Pisiidae

THE XERCES SOCIETY FOR INVERTEBRATE CONSERVATION

Aquatic Invertebrates in Pacific Northwest Freshwater Wetlands
An Identification Guide and Educational Resource

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Identify taxa

Mollusca

Veneroida

Sphaeriidae


	Sphaeriidae (fingernail clams, pea clams) Key to Genera \| List of Genera

The fingernail clams are abundant and ubiquitous and can be found in many wetlands and other aquatic habitats. They are often overlooked when scanning a sample because they look like sand or pebbles. They spend most of their time burrowing through mixed sediments of gravels, sand, and/or fine organics, and filtering out organic material as they go. They can be identified by their, small, relatively round, white to tan shells.

The Sphaeriidae are sometimes called Pisidiidae, as is a family of tiny beetles. We use the family name Sphaeriidae here, but Pisidiidae is used in many references. Three genera of Sphaeriidae were collected in a study in the Willamette Valley in Oregon; they are described briefly below.

Size: small to medium
Identifying feature(s): small, delicate, round shells without raised growth rings
Habitat: sand or gravel of the wetland substrate; also found in moving waters
Tolerance: moderate


	Key to Sphaeriidae
	Pisidium (pea clams) The umbo of the shell is not prominent and is off center (posterior). Pisidium clams are very common and can be identified by their small umbo that is slightly off-center. They are unique in that they have only one siphon.

	Musculium (fingernail clams) The umbo is very prominent and near the center of the shell; shell is fragile; body has two long siphons that are free from each other along much of their length. Musculium can be identified by the prominent umbo that is centered at the back of the shell. The shell is fairly delicate. The body has two siphons that tend to be long and are free from each other for part of their length.

	Sphaerium (fingernail clams) Umbo is not prominent, but is near the center of the shell; shell is fairly tough; body has two siphons that are connected along their length. Sphaerium can be identified by the rounded umbo that is not at all prominent and is centered at the back of the shell. The shell is fairly sturdy. The body has two siphons that are fused along their length. The photo at right was used with permission by Michael Clapp and is from his website www.nwnature.net.
	List of Genera Sphaerium Musculium Pisidium

Resources

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