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Fingernail clams (Sphaeriidae)

Taxonomic group

Family: Sphaeriidae

Diagnostic features

Sphaeriids are tiny bivalves shaped a bit like miniature cockles. Their shells are often pale, but larger individuals may become brown as growths of biofilms or inorganic crusts form on the outer surfaces. They can be easily missed in field stream studies as they tend to be burried in fine sediment, and their small size and lack of movement might make them look like small stones.

Typical habitats

Sphaeriids are most common in the sandy or muddy sediments of lakes, slow-flowing streams, seepages and swampy habitats.


They are filter feeders, extracting fine particles of organic matter (including plankton) from the water.

Indicator value

  • Hard bottom: 3
  • Soft bottom: 2.9

The tolerance values (ranging from 0 to 10) give an indication of which are the sensitive taxa (values of 8 or more) and which are the tolerant taxa (values of 3 or less). For more information see: Indicator species

Sphaeriids can be abundant in pristine and degraded waters. We know little about their ability to tolerate specific types of pollution. They have been assigned low tolerance values of 3 (hard bottom sites) and 2.9 (soft bottom sites).