Non-native means originally from elsewhere, and it could have reached the new location naturally or by human means. Introduced means transferred to a new locality by humans, either intentionally or by accident. Invasive refers to a non-native species is causing problems in the new location, directly to human health or economy, or to cultivated plants or farm animals. Sometimes the term invasive is applied ecologically to include harm to other local species that may or may not directly affect humans.

Introduced species have often arrived from another continent, such as the Arion spp. slugs from Europe that ravage our gardens, but occasionally they are from other parts of the same continent, e.g., Prophysaon andersonii slugs from the west coast of the USA introduced to Quebec (Nicolai & Forsythe 2020), and Triodopsis hopetonensis that is spreading both northward and eastward from the mid-Atlantic coastal areas where it used to occur {Hubricht 1985, GBIF 2023, iNaturalist 2023).

Introduced species usually arrive at ports or large urban areas, often arriving on cargo, especially agricultural products. There is evidence that the grove snail Cepaea nemoralis has often arrived on tiles from Mediterranean areas (check a vacant lot near a tile store and you might find a colony of C. nemoralis). Many past introductions are likely via the soil of potted plants and modern introductions continue in the horticulture trade. The Cuban snail Zachrysia provisoria was established in the 2000s in the rainforest exhibit in the Pittsburgh Zoo, undoubtedly transferred on tropical plants from Florida, where Z. provisoria has become established. Plant nurseries can be important hubs for the spread of non-native gastropod species (Bergey et al. 2014).

How long must an introduced species be present in a new location before it becomes accepted as part of the native fauna? In North and South America, we can use the year 1492 as the year when modern Europeans began traveling between Europe and the Americas, carrying species between the continents. If a species is known to be present in the Americas before 1492 (many people use the year 1600 after which trade dramatically increased between continents), we can confidently call it native. In contrast, Europe, Asia, and Africa lack such a convenient date for defining introduced species and must define introduced species in other ways (e.g., several species of edible snails in the family Helicidae are known to have been moved around Europe by Romans thousands of years ago).

Even with the convenient start date of 1492 (or 1600) for arrival of introduced species in the Americas, some species can be challenging to classify. For example, Cepaea hortensis appears to have arisen in northern Europe (its range is spread widely across Europe (not including the southernmost parts); in contrast it is spread thinly along the NE coast of North America from Long Island to Newfoundland (and into the St. Lawrence Seaway), suggesting that it arrived in North America relatively recently and has not had a chance to spread. Evidence indicates that C. hortensis was present in North America before Columbus arrived (in 1492) and evidence from a cave on the Gaspe Peninsula, Quebec indicates it was present in North American before the Vikings (Pearce et al. 2010).

How do we know whether a species is native or introduced? Sometimes we do not know for sure, so we rely on clues. Many non-native species occur in urban areas, especially seaports but also airports receiving air freight, or their populations appear to be spreading from those areas. Many of those localities are disturbed areas. Many recently introduced species occur in small, localized populations in contrast to larger widespread populations elsewhere on earth. If a fossil record exists, presence of a species before 1492 (or 1600) can argue for native status. Improved genetics capabilities have allowed us to examine population-wide genetics patterns, which typically show reduced genetic variability in introduced species (resulting from a founder effect), in comparison to much greater genetic variability in populations in the source area; in fact, genetic analysis can sometimes pinpoint the geographic location from which the introduced species was translocated.

Despite these useful clues, in some cases recognizing introduced species can be challenging. Arion spp. are common agricultural and garden pest slugs in North America and they are known to be widespread in Europe (where they are also pests). However, they are also widespread in northeastern North America (including in relatively wild areas), raising the question whether they could be native species that take advantage of the tasty human crop plants that appeared with humans. I’m not proposing that they are native to North America and were introduced to Europe (I expect they were known pests in Europe before 1492). I argue that their widespread occurrence in North America could be via dispersal on logging equipment during the late 1800s and early 1900s when trees across the northeastern United States were cut down for timber. Furthermore, genetic testing shows that North American Arion spp. have reduced genetic variability compared to European Arion spp., arguing that the North American populations originated from (and are a subset) of the European populations.

