Skip to content

Snail (Physa)

Taxonomic group

Physidae: Physa

Diagnostic features

Physa is a very common introduced snail. They are small, oval, thin-shelled, and they lack any rings of nodules or shouldering around the edges of the whorls. Sometimes finger-like extensions are visible from the mantle (just inside the shell). There is no operculum (protective shield) covering the aperture (shell opening) when the animal withdraws, and the aperture is on the left when facing toward you and when the spire points upwards (see lower photo).

Typical habitats

Physa are common in many slow flowing, hard bottom or soft bottom streams or ponds.


Like most snails they are grazers, scraping biofilms from submerged surfaces.

Indicator value

  • Hard bottom: 3
  • Soft bottom: 0.1

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

Physa can be found in habitats with good water quality, but they can also be abundant in streams with very poor water quality. Therefore they have been assigned low tolerance values of 3 (hard bottom sites) and 0.1 (soft bottom sites).