Punctum - Spot Snails (Family Punctidae)

a Punctum snail

Species of the genus Punctum, the Spot snails are among the tiniest of land snails, about 1-1.5 mm in diameter (about 1/16 inch)! They are called the Spot snails, but they are not easy to spot – litter and soil samples need to be sifted and examined with a hand lens or extremely good eyesight.

Five species have museum records in Indiana. Two species were found in more of the counties: Punctum minutissimum, the Small Spot Snail (imaged here) and P. pygmaeum, the Dwarf Snail. The other three, P. blandium (the Brown Spot Snail), P. smithi (the Lamellate Spot Snail), and P. vitreum (the Glass Spot Snail) were found in fewer counties in the databases. Curiously, two species are not listed in NatureServe for Indiana - one of the less common, the Brown Spot, and one of the more common, the Dwarf Snail. All three of these last species are found in the counties bordering the Ohio River across from KY where they are also found.

The Small Spot, Punctum minutissimum, is a "good" species for Indiana, but the Dwarf Snail, Punctum pygmaeum, is particularly problematic. It is given on Animal Base as a species of  Europe and W and N Asia. NatureServe shows it as exotic in Vermont but status unknown out west in Colorado. 

General Recognition of Spot snails and Tentative Key to Species

Characters:  tiny snails (~1 - 1.5 mm in diameter); subglobose to depressed; simple lip; widely umbilicate; radial striae; about 4 whorls; thin, unreflected lip

Comparison: a hand lens or better is needed to distinguish the features in the key: teeth, riblets, and striae. Oesch et al. (2013) has great images of the surface sculpture. Also see  Punctum in Hotopp et al. (2013)

Habitat: in general - woodland habitats among leaf litter and woody debris, especially if substrate is well‐decayed, under rocks and such.

Status: widely scattered, likely under collected, basically  status not known.

Key to Punctum based on Burch (1962) and Oesch et al. (2013) and excluding Punctum pygmaeum

1a. Shell aperture without teeth or lamina – go to 2
1b. Shell aperture with a broad lamina – Punctum smithi

2a. Shell with major riblets low and relatively closely spaced with 1–3 minor ridges lightly crossed by spiral striae, not forming distinct microscopic squares – go to 3
2b. Shell with major riblets high and widely spaced with 3-7 minor ridges crossed by spiral striae forming distinct microscopic squares. – Punctum vitreum

3a. Shell somewhat depressed; umbilicus narrower – Puctum minutissimum
3b. Shell more depressed; umbilicus wider – Punctum blandianum

Punctum minutissimum (I. Lea, 1841)

Punctum minutissimum (I. Lea, 1841) 

Small Spot
Punctum minutissimum (I. Lea, 1841) 

Dwarf Snail

Punctum pygmaeum (Draparnaud, 1801)

Brown Spot Snail
Punctum blandianum Pilsbry, 1900

Lamellate Spot Snail
Punctum smithi Morrison, 1935

Glass Spot Snail
Punctum vitreum (H. B. Baker, 1930)