Another species challenging to classify as native or introduced is Cochlicopa lubrica, a small (5-6 mm) glossy shelled snail common in human-modified areas. It is widely distributed across North America (north of Mexico) and throughout Europe. Many people in North America consider it to be native (Hubricht 1985). However, another possibility is that it might have arrived in North America shortly after the Europeans arrived (unseen), then was noticed later and assumed to be native. I am not aware of a fossil record arguing for its native status and I consider the matter to be unresolved. Another similar (and slightly amusing) story is the genus Hawaiia. These small (2 mm) white snails are actually native to North America but were inadvertently introduced to the Hawaiian Islands where they were first discovered and named Hawaiia after their presumed homeland. Only later were they discovered to be widespread in North America in their true homeland, but in taxonomy (the science of naming species), once a valid name has been established, it persists even if it was misleadingly named.

Literature Cited

Bergey, E.A., Figueroa, L.L., Mather, C.M., Martin, R.J., Ray, E.J., Kurien, J.T., Westrop, D.R. & Suriyawong, P. 2014. Trading in snails: plant nurseries as transport hubs for non-native species. Biological Invasions 16:1441-1451.

GBIF. 2023. Triodopsis (Triodopsis) hopetonensis (Shuttleworth, 1852). https://www.gbif.org/species/2295718.

Hubricht, L. 1985. The distributions of the native land mollusks of the eastern United States. Fieldiana: Zoology, New Series, No. 24:1-191.

iNaturalist. 2023. Observations on Magnolia Threetooth https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=232995.

Nicolai, A. & Forsyth, R.G. 2020. Introduced Prophysaon andersonii (J.G. Cooper, 1872) in Quebec, Canada: first record of Prophysaon (Gastropoda, Eupulmonata, Arionoidea) in eastern North America, confirmed by partial-COI gene sequence. Check List 16 (2): 307–316. https://doi.org/10.15560/16.2.307.

Pearce, T.A., Olori, J.C. & Kemezis, K.W. 2010. Land snails from St. Elzear Cave, Gaspé Peninsula, Quebec: antiquity of Cepaea hortensis in North America. Annals of Carnegie Museum 79(1): 65-78.

A variety of predators eats land snails and slugs, as they are small and slow-moving invertebrates. Abundant and nutritious prey, land snails might be considered “low” on the food chain. In response to this predation, land snails have evolved an array of sophisticated defenses.

Invertebrate predators of land snails include beetles and their larvae, millipedes, flies, mites, nematodes, and other snails. Vertebrate predators of snails and slugs include shrews, mice, squirrels, and other small mammals; salamanders, toads and turtles, including the uncommon Blandings Turtle Emydoidea blandingii; and birds, especially ground-foragers such as thrushes, grouse, blackbirds, and wild turkey. By far the most comprehensive account of these land snail predators is found in Natural Enemies of Terrestrial Molluscs, edited by G.M. Barker (2004).

Land snail defenses against predators include cryptic coloration and texture; thickened shells and aperture barriers; defense mucus production including irritating smells and tastes; hiding behaviors, and rapid withdrawal or dislodging movements.

How predators eat land snails differs according to their size and capabilities. For shelled land snails, beetle larvae may enter the aperture of a snail’s shell, while larger beetles can crush small snails. Predatory snails can rasp a hole in a victim’s shell, then insert their head to feed. Small mammals will break a hole in the shell to pull out the snail’s body, sometimes leaving the empty shell on a rock or log. Larger predators such as wild turkey can swallow even big snails whole.

While most land snails are herbivores or detritivores, some are specialized predators themselves. The Gray-foot Lancetooth Haplotrema concavum is a large predatory land snail that attacks nematodes and other snails, and is a widespread native in Pennsylvania. This predator will insert its head into the aperture of a prey snail’s shell, or can drill a hole in the shell to gain access. Pearce and Gaertner (1996) were able to predict which snail prey were most preferred by the lancetooth using information about how well-armed those prey were. The Oval Ambersnail Succinea ovalis has a large aperture and thin shell, and was attacked most often, but the small Maze Pinecone Strobilops labyrinthica with its thick shell and narrow aperture, was least preferred.

