Iowa Pleistocene Snail
Iowa Pleistocene Snail
|Listed||July 3, 1978|
|Family||Discidae (Forest Snails)|
|Description||Six-whorled, lightly coiled, domeshaped snail, brown or greenish white.|
|Habitat||Talus slopes under logs or in leaf litter.|
|Food||Leaves of birch and maple trees.|
|Reproduction||Hermaphroditic; clutch of 2-6 eggs.|
|Threats||Restrictive habitat, low numbers.|
The Iowa Pleistocene snail is an average-sized member of its genus with an adult width of 0.5 in (8 mm). The dome-shaped shell is tightly coiled, typically with six whorls. Shell color may be brown or off-white with a greenish cast. Ribs are relatively fine and confined to the upper half of each whorl.
This snail feeds on the leaves of white and yellow birch, hard maple, or occasionally dogwood and willow. It is active from spring through summer but becomes lethargic in August when the habitat dries. It remains near the soil surface until the first hard freeze, then burrows into the soil to hibernate.
Like most North American land snails, the Iowa snail is hermaphroditic (with both male and female reproductive organs) but not self-fertilizing. Adults can apparently lay eggs as well as fertilize the eggs of other snails. Breeding occurs from late March to August. Eggs are laid under logs and bark, in protected moist rock crevices, and in the soil. Clutch sizes vary from two to six eggs with an incubation time of 28 days. Life span is about five years.
The Iowa Pleistocene snail is found in pockets of a very specialized microhabitat known as an algific talus slope. These cool, moist areas develop around the entrances to fissures and caves where circulating air and infiltrating water create a condition of nearly permanent underground ice. The Iowa snail lives on the surface in deep, moist leaf litter that is cooled throughout the summer by this icy substrate. The snail prefers deciduous leaf litter and typically avoids mossy ground cover or coniferous litter. Most algific talus slopes are steep and north-facing, composed of fragments of a porous carbonate rock. This cool, moist habitat reproduces conditions that were more common during previous glacial epochs. This snail is often associated with northern wild monkshood (Aconitum noveboracense ), a federally endangered plant.
The geologic record of the Iowa Pleistocene snail goes back over 300,000 years, when it was fairly widespread throughout the Midwest. Its maximum range during cooler glacial periods included Iowa, Nebraska, Missouri, Illinois, Indiana, and Ohio. The center of distribution apparently was once Illinois.
The snail survives at 18 known locations in a region known as the Driftless Area, which encompasses portions of Clayton and Dubuque Counties, Iowa, and Jo Davies County, Illinois. These populations are estimated to number no more than 60,000 snails.
The major long-term cause of decline is cyclic climatic change. The species has survived several such cycles in the past and, with a return of glacial conditions, would certainly replenish itself over a large range. The most immediate threat to its survival is human disturbance. An estimated 75% of its specialized habitat has been destroyed in the last 150 years by agriculture, road construction, quarrying, and other human intrusions.
Conservation and Recovery
In 1986, The Nature Conservancy, the Iowa Conservation Commission, and the Fish and Wildlife Service implemented the Driftless Area Project in northeast Iowa to protect remaining pockets of algific talus slope habitat. Over two-thirds of all landowners that were contacted, agreed to register a commitment to conserve habitat on their properties. Registration is voluntary and is considered an interim solution, until land can be acquired and protected permanently. Public support for habitat conservation in the region is very high.
Baker, F. C. 1928. "Description of New Varieties of Land and Fresh Water Mollusks from Pleistocene Deposits in Illinois." Nautilus 41:132-137.
Ferst, T. J. 1981. "Final Report, Project SE-1-2, Iowa Pleistocene Snail." Iowa State Conservation Commission, Des Moines.
Hulbricht, L. 1955. "Discus macclintocki (F.C.Baker)." Nautilus 69:34.
U.S. Fish and Wildlife Service. 1984. "Recovery Plan for the Iowa Pleistocene Snail (Discus macclintocki )." U.S. Fish and Wildlife Service, Twin Cities.