H. 6.5-1.3, D. 4.5-0.7 mm (La Rocque 1967).

Range Comments
La Rocque (1967) includes Montana in range, indicates the species occurs throughout the United States. Burch (1972) reports that the species has been reported for all of the United States except Hawaii, Kentucky, and North Dakota. Clarke (1981) states this is the most widely distributed species of freshwater mollusk in the world and in found throughout North America.

Summary of Observations Submitted for Montana
Number of Observations: 20

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Relative Density



(Observations spanning multiple months or years are excluded from time charts)

The species is adapted to a wide variety of habitats and considered to be the most common Pisidium. It is found in all aquatic environments including "bog ponds, ponds, and swamps that dry up for several months each year" and temporary streams or seepages. It can be found wherever any other Pisidium is found, except in deep water. The species exhibits a variation in shell characteristics which corresponds to the environment in which is it found. "Shells with thin walls and smooth outlines come from ponds, swamps, lagoons, bog ponds, and small lakes that are filling up with marl. The heavier shelled, typical Casertanum lives in rivers or fairly large creeks." It has been collected in sand, mud, and clay bottoms at depths ranging from 0.5-3.0 m, with pH levels of 5.8-7.95, and fixed carbon dioxide levels of 5.5-30.56 ppm (La Rocque 1967). Clarke (1981) indicates the species is found in ponds, lakes, small streams, rivers, ditches, swamps, and in temporary-water habitats.

Food Habits
Fingernail clams 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 from the bottom with the foot muscle has also been observed.

Reproductive Characteristics
Litter size varies from 1 to more than 40. The life span of individuals in a population studied in Michigan is 1 year (Clarke 1981).

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Citation for data on this website:
Ubiquitous Peaclam — Pisidium casertanum.  Montana Field Guide.  .  Retrieved on , from