Another predator and prey relationship of note is an apparent “arms race” between land snails and Cychrine beetles (Carabidae), which feed mostly on land snails (Symondson, 2004) – while the beetles have evolved narrower heads to extract snails from their shell aperture, the snails have evolved more obstructed apertures (which would also help in defense against the lancetooth).

Cychrine beetles have an elongated head, thorax, and mouthparts that help them gain entry through the aperture of snail shells, while less-specialized beetles may simply crush snail shells when possible (e.g., Digweed, 1993). Mouthparts of the Cychrini have been described as having “hooks” or “spoons” that help extract mollusk flesh. Some snails in turn, have barriers of shell calcium, called denticles or lamellae, which partially obstruct their aperture (e.g. Solem, 1972). The plasticity of the land snail head and foot allow their protrusion through the aperture despite the obstacles. However, these “teeth” obviously thwart the entry of beetles and other small predators having hard exoskeletons. Some snails in the slitmouth genus (Stenotrema: Polygyridae) are so well armed – with a hard round shell, covered by hair-like processes, and a slit-shaped aperture blocked by lamellae – as to appear impervious to attack.

Further research is needed to demonstrate whether barrier adaptations have evolved specifically for defense purposes. Other hypothesized functions for barriers include storing calcium for other physiological needs, trapping air if the animal becomes immersed in water (Emberton, 1995); or providing leverage “handles” on the shell so it can be moved and balanced by the animal (Suvorov 1993, 1999).

Other land snail defenses include cryptic coloration, which hides them from predators that search by sight. Few land snails in North America are brightly colored, most are brown or tan, and many have finely-textured shells that do not reflect light. For example, the tiny pinecone snails (genus Strobilops) are almost indistinguishable from the fruiting bodies of fungi on rotten logs. Some species such as the velvet wedge Xolotrema denotata have hairlike processes on their periostracum that hold onto dust and spider webs, making an effective camouflage.

Another typical land snail defense mechanism is the production of mucus. Although mucus normally aids in maintaining a snail’s skin and promoting locomotion, a defensive version is exuded in large amounts when a snail is attacked. The smothering or confusing effect of the sticky secretions can help to stop predators (e.g., Eisner and Wilson, 1970; Parkarinen, 1994). Many slugs have even more copious and stickier defense mucus, suggesting an anti-predator function compensating for the lack of a shell.

Defense mucus likely contains chemicals repellent to predators, but this is not well studied in terrestrial gastropods (this phenomenon has been established for marine snails). Domestic dogs and cats avoid slugs, and a pet lizard fed the introduced European slug Arion subfuscus was rendered comatose for several days. Some color forms of A. subfuscus are quite orange, suggesting that they may be advertising their toxicity to would-be predators. Defense mucus of several snails is visible under ultraviolet light, suggesting their toxicity is being advertised to insect predators that see in that portion of the light spectrum (D. Dourson, pers comm). Some snails and slugs are attacked by ants, while others are ignored, so there may be ant-specific defense chemicals that have evolved only in some snails (Hotopp, pers obs).

For defense against much smaller attackers, land snail mucus is known for its antibacterial properties, with some constituents used to treat acne.

During periods of inactivity land snails appear to hide from potential predators. Land snails that live among rock talus or cliffs tend to remain immobile on ceilings, vertical surfaces and within cracks, rather than in locations that might be more accessible to small mammals. In the Northeast during summer months, the land snail Neohelix albolabris is typically found singly upon woody debris and snags above ground level. For aestivation, some larger land snails move from leaf litter to sticks, logs or rocks, though in addition to predator avoidance, this behavior may also aid in retaining moisture, avoid pooling water, or other functions.

