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Wild About Illinois Land Snails and Slugs!

WALSTop.jpgLand snails and slugs, hereafter referred to as “land snails” or “snails,” are among the least-studied groups of animals and are commonly misunderstood to be pests or undesirable creatures. While it is true that some snails, particularly species introduced from other areas, can be damaging to crops or the environment, the snails native to an area play an essential role in the functioning of its ecosystem. Numbering almost 40,000 species worldwide, land snails are found in nearly every type of habitat, from tropical rain forests to mountains, arctic regions and deserts. Land snails are abundant in Illinois. They are represented by approximately 124 species, ranging in size from about 0.05 inch to approximately 1.80 inches.

These organisms, as well as aquatic snails, are members of the Class Gastropoda, a classification category meaning “stomach-foot.” They move on their ventral side on a large, muscular foot. For those gastropods with a shell, their organs are located in a visceral sac that is covered with mantle tissue, inside the shell. Gastropods are part of a larger group of animals, the Mollusks, that includes clams, mussels, limpets, chitons, scaphopods, octopuses, squids and others.

 Family Species and Gallery

Family Agriolimacidae
     gray fieldslug Deroceras reticulatum [nonnative]

Family Arionidae
     dusky arion Arion subfuscus

Family Cochlicopidae
     Appalachian pillar Cochlicopa morseana

Family Discidae
     tigersnail Anguispira alternata
     Iowa Pleistocene snail Discus macclintocki

Family Ellobiidae
     ice thorn Carychium exile

Family Gastrodontidae
     globose dome Ventridens ligera

Family Haplotrematidae
     gray-foot lancetooth Haplotrema concavum

Family Helicinidae
     cherrystone drop Hendersonia occulta

Family Helicodiscidae
     compound coil Helicodiscus parallelus

Family Orthalicidae
     whitewashed rabdotus Rabdotus dealbatus

Family Oxychilidae
     brittle button Mesomphix friabilis
     domed supercoil Paravitrea significans

Family Philomycidae
     Carolina mantleslug Philomycus carolinianus

Family Polygyridae
      broad-banded forestsnail Allogona profunda
      carinate pillsnail Euchemotrema hubrichti
      toothed globe Mesodon zaletus
      striped whitelip Webbhelix multilineata
      bladetooth wedge Xolotrema fosteri

Family Punctidae
      small spot Punctum minutissimum

Family Strobilopsidae
      bronze pinecone Strobilops aeneus

Family Valloniidae
      thin-lip vallonia Vallonia perspectiva

Family Vertiginidae
     armed snaggletooth Gastrocopta armifera
     Roger's snaggletooth Gastrocopta rogersensis

 Anatomy

Snails have a complex system of organs. The mouth contains a radula, a flexible, ribbonlike structure lined with rows of teeth, used to scrape food. On the snail’s head are tentacles, which serve as chemosensory structures. Most snails in Illinois have an eye at the tip of each upper tentacle. All internal organs are contained within the mantle, inside the shell. Food travels through a digestive system consisting of a stomach, di-gestive gland and intestine. The circulatory system is semi-closed and has a heart with one ventricle and one auricle. Breathing is accomplished by drawing air through the pneumostome, a small opening in the mantle. Gas exchange occurs within the mantle cavity.

A snail’s shell develops in the egg along with the rest of its body and continues to grow until the snail reaches sexual maturity. The shell is formed by deposits of calcium laid down by the mantle. As the shell grows in its coiled shape, whorls are added. A snail cannot leave its shell. It has a strong muscle inside that is firmly attached to the shell. Snail shells grow in a variety of shapes, including discoid, conical and beehive. Shell shape, number and type of whorls and shell ornamentation, such as ribs or hairs, aid in identification of species. Some snails have denticles, or “teeth,” that protrude towards the center of the shell opening to prevent insect predators from entering and eating the snail. Snail shells may persist long after the snail has died and often can be used to identify species. Slugs have either a much reduced shell, located under the mantle on the dorsal side, or no shell.

snail diagram     shell diagram     disection diagram
Illustrations © Marla L. Coppolino

 Conservation

In today’s rapidly changing world, land snail populations are susceptible to decline. Snails are particularly affected in areas where acid rain is prevalent. The acidic soil contains less-than-normal amounts of available calcium that snails need for proper growth and life functions. A research study in Europe documented reduced populations of certain types of forest-dwelling birds attributed to reduced populations of their main food source, land snails. Acidic rainfall led to the decline in snails, and the decline in the bird populations followed. Other threats to snails include habitat loss due to agricultural use or construc-tion and the application of salts and chemicals to roads in the winter. It is believed that forest fires and floods can be detrimental to populations of land snails, too.

Unfortunately, some snail species living in North America are not native. These nonna-tive species are mainly from Europe, arriving in shipments of food or plants and escaping to establish populations. Some do little damage, but others, such as the giant gardenslug (Limax maximus) can damage crops and compete with native land snails for resources. Any animal or plant that is not native to an area can upset the natural balance of the ecosystem.

