Carychium - Thorn Snails (Family Ellobiidae)
Carychium exile
Carychium - Thorn Snails
Family Ellobiidae
The Thorn snails are easily distinguished from other land snails by their small size (~ 2 mm high), columnar/conical shell, and one small tooth on the parietal wall.
As well, these small land snails belong to a different order of gastropods, the Basommatophora, than most common land snails. Their eyes are at the base of the tentacles as found in the familiar aquatic snails, rather than at the ends.
The two main species are Carychium exile, the Ice Thorn and Carychium exiguum, Obese Thorn. The others are Carychium riparium, the Floodplain Thorn Snail; Carychium nannodes, the File Thorn Snail, and Carychium clappi, the Appalachian Thorn Snail.
Habitat: In general, Thorn snails can be found within and around packed leaves and small or larger woody debris on wooded hillsides. They are very tiny and can be overlooked.
Working Key (based on multiple sources)
All but Carychium riparium are described in Dourson (2015) and at Land Snails and Slugs of the Mid-Atlantic and Northeastern United States. Identification requires good magnification. This group, as with many land snail groups, probably needs revision.
1a. Shell without rib-striae, or striae weak and irregular; parietal lamella not deflected sharply downward - go to 2
1b. Shell, especially the last several whorls, sculptured with relatively strong, evenly spaced transverse rib-striae; parietal lamella bent sharply downward; shell long and slender and usually more than 1.7 mm in height; outer apertural lip usually with a thickening, but not a distinct tooth - go to 4
2a. Shell very small, 1.5 mm or less in height - Carychium nannodes Clapp
2b. Shell larger, 1.5 mm or more in height - go to 3
3a. Shell generally ovate in shape and with fewer (about 4 1/2) whorls - Carychium exiguum (Say)
3b. Shell narrow and conical, smaller (2.0 mm or less in height) - Carychium riparium Hubricht
4a. Shell generally larger, nearly always at least 1 .85 mm in height - Carychium clappi Hubricht
4b. Shell smaller, averaging 1.75 mm in height - Carychium exile Lea
Identification: Shell without rib-striae or striae weak and irregular; parietal lamella not deflected sharply downward; shell larger, 1.5 mm or more in height; shell generally ovate in shape and with fewer (about 4 1/2) whorls.
Comparisons: Carychium exile has radial striae, as does C. clappi. C. riparium is more narrow and conical, less ovate. C. nannodes is smaller.
Habitat: In general, Thorn snails can be found within and around packed leaves and small or larger woody debris on wooded hillsides. They are very tiny and can be overlooked.
Status: The Obese Thorn Snail has been found, in suitable habitat, in various regions of the state.
Carychium exile
Ice Thorn Snail
Carychium exile I. Lea, 1842
Characters: Shell, especially the last several whorls, sculptured with relatively strong, evenly spaced transverse rib-striae; parietal lamella bent sharply downward; shell long and slender and usually more than 1.5 mm in length; outer apertural lip usually with a thickening, but not a distinct tooth; shell smaller, averaging 1.75 mm in length.
Comparisons: Carychium exiguum and C. riparium lack radial striae. C. clappi is larger and C. nannodes is smaller.
Habitat: In general, Thorn snails can be found within and around packed leaves and small or larger woody debris on wooded hillsides. They are very tiny and can be overlooked.
Status: The Ice Thorn Snail has been found, in suitable habitat, in various regions of the state but not, to-date in the northwest or southeast.
Appalachian Thorn Snail
Carychium clappi Hubricht, 1959
File Thorn Snail
Carychium nannodes G. H. Clapp, 1905
Floodplain Thorn Snail
Carychium riparium Hubricht, 1978
Identification: All but C. riparium are described at Dourson (2015). (Some images)
Characters for Carychium clappi: Shell, especially the last several whorls, sculptured with relatively strong, evenly spaced transverse rib-striae; parietal lamella bent sharply downward; shell long and slender and usually more than 1.5 mm in height; outer apertural lip usually with a thickening, but not a distinct tooth; shell generally larger, nearly always at least 1 .85 mm in height
Characters for Carychium nannodes: Shell without rib-striae, or striae weak and irregular; parietal lamella not deflected sharply downward; shell very small, 1.5 mm or less in height.
Characters for Carychium riparium: Shell without rib-striae, or striae weak and irregular; parietal lamella not deflected sharply downward; shell larger, 1.5 mm or more in length; shell narrow and conical, smaller (2.0 mm or less in height).
Status: These are three less-often found species of Thorn snails in Indiana, all in the southeast.