Many shelled land snails will rapidly withdraw into their shell when attacked, or sometimes when merely sensing wind or movement. For example, the land snail Webbhelix multilineata frequently drops from low vegetation when disturbed. Large Neohelix sp. land snails kept in a terrarium will sometimes be seen rapidly twisting their shell when another snail is crawling upon it. This behavior appears to be an attempt to dislodge the attached snail.

Finally, if a land snail does survive a predator’s attack, it has impressive powers of regeneration. Damage to shells can be repaired by the snail’s shell-building mantle, although it often appears that repairs are most complete when the break is in the final “body” whorl of the shell. Field collectors are familiar with this phenomenon, as recovered shells often show irregularities where “patches” have been made. Land snails are also able to regenerate some tissue that has been lost, including parts of the foot and tentacles (e.g., Bobkova, et al., 2004).

As litter-dwelling herbivores and decomposers, land snails are crawling across and eating organic material in their environment each time they become active. Marine and freshwater mollusks are known to take up various contaminants, and mussels are even used in bioremediation of radioactive material. Researchers have discovered that land snails are also taking in environmental toxins from their surroundings, with implications for snails, other wildlife, and people.

Zinc, cadmium, lead, and copper can all be taken up by land snails, as shown by laboratory experiments wherein snails were fed toxin-laced lettuce (e.g., Dallinger and Wieser, 1984). But in more recent lab tests, snails incorporated cadmium when simply raised upon contaminated soil, raising concerns that toxins in polluted soils may be more bio-active than previously believed (Scheifler et al., 2003a). Aquatic environments had previously been thought to be the only environments providing the conditions for chemical reactions that allow certain toxins to bio-magnify, moving up the food chain.

Most recently, Italian scientists showed how the land snail Helix aspersa accumulated a suite of trace metals and polycyclic aromatic hydrocarbons along roadsides (Regoli et al., 2006). The pollutants were not only carried by the snails but damaged their physiological defenses, cells, and DNA.

In the case of mercury, this neurotoxin is well-known as a problem in aquatic systems, where it bioaccumulates, poisoning top predators and sometimes people. Aquatic snails are one possible vector for mercury contamination in water (e.g., Leady and Gottgens, 2001). However, mercury has now turned up in a terrestrial ecosystem as well.

Researchers have found elevated levels of mercury in the blood of Bicknell’s thrush on mountains in New England (Rimmer et al., 2005). These thrushes are neotropical migrants that forage upon a wide variety of prey in wintering as well as breeding habitats. But thrushes are generally known to eat snails (Martin et al., 1951), presumably to obtain calcium for egg laying, so the role of land snails here is one possibility for future research.

Lift up a chunk of wood from the forest floor and a world of miniature organisms appears—camouflaged camel crickets, armored beetle larvae, fierce-looking pseudoscorpions, spiders, springtails, and snails. Here and there you will see tiny whitish balls, mysteriously translucent.

These are land snail eggs, laid in a damp and protected place where they will develop and hatch in a few weeks. They come in different sizes, at most a few millimeters, and may be clustered or single, rubbery or hard, depending upon their species.

The tiny hatching snail that emerges will be almost completely transparent, and if a shelled species, will have a fragile transparent shell—the “nuclear whorl” of an adult’s shell.

Land snail eggs usually result from sexual reproduction, in which genetic material from two individuals is combined. The genetic material in eggs and sperm are brought together by internal fertilization, which involves copulation, though it is much different from that in mammals.

Almost all Pennsylvania land snails are naturally hermaphroditic, that is, they have both male and female reproductive organs. That means that both mating snails may produce sperm and lay eggs. In Pennsylvania the only exceptions to this generality are the cherrystone drop (Hendersonia occulta), and the slender walker (Pomatiopsis lapidaria) in which individuals are male or female.

In the hermaphroditic land snails a single gonad produces both egg and sperm. An in-depth review of land snail reproductive organs is by Gomez (in Barker, 2001). Sperm are often put into a package called a spermatophore, which is transferred by the donor’s penis into the receiver’s vagina. The penis and vagina share a common opening called the atrium, whose opening can be seen as the small “genital pore” on the right side of the animal’s head, usually behind the right eye tentacle. Once inside the receiving animal, the spermatophore releases the sperm, then sperm and eggs meet in the fertilization chamber, where genetic material is combined.