Because land snails are not sufficiently studied, it is difficult for conservation specialists to assess the measures needed to protect them. You can help to conserve land snails, though. Learn to identify the snails in your area. Snails can be found in many habitats, even a small back yard. Look under rocks, logs and in leaf litter. Keep records of the snails you find. Write down the name of the snail, the date you observed it, precisely where you found it and in what type of habitat it was found (woods, wetland, urban, etc.). Take a photograph, if you can. Put the snail back where you found it and return any logs and rocks to their original position. Note any trends in your observations and report them to scientists in your area. Allow part of your property to remain natural with fallen logs and leaf litter to provide snail habitat. Join a shell club or malacological society, such as the Chicago Shell Club or the Conchologists of America.

 Life History

Most land snails are hermaphrodites, possessing both male and female sex organs and gametes. When snails mate, fertilization often occurs in both individuals, and both lay a clutch of from one to at least 20 eggs. Clutch number varies by species. Generally, the larger the snail, the more eggs it will lay. Snails lay their eggs in spring and fall. Eggs are deposited in a cool, damp place, often just under the soil’s surface. The adult snail provides no care for the eggs. Hatching takes place in about seven to 10 days. The young snails emerge and begin to search for food immediately. Young slugs have the same appearance as the adults, while young snails have one body whorl at first.

Several adaptations help snails to survive. A snail’s external skin contains glands that produce mucus. The mucus prevents the snail from drying out and also helps it move. During very hot, dry weather and during very cold periods, snails and slugs may become inactive. When a snail aestivates (hot weather) or hibernates (cold weather), it produces a thick mucus membrane over the opening of the shell to prevent further desiccation. Homing tendencies help snails to return to the same sheltered area after activities, reducing the risk of traveling to a potentially dangerous habitat.

Most Illinois land snails are detritivores, eating decaying vegetation, such as leaves. Some snails are carnivores, feeding on other snails or carrion. In Illinois, the gray-foot lancetooth snail (Haplotrema concavum) eats other land snails. It has a thin, elongated “neck” that it inserts into the shell of other snails. Its special barbed teeth then attack the flesh of the prey snail.

Snails need to seek sheltered places to live, eat and rest. They prefer to live in moist areas and are commonly found under logs, loose bark or coarse woody debris, and in leaf litter on the forest floor. In general, snail populations are greatest in areas that have high soil calcium levels. Calcium is needed by snails to produce the shell and to regulate body functions.

Land snails move by gliding on a large, muscular foot. The muscles in the foot con-tract in waves in the same direction that the animal is moving. Each wave allows the muscles to grip the substrate and pull the snail forward. Glands in the foot produce mucus to help the snail move along. Snails do move very slowly. One snail was recorded to be moving at 0.0023 mph.

Snails are a food source for many animals. Some insects eat land snails. Firefly larvae feed almost exclusively on snails. Land snails are an essential part of the diet of many birds. During periods of egg-laying, female birds that normally eat snails increase their snail consumption. The calcium in snail shells is a nutrient vital to embryonic devel-opment and egg shell production in birds.

 Glossary

aestivation - period of inactivity when temperatures are high enough to be stressful to an organism

auricle - a chamber of the heart that receives blood from the veins

carnivore - animal that preys upon and eats other animals

chiton - also called sea cradle, this mollusk lives on rocks in the ocean and has a shell made of hard, overlapping plates

conical - cone-shaped

denticle - a small, toothlike projection

detritivore - an organism that eats dead plant or animal materials

discoid - having a flat, circular shape, like a discus

dorsal - the back or upper side of an animal

hermaphrodite - organism possessing both male and female reproductive organs

hibernation - period of inactivity in winter during which an organism has reduced metabolic functions

mantle - the part of a mollusk that surrounds the organs and lines and secretes the shell

mucus - a thick fluid secreted by mollusks that assists them in movement and protection; helps prevent the body from drying out

pneumostome - an opening in the mantle of a gastropod where air can enter for breathing

radula - a flexible band with small teeth in the mouth of most mollusks; used to scrape food

scaphopod - a burrowing, marine mollusk with a long, tapering shell

tentacle - an elongated, unsegmented extension from a snail’s head used for sensory

ventral - belly or lower side of an animal

ventricle - lower chamber of the heart; receives blood from the auricle and pumps it to the snail’s tissues

whorl - one of the turns of a spiral shell

 Agency Resources

Although there is little information available about snails and slugs, several institutions in Illinois maintain research collections. The Field Museum of Natural History and the Peggy Notebaert Nature Museum, both in Chicago, and the Illinois Natural History Survey in Champaign, have significant collections and conduct studies. Other organizations, such as the U.S. Fish and Wildlife Service and the Illinois Department of Natural Resources, support research of these species. The Illinois Wildlife Preservation Fund, com-prised of taxpayer donations, helps to support further understanding of these creatures by providing grants for projects, such as the development of this poster.

WALSPenny-MC.jpg The snail species shown in this photograph occur in Illinois.
Photo © Marla L. Coppolino.