Carychium exiguum

Geographic Range

Carychium exiguum, the obese thorn snail, is native to the Nearctic region. It ranges north from Newfoundland to central Manitoba (areas including Alberta, New Brunswick, Nova Scotia, Ontario, Quebec, and Newfoundland), west to New Mexico and the Sierra Madre mountains in Durango, Mexico, south to Costa Rica, and east to the Atlantic coast. It is common through the middle and eastern United States, and in many states is the most abundant species of land snail. This species has also been recorded in central Oregon. In Michigan, obese thorn snails may be found throughout the state. ("Carychium exiguum", 2003; Burch and Jung, 1988; Hubricht, 1985; Nekola, 2005; Perez and Cordeiro, 2008)


Obese thorn snails are found in very damp (but not frequently flooded) areas, such as floodplains, swamps, and moist woodlands. They are considered to be hygrophilous. These snails are generally found in areas with moss, ferns, and plant debris, in shaded areas such as under leaf litter or rotting logs. (Bashynski, 2008; Burch and Jung, 1988; Harry, 1997; Miller, 1970; Perez and Cordeiro, 2008)

In Michigan, three macrohabitats have been described for this species: small depressions in cypress (Thuja sp.) forests, grasslands (possibly former Thuja sp. forests), and hardwood forests. (Bashynski, 2008; Burch and Jung, 1988)

Physical Description

Obese thorn snails have translucent to whitish elongate shells, which are 1.2 to 2.5 mm tall, with about 4.5 convex whorls. The columular lamella, found at the left side of the aperture, is S-shaped (curving down and then up). There is a small tooth on the parietal wall of the shell opening. The aperture lip is thickened and reflected. (Burch and Jung, 1988; Harry, 1997; Perez and Cordeiro, 2008)

The soft anatomy of obese thorn snails consists of a smooth integument and a single pair of contractile tentacles with no pigmentation at the base. Eyes are located at the base of the tentacles. This species has no pedal groove, but does have a reproductive groove. These snails have approximately 100 transverse rows of teeth on their radular membranes. Sixteen teeth are located on either side of the median tooth. The radula is covered with a thick cuticle. (Bashynski, 2008; Burch and Jung, 1988; Harry, 1997)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    1.2 to 2.5 mm
    0.05 to 0.10 in


In general, land snails deposit their eggs in moist areas. Snails in the genus Carychium have a shell gland, which produces a leathery coating for the eggs. Timing for egg hatching depends on moisture and temperature, and some individuals may develop much slower than others, even if they hatch at the same time. Evidence suggests that populations of these snails in Michigan grow throughout the summer after hatching and attain near adult size by mid-fall. The snail reabsorbs the shell partitions and the shell's lamellae form during the post-embryonic stage. Thickened, reflected lips at the aperture indicate maturity. (Burch and Pearce, 1990; Geraerts and Joosse, 1984; Harry, 1997)


Carychium exiguum is hermaphroditic, and mainly reproduces by cross-fertilization. Although specific stimuli are not known, basommatophoran snails, such as obese thorn snails, may be cued to mate by encounters with other partners, increased temperatures, and/or changes in photoperiod. Two snails may locate each other through mucus trails. After a courtship ritual, snails copulate, with each snail inserting spermatophores into the other. (Burch and Pearce, 1990; Geraerts and Joosse, 1984; Harry, 1997)

Obese thorn snails likely hatch in late spring or early summer and reach maturity in the fall, particularly in temperate areas such as Michigan. Thickened, reflected lips at the aperture indicate maturity. These snails are oviparous and produce about three eggs during a reproductive cycle. (Geraerts and Joosse, 1984; Harry, 1997)

  • Breeding interval
    Obese thorn snails reproduce once yearly.
  • Breeding season
    Obese thorn snails likely reproduce in the spring and early summer.
  • Average number of offspring

Land snails leave eggs after they are deposited and do not exhibit any parental care. (Burch and Jung, 1988)

  • Parental Investment
  • no parental involvement
  • pre-fertilization
    • provisioning


The lifespan of Carychium exiguum is likely about one year; there is evidence that mortality is highest in late summer/early fall, mainly for juveniles. (Harry, 1997)

  • Average lifespan
    Status: wild
    1 years


Light intensity, relative humidity, and temperature influence much of a land snail's behavior, since these factors affect the snail's water retention. In general, land snails are nocturnal, and more active with increased relative humidity and decreased temperature. In temperate climates, an epiphram will form over the shell's aperture as the snail aestivates over the winter. During dry periods, this same membrane will form to prevent dessication. Several individuals of this species are usually found together, under leaves. They are negatively phototactic. (Burch and Pearce, 1990; Harry, 1997)

When crawling, the head of an obese thorn snail emerges from the base of the shell aperture; the shell is elevated with its columnar axis at about 45 degrees. The shell moves slightly side to side as the animal goes forward and the tip of the tail jerks slightly. These snails move slower when wet, and may lope to avoid rough substrates or to retain water. (Harry, 1997; Pearce, 1989)