Self-fertilization is possible for some Pennsylvania land snails such as the whitelip (Neohelix albolabris; McCracken and Brussard, 1980), the black gloss (Zonitoides nitidus; Jordaens et al., 1998), and others, an apparent advantage if populations are small or scattered.

Aside from the reproductive organs already mentioned, several other organs play important roles in reproduction, again, well-covered by Gomez in Barker, 2001, and there are interesting variations in reproductive anatomy according to family and species. For example, land snails of some families, including the belly-tooth snails (Gastrodontidae) such as the quick gloss (Zonitoides arboreus), have a “dart sac” containing a “dart.” The dart is a small, sharp spear of calcium carbonate that is rapidly injected into the flesh of a partner before mating. The dart is believed to inject reproductive hormones that increase the shooter’s odds of paternity (Davison et al., 2005; Koene and Chase, 1998; Schilthuisen, 2005).

After fertilization, the egg passes down the spermoviduct, which coats the egg with jelly-like albumen to feed the developing snail and a protective outer coating. Then the egg is passed out of the snail’s body, or oviposited, in a damp area, sometimes in a hole dug by the parent.

Laying pattern varies by species, for example, the quick gloss (Zonitoides arboreus) lays eggs singly while the dusky arion (Arion subfuscus) lays eggs in clusters. Adults may lay from one to dozens of eggs, but details about oviposition are almost completely unknown for Pennsylvania species. An exception is the flamed disk (Anguispira alternata) for which Elwell and Ulmer (1971) provide many details from a captive colony.

The flamed disk burrows into soil, gravel, or decayed wood and lays eggs 2-3mm in diameter at a depth of 1.5 to 2.5cm. The number of eggs laid by an individual varied from 2 to 25, but may have been as high as 40. Eggs that were left where they were laid hatched in 30-45 days, with almost all hatching. Cannibalism of eggs by adults was observed by Elwell and Ulmer, and is known to occur in other land snails as well.

Mating behavior between land snails begins when two adults of the same species find each other at the right time of year or in the proper conditions. Land snails have limited vision and are often in dark habitats, but they are well-adapted to receive chemical signals and feel objects. The four antennae on a snail’s head are sensitive chemoreceptors (like the inside of your nose) and a snail’s head and foot are covered with touch sensors.

It is unclear if land snails are able to “smell” other snails from a distance, though they can be attracted to food items in this way. Once snails are in an area with other snails, they are able to detect and follow another’s slime trail.

When two potential mates meet, they exhibit a variety of crawling and touching courtship behaviors, which may take hours. This is when the dart is employed, for those species that have them, and the genital pore may become more prominent. For copulation, most species orient in opposing directions with the right sides of their heads together, in order to bring the genital pores into close proximity.

In land snails, reproduction is linked to life history, which may be separated into two primary strategies – semelparous, in which adults reproduce once and then die, and iteroparous, in which adults reproduce then survive to reproduce again (e.g., Heller, 2001). Of the Pennsylvania species that are semelparous, some of the most noticeable are the ambersnails (Succineidae), which may reach high densities along stream floodplains or damp fields, reproduce and then begin quickly dying off.

In temperate climates such as Pennsylvania’s, reproduction of some snails appears to be linked to the seasonal rhythm or favorable weather, with much reproductive activity in early summer, such as that of the button snails (Mesomphix spp.) Snails may also have long periods of inactivity during unfavorable weather.

If you are hunting land snails in Pennsylvania during the late fall or winter, or during a dry summer spell, you may come across a land snail shell with a sealed-up aperture and a live snail still inside. To conserve water and energy, land snails will become completely inactive. The seal on the shell is a layer of thick, dried mucus called an epiphragm. It is secreted by the snail and usually has a tiny hole that allows some air exchange. One Pennsylvania snail, the flamed disk, is able to form an epiphragm in as little as five minutes (Elwell and Ulmer, 1971).

