Snail, Iowa Pleistocene

Discus macclintocki

phylum: Mollusca

class: Gastropoda

order: Stylommatophora

family: Discidae

status: Data deficient, IUCN Endangered, ESA

range: USA (Illinois and Iowa)

Description and biology

The Iowa Pleistocene (pronounced PLICE-ta-seen) snail is a small forest snail measuring 0.3 inch (0.8 centimeter) wide. Its domed shell is brown or off-white with a greenish cast. The shell is marked by tightly coiled whorls (spirals), usually six.

These snails eat tree leaves, including those of maples, white and yellow birches, willows, and dogwoods. After the first hard freeze in late fall, they burrow in the soil and hibernate through the winter. They are active in the spring and summer, but become sluggish in late summer when their habitat begins to dry out.

Like all land snails, Iowa Pleistocene snails are hermaphroditic (pronounced her-ma-fra-DI-tick). This means that each snail has both male and female reproductive organs. They can each lay eggs and also fertilize the eggs laid by other snails.

Between March and August, the snails lay two to six eggs apiece under logs or bark, in the soil, or in moist rock crevices. After the eggs are fertilized, they incubate (develop) for 28 days before hatching. On average, Iowa Pleistocene snails live for five years.

Habitat and current distribution

The Iowa Pleistocene snail is found only in one county in Illinois (Jo Davies County) and two in Iowa (Dubuque and Clayton Counties). Biologists (people who study living organisms) estimate that the total snail population, scattered over 18 locations, numbers about 60,000.

This snail lives in a very specific habitat. It inhabits cool, moist areas found around the entrances to caves or fissures (long narrow cracks or openings in the ground). Underground ice, formed by water bubbling up from underground, cools the surface where the snail lives in deep, moist, deciduous (shedding trees) leaf debris.

History and conservation measures

The Iowa Pleistocene snail has existed as a species for more than 300,000 years. It receives its name from the geologic time period in which it arose, the Pleistocene epoch, which covers the period from 2,000,000 to 11,000 years ago. The Pleistocene is the best known glacial period (Ice Age) of Earth's history.

The Iowa Pleistocene snail's present-day habitat recreates the conditions of the environment inhabited by its ancestors. If glacial conditions returned to the midwestern United States, it is likely that the number of Iowa Pleistocene snails would increase and their range would expand. During cooler periods in Earth's history, the snail's range extended over the present-day states of Nebraska, Iowa, Missouri, Illinois, Indiana, and Ohio.

Humans pose the greatest threat to this snail. Over the last 150 years, almost 75 percent of its habitat has been converted into farms and stone quarries (excavation sites where stone is dug, cut, or blasted out of the ground).

In 1986, the Nature Conservancy, the Iowa Conservation Commission, and the U.S. Fish and Wildlife Service established the Driftless Area Project in northeastern Iowa. The aim of this voluntary project is to protect any remaining Iowa Pleistocene snail habitat. Private landowners have been asked to conserve any such habitat on their land. So far, more than 65 percent of the landowners contacted have agreed to take part in the project.

DID YOU KNOW?

Ice ages were periods in Earth's history when average annual temperatures dropped low enough to allow glaciers and vast sheets of ice to spread and cover large portions of the planet's surface. Over the last 2.5 million years, about two dozen ice ages have occurred.

Scientists do not know exactly what causes ice ages or periods of glaciation, but they have offered many theories. Some scientists believe that changes in the geometry of Earth's orbit around the sun affects the amount of solar radiation reaching the planet, making it warmer or cooler. Others believe changes on Earth are responsible. They point out that volcanic eruptions may have contributed to significant temperature variations on the planet. Dust particles thrown into the air during an eruption can reflect sunlight back into space, reducing the amount of heat reaching Earth's surface.