  • Range territory size
    10 (high) m^2

Home Range

Home ranges of land snails vary, but smaller species such as obese thorn snails may have home ranges of 10 square meters or less. (Burch and Pearce, 1990; Nekola, 2012)

Communication and Perception

Carychium exiguum has sensory tissue at the tips of its tentacles and anterior margin of the labial palps. Land snails leave mucus trails which are used as a form of communication. The mucus allows the snails to detect individuals of their own and other species. They also have eyes located at the bases of their tentacles and are negatively phototactic. (Burch and Pearce, 1990; Harry, 1997)

Food Habits

An obese thorn snails uses its radula, a toothed feeding organ, to scrape or grind food. These snails likely feed on decaying plant material and fungi. (Bashynski, 2008; Burch and Jung, 1988; Burch and Pearce, 1990; Harry, 1997)

  • Plant Foods
  • leaves


Specific predators are not known for this species, but in general, land snails are preyed on by lampyrid beetle larvae or other insects, birds, rodents, and small mammals, particularly voles and shrews. (Burch and Pearce, 1990)

Ecosystem Roles

Generally, land snails disperse fungal spores and plant seeds, and break down detritus in the forest. While some land snails are vectors for nematodes, none are currently recorded in the literature for this species. Carychium exiguum also serves as prey to a variety of species, including insects, birds, and small mammals. (Burch and Pearce, 1990; Harry, 1997)

Economic Importance for Humans: Positive

There is no known positive effects of Carychium exiguum on humans.

Economic Importance for Humans: Negative

There are no known adverse effects of Carychium exiguum on humans.

Conservation Status

Obese thorn snails have not been evaluated by the International Union for the Conservation of Nature and Natural Resources and hold no special conservation status. (IUCN, 2013)

Other Comments

The family Carychiidae is sometimes recognized as a subfamily of the family Ellobiidae. The taxonomy of species found within the genus Carychium is still under review. ("Carychium exiguum", 2003; Burch and Jung, 1988; Burch and Pearce, 1990)


Renee Mulcrone (author), Special Projects, Angela Miner (editor), Animal Diversity Web Staff.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


helps break down and decompose dead plants and/or animals


a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.


uses smells or other chemicals to communicate


an animal that mainly eats decomposed plants and/or animals


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


marshes are wetland areas often dominated by grasses and reeds.


having the capacity to move from one place to another.


an animal that mainly eats fungus

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


associates with others of its species; forms social groups.


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


uses sight to communicate


2003. "Carychium exiguum" (On-line). Encyclopedia of Life. Accessed May 16, 2013 at

Bashynski, S. 2008. "Carychium exiguum (Say, 1822)- Obese Thorn" (On-line). BioWeb. Accessed December 15, 2013 at

Burch, J., Y. Jung. 1988. Land snails of the University of Michigan biological station area. Walkerana, 3/9: 1-177.

Burch, J., T. Pearce. 1990. Terrestrial gastropoda. Pp. 201-310 in D Dindal, ed. Soil Biology Guide. New York: John Wiley and Sons.

Geraerts, W., J. Joosse. 1984. Freshwater snails (Basommatophora). Pp. 141-207 in A Tompa, N Verdonk, J van den Biggelaar, eds. The Mollusca, Vol. 7, reproduction. London: Academic Press, Inc.

Harry, H. 1997. Carychium exiguum (Say) of lower Michigan; morphology, ecology, variation and life history (Gastropoda, Pulmonata). Walkerana, 9/21: 1-104. Accessed August 19, 2013 at

Hubricht, L. 1985. The distributions of the native land mollusks of the Eastern United States. Fieldiana, Zoology New Series, 24: 1-191. Accessed August 19, 2013 at

IUCN, 2013. "The IUCN Red List of Threatened Species. Version 2013.2" (On-line). Accessed August 19, 2013 at

Miller, B. 1970. The Sandahl molluscan fauna (Illinoian) from McPherson County, Kansas. Ohio Journal of Science, 70/1: 39-50. Accessed December 15, 2013 at

Nekola, J. 2005. Geographic variation in richness and shell size of eastern North American land snail species. Records of the Western Australian Museum, Supplement 68: 39-51. Accessed December 15, 2013 at

Nekola, J. 2012. The impact of a utility corridor on terrestrial gastropod biodiversity. Biodiversity and Conservation, 21: 781-785. Accessed August 19, 2013 at

Pearce, T. 1989. Loping locomotion in terrestrial gastropods. Walkerana, 3/10: 229-237. Accessed August 19, 2013 at

Perez, K., J. Cordeiro. 2008. A guide for terrestrial gastropod identification. Terrestrial Gastropod Identification Workshop, 1: 1-72. Accessed December 15, 2013 at