During dry summer periods, large slugs will aestivate deep within rotten logs, and shelled snails may be adhered by their epiphragm to the underside of a stick or rock. In winter, shelled snails may develop a thicker epiphragm, and will be said to be hibernating (although it is not hibernation as in mammals). These shells will often be in the leaf litter or under cover, with their sealed aperture facing upward. In fact, over the course of a year some land snails may be active for only a handful of days when temperature and moisture conditions are most favorable.

Life span of land snails can be brief, a few weeks or months, as in the Succineids mentioned above, or several years, as in some of the larger Polygyrid and Endodontid snails. One large Pennsylvania species, the broad-banded forestsnail (Allogona profunda), was found in an Illinois study to mature in one year and live up to four years (Blinn, 1963), and some species can live perhaps twice as long.

If you look at an adult shell of some of the larger land snail species you may see that one or two areas of growth rings are more pronounced, indicating periods when growth was interrupted during winter hibernation. From these “rings” we can deduce that these animals took two or three winters to mature.

Land snail shells, for those species that have them, are made mostly of calcium carbonate with a protein outer coating. Some wildlife obtain the nutrient calcium by consuming live land snails or their empty shells.

Calcium plays a variety of roles in the body of land snails and slugs, including parts in fluid regulation, cell wall function, muscle contraction, and egg laying, and of course, the shelled animals use a large amount of calcium in forming their shell structure. The shell-building organ, the mantle, develops a pH gradient to create a small electric current to move calcium ions into place. A few slugs such as the non-native Limax maximus have a small, vestigial shell that can be found within the mantle.

Land snails obtain calcium from their environment in a variety of ways, depending upon their autecology. They eat live and decaying leaves and wood, fungi and algae on wood and rocks, sap, animal scats and carcasses, nematodes, and other snails. They can be found rasping old or occupied snails’ shells, bones and antlers, rock particles or larger stones and outcrops, and the soil that they regularly consume contains calcium. In captivity they consume lime and paper. Land snails also absorb calcium directly through the sole of their foot (Kado, 1960).

Calcium availability in forest environments generally is positively correlated with the number and species richness of land snails (Burch, 1955; Hotopp, 2002; Beier et al, 2012). However, note that at the greatest levels of site calcium, snail numbers may actually be slightly reduced (Valovirta, 1968). Acid precipitation can reduce the amount of calcium in forest soils, and this in turn can depress snail numbers as much as 80% on sensitive sites (Wäreborn, 1992). Conversely, land snail numbers respond positively to the addition of calcium (Johannessen and Solhøy, 2001).

Calcium in forest soils can come from below, from the breakdown of bedrock and other parent material that contains calcium, but it also comes from above, in the form of plant debris (noting that plants have also obtained their calcium from soil and rock via roots). Plants use calcium in nutrient and water translocation, cell division and cell walls. The amount of calcium in tree leaves and other litter varies, so forest species composition influences soil calcium (Boettcher and Kalisz, 1990; Vesterdahl and Rauland-Rasmussen, 1998).

Soil profiles may exhibit declining or increasing calcium with depth, depending upon the calcium content of bedrock versus leaf litter. For our purposes the important issue is the amount of calcium available to land snails at the upper soil horizons where they live, generally the lower O and A1 horizons. Soil horizon effects may also be interrupted, by calcium-rich limestone outcrops or limestone scree that can make large quantities of this nutrient available to land snails (and other animals).

Tree species composition can be an important factor influencing leaf litter calcium, as some trees are better “calcium pumps,” having relatively more Ca in their leaves, or in the bark cambium layer of decomposing logs. This calcium must also be available to decomposers, in the form of water-soluble calcium citrate (as in ashes, maples, and birches), rather than tightly-bound calcium oxalate (as in oaks and laurels). Aspens, sugar maples, and flowering dogwood (e.g. Nation, 2007) are examples of high-calcium species. Circumstantial evidence suggests that fungi and other decomposers may also play an important role in mediating calcium available for land snails. Examples of leaf litter snails whose populations are finely-attuned to calcium levels are Punctum minutissimum and Striatura meridionalis. Many snails are calciphiles only found on the richest forest sites, such as Gastrocopta armifera.

