Helicodiscus - Coil Snails (Family Helicodiscidae)

The coil snails are easily distinguished from other land snails their small size (< 4 mm), coiled cinnamon roll shape, and spiral ridges giving it a tire-like appearance. The most common is Helicodiscus parallelus (Say, 1817), the Compound Coil. Two others (H. shimeki Hubricht, 1962, the Temperate Coil and Helicodiscus notius Hubricht, 1962, the Tight Coil Snail) are possibilities.

The Tight Coil Snail (Helicodiscus notius Hubricht, 1962) was listed in NatureServe for Indiana, but no museum records were found. The Temperate Coil Snail (Helicodiscus shimeki Hubricht, 1962) is presumed extirpated by NatureServe, but there are two separate observations from Posey County. It is also considered extirpated from KY (Dourson 2010).

Helicodiscus parallelus (Say, 1817)

Helicodiscus parallelus (Say, 1817)

Compound Coil Snail
Helicodiscus parallelus (Say, 1817)

Tight Coil Snail
Helicodiscus notius Hubricht, 1962

Temperate Coil Snail
Helicodiscus shimeki Hubricht, 1962

Characters: Heliciform, simple lip, discoidal, umbilicate, 3-4 mm. The Coil snails have a coiled cinnamon roll shape and distinct multiple spiral ridges. 2-3 small teeth are usually visible deeper inside on the interior wall.

Comparison: The Compound Coil has a more narrow and deeper umbilicus comared to the Temperate coil (Dourson 2015). Helicodiscus notius Hubricht, 1962, the Tight Coil, may be present but it can be confused with the Compound Coil. Spiral ridges are absent from the embryonic whorls of the Compound Coil compared to the Tight Coil. Find images at Hotpp et al. 2020. According to Nature Serve, the Temperate Coil is "presumed extirpated" in the state. The two existing specimens were collected in 1956 and 1958 in extreme southwest Indiana by L. Hubricht, who named the species (Hubricht 1962).

Habitat: In general, Coil snails can be found associated with leaf litter and decaying wood. There may be preferences among the species for amount of moisture. The Compound Coil has been found in grasslands and verge habitats. The Temperate Coil has been reported as preferring acidic environments, whether dry or damp (Hubricht 1985, Nekola 2008).

Status: The Compound Coil is scattered throughout the state while there is only one county with the Temperate Coil snail. The presence of the Tight Coil is not documented to date.

revised: 1/23