Although calcium-rich areas have many species, an interesting variety of land snails exist across the calcium gradient, including calcium-poor sites. Some shelled snail species persist on poor sites in refuge habitats such as deep leaf litter, logs, or around springs. Others are acidophiles, specially-adapted to gleaning calcium from poor habitats. An example of an extreme acidophile is the bog and pocosin-dweller Vertigo malleata. Even large snails such as certain Triodopsis and Mesomphix species are adapted to calcium-poor environments.

Moving up the “food chain,” a variety of animals eat land snails. Many predators are after the protein in snail flesh, especially invertebrates such as beetle or fly larvae, while other predators are consuming calcium-rich eggs or shells as well. Gastropods are eaten by herptiles including turtles and salamanders; by mammals including shrews, mice, and squirrels; and by birds including thrushes, ruffed grouse and wild turkey (Martin et al., 1951). Ruffed grouse are reported to consume the slug Deroceras laeve, relatively low in calcium, but also the whitelip Neohelix albolabris, a large shelled snail.

Snail availability may be critical to calcium provisioning for some of these animals. Changes in snail numbers can have ripple effects through an ecosystem, as demonstrated for the great tit (Parus major) in the Netherlands (Graveland, 1996; Graveland et al. 1994). There, reduction in soil calcium due to acid rain resulted in fewer snails, which caused eggshell thinning and reduced reproductive success for the birds. Researchers in North America have also found negative correlations between acid rain and wildlife, such as wood thrush (Hylocichla mustelina; Hames et al, 2002) and salamanders (Beier et al, 2012). However, songbirds in high-elevation New England forests – where there are few snails – are apparently obtaining calcium from other invertebrates such as spiders.

The acid rain-soil calcium picture may be even more complex. Some work suggests that forest soils in Northeastern North America have become more acid as stands age, overriding the influence of acid rain by a wide margin (Hamburg, et al. 2003). In Sweden a study of clearcutting in boreal forest found a long-term increase in calcium and the number of most land snails (Ström, 2004), despite an initial snail decline. These studies show how younger trees are more effective “calcium pumps.”

At the other end of the forest age continuum, research suggests higher land snail numbers and species richness in old forest. In two of three old growth forests in Kentucky, land snail species richness and abundance were higher than in younger stands (Douglas et al, 2013). In old European beech stands, land snail numbers and species richness rose significantly after 187 years (Müller et al, 2005).

Important outstanding questions about soil calcium and forest age are the relative importance of tree species composition shifts; changes in coarse woody debris; differences in fungi and decomposers; and change in site moisture with age or management.

Eastern land snails will eat or at least taste many organic and even inorganic materials that they can crawl to or on. Eating and crawling, looking for food, are the primary activities during much of land snails’ active periods. Various aspects of diet and feeding behavior are discussed in great detail in Barker (2001).

Most land snail species are herbivorous or omnivorous, with only a few mainly predatory species. In snail diets, plants are the dominant food item, then fungi, animal matter and soil, though these preferences are deduced from studies of larger snails and slugs mostly from Europe (see Speiser 2001). Even within a species, snail diets vary widely, as animals take advantage of the foods that are available within crawling distance. Unlike most invertebrate herbivores that may be specialist feeders, including many insects, most land snails are generalists, sampling and assessing a large variety of food items in their path (see Speiser 2001).

Land snails are most often active at night and during damp weather because crawling requires mucus, which is mostly water, and humid air minimizes water evaporation. Once active, snails find food by using the chemoreceptors on their four tentacles, much as mammals use their nose. One Pennsylvanian species, the flamed disk (Anguispira alternata), learned to detour around a barrier to find food that it could smell (Atkinson, 2003).

If you watch some of the larger land snail species, as a snail is travelling it will often have its upper tentacles fully extended, sometimes waving, to sense chemical gradients in the air. When food is reached, the smaller lower tentacles become more active, often curved downward or touching the food item. The snail will also touch food with their mouth and foot, then begin rasping with the radula in its mouth (see Mackenstedt and Märkel, 2001).

The radula is a membrane covered with series of tiny teeth made of chitin, so it is coarse like sandpaper. The shapes of these small teeth are used to help identify some snail species. However, the teeth of the radula should not be confused with denticles in the aperture of a snail’s shell, which are often referred to as “teeth” as well.

The radula is drawn over a ridge of cartilage (the odontophore), somewhat like a chainsaw chain slides around its bar – though it moves back and forth rather than in a circular motion. Bits of food are broken off and drawn into the snail’s esophagus for digestion. Several bouts of crawling and feeding can occur during an outing.

If you allow a larger snail to crawl on your hand, you may be able to feel it “taste,” or rasp, your skin – the sensation is painless, but feels like a cat’s licking.

Saliva aids digestion in land snails, and muscular contractions move food along the esophagus as in people (see Dimitriadis, 2001). Digestive juices begin to break down food items here, and as they move into the gastric pouch. Connected to the gastric pouch is the large digestive gland that serves to absorb food, excrete waste, and regulate body chemistry.

From the gastric pouch, waste enters the intestine and rectum on its way back out of the body. Land snails excrete the undigested parts of their food from the anal pore, located in the mantle, at the edge of the shell in shelled species. Snail excrement may appear as a tiny folded rope. Microscopic examination of its contents can reveal what a snail has been eating, but most of what we know is from observed feeding behavior.

Land snails and slugs may eat herbaceous plant leaves or stems; rotting herbaceous plants, leaves, wood or bark, including the fungi that live within these items; fungal fruiting bodies such as mushrooms or conchs; and coatings of fungi or algae on rock or bark (e.g. Grime and Blythe, 1969; Mason, 1970; Hanley et al., 1995). Some snails such as the ambersnails (Succineidae) can be found in numbers upon lush floodplain herbs during June, while pinecone snails (Strobilops spp.) are found under the bark of a rotten log. Some herbivorous snails, especially introduced species, can be agricultural pests, which is why growers raising European snails for the escargot market usually have strict containment rules.

Snails and slugs are also found eating animal scats and carcasses; nematodes; old shells of other snails; or snail eggs, shells, and flesh. In the woods large snails such as the toothed globe (Mesodon zaletus) might be found upon white-tailed deer scats, while the gray-foot lancetooth (Haplotrema concavum) hunts and consumes live snails and slugs.

Organic and inorganic soil and rock particles are also ingested by snails. Consumption of calcium-bearing minerals provides the nutrient that snails need to build their shells, which are mostly calcium carbonate with a protein outer coating, the periostracum.

Land snails can ingest environmental contaminants and hold, or sequester, those contaminants in their tissues (e.g. Dallinger and Wieser, 1984), which makes snails useful indicators of pollution.

Snails that are tiny usually live very near or on their food – a drifted pile of leaf litter, or a rotten log. Others may move short distances from cover to food at night, which is often how slugs that feed in your garden escape detection. Snails are often found at the base of a plant or tree upon which they feed at night or in damp weather.

Large snails and slugs can make seasonal movements (e.g. Lloyd, 1967), perhaps traveling several meters to congregate on a rotten tree snag and then dispersing again. One eastern species, the broad-banded forestsnail (Allogona profunda) exhibited homing behavior in an Illinois study. It moved to a winter hibernation spot and returned in springtime to an area of fragmented log mould (Blinn, 1963).

In the field it is sometimes possible to see where snails have been feeding as indicated by slime trails, casts (snail excrement), and holes or feeding “tracks.” Squiggly lines or tiny fan patterns on rock or tree bark show where a snail has scraped off algae or fungi, leaving a paler spot. Smooth-barked red maple or American beech are good trees to check for snail or slug feeding tracks. You can look closely at mushrooms to see if a chewed area is found along with a slime trail.