Terrestrial Snails

Gastropoda Cuvier, 1795

Cheese snail (Helicodonta obvoluta): Valganna (Lombardy, Italy), 440m.
Photo: Alberto Colatore (iNaturalist).
Snails (Gastropoda) are the only molluscs that succeeded in the course of evolution in making the transition to life on dry land – not just once, but several times independently. Yet terrestrial snails are not the kind of land animals one might expect. They lack a hard outer skin and are constantly threatened by desiccation as well as by a wide variety of predators. Their shell, their most obvious means of protection, is not as thick or strongly armoured as that of many of their marine relatives; some land snails have even reduced it partially or completely. However, these slugs also survive very well.

Nevertheless, there are an estimated 25,000 species of land snails worldwide. How, then, have snails managed not only to survive on land throughout Earth’s history, but also to spread successfully into almost every terrestrial habitat?

Terrestrial Snails (Gastropoda): An introduction. ( new page).
Snails on Dry Land.
Adaptation to Terrestrial Life.
Slugs - Life (Almost) Without A Shell.
Identification of Terrestrial Snails.
Links.
Literature.

Snails on Dry Land

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Helicina unizonata (Helicinidae): Orellana, Ecuador.
Photo: Stephen Luk (iNaturalist).

Land snails evolved several times independently, and along different evolutionary pathways. In almost all snail groups that otherwise live in water, there are also terrestrial representatives. A single, unified group of “land snails” therefore cannot easily be defined: different types of land snails are each more closely related to certain marine or freshwater groups than to other land snails.

This also explains why some groups of land snails still carry a shell lid (operculum), while most others lack it. Operculate land snails also share with their aquatic relatives the feature of having only one pair of tentacles, with the eyes located at their bases.

Striped needle snail (Acicula lineata) from Serpiano, Canton
Ticino, Southern Switzerland.
Dentate thorn snail (Carychium tridentatum): Canton St. Gal-
len, Switzerland. Photos: © Stefan Haller.

These examples illustrate well the many routes by which snails have colonised land: The round mothed snail (Pomatias elegans) belongs to the Littorinimorpha, and is thus related to the periwinkles (Littorinidae). It probably evolved from similar coastal ancestors, which enabled it to colonise dry shore areas.

The needle snails (Aciculidae) and Cochlostoma snails (Cochlostomatidae) belong to the Architaenioglossa and are related to freshwater snails such as the mud snails (Viviparidae) and apple snails (Ampullariidae). They likely developed from freshwater ancestors, for example along swampy river and lake shores.

Finally, the tropical Helicinidae belong to the Neritopsina, together with the marine and freshwater nerites (Neritidae). They are assumed to have adapted to life on land via tropical mangrove swamps.

By far the most successful group of land snails, however, are the Pulmonates (Eupulmonata). They have transformed their mantle cavity into a lung, perfecting their ability to breathe air. The order Ellobiida includes amphibious coastal snails (Ellobiidae) as well as the entirely terrestrial thorn snails (Carychiidae).

Another partial group of the Eupulmonata are the Stylommatophora, the order that has adapted best to terrestrial life and today comprises the largest number and diversity of land snails. Their scientific name refers to their second, longer pair of tentacles, which carry the eyes and provide the snail with a wider field of vision. The Stylommatophora include both extremely small species such as the dot snail (Punctum pygmaeum), with a shell diameter of only 1.6 mm, and giants such as the Giant African snail (Achatina achatina), with shells up to 20 cm long. In total, about 23,000 extant species of Stylommatophora are known.

Terrestrial Snails: Different Groups.
How did Snails Come to Live on Land?

Adaptation to Terrestrial Life

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Eastern heath snail (Xerolenta obvia): Puławy,
Poland. Photo: Ewa Rauner-Bułczyńska
(iNaturalist).
For snails, multiple adaptations made it possible to live on dry land as opposed to the sea they came from. The requirements are different ones.

Common door snail (Alinda biplicata): Lambrecht, Rhineland-
Palatinate, Germany. Photo: Matthias Buck (iNaturalist).
Final whorls of Alinda biplicata, laid open. The shell spire (Co-
lumella), as well as the Clausilium, are visible.
Photo: Mathijs Zonneveld (iNaturalist). Enlarge!

Humidity: Since they lack a hard outer skin, adapting to the changing humidity conditions on land was rather challenging for the evolving terrestrial snails. However, they are protected by their hard calcareous shell, and, on the other hand, by the slime or mucus covering their bodies entirely. Snail slime is hygroscopic and thus binds moisture rather than losing it. Many snails also have adapted their behaviour, they are active in the evening, night and early morning, instead of during the day and sunny weather.

There are however xerophilous species, fond of sunlight and dry weather, such as the heath snails (Geomitridae) and the zebra snail (Zebrina detrita: Enidae), which are protected against the sunlight by their reflective black-and-white shells. But even they can often be seen aestivating on plants, enduring periods of dryness by sleeping.

A very special adaptation is shown in the door snail family (Clausiliidae), a family of small snails with a highly coiled shell. Those have developed a special closing aparatus called a clausiliar, consisting of several folds (lamellae) in the terminal shell whorls, as well as a closing plate called a clausilium (cf. photo on the right). When the snail withdraws into its shell, the clausilium connected to the shell spire closes the shell mouthe (aperture) and so protects the snail against evaporation.

Terrestrial Snails: Protection against Evaporation.

Breathing Air: Most terrestrial snails are well adapted to breathing oxygen from dry air. In pulmonate snails, the pallial cavity has been developed to a simple lung, absorbing oxygen through its wall. The mantle (pallium) protects the pulmonary cavity against outside and the pulmonary opening (pneumostome) can be actively closed by the snail to minimize evaporation. Terrestrial operculate snails (i.e. snails with a shell lid or operculum) do not habe a pneumostome: They breathe by secondary air respiration, a mantle fold serving as lung.

Terrestrial Snails: Breathing Air.

Cuban tree snail (Polymita picta): Maisi, Kuba.
Photo: Miguel Ernesto Suárez Blancart (iNaturalist).
Common hair snail (Trochulus hispidus): Zuid Holland,
Netherlands. Photo: Rob Westerduijn (iNaturalist).
Orientation: The ability of terrestrial pulmonate snails (Stylommatophora) to orientate themselves in their environment, has greatly been enhanced by another pair of tentacles. At the end of those, each carries one eye, noticeably better developed than most water-dwelling snails' eyes. Also, snails' mouth flaps are covered with taste sensing cells helping with finding food and mating partners. In some predator snails (e.g. the rosy wolf snail, Euglandina rosea), the mouth flaps are extended to form what appears to a third tentacle pair. This enables the wolf snail to pursue its prey, other snails, along their slime thread. Additional light sense cells distributed over the snail's foot surface also provide it with a diffuse light sense, so snails are able to detect a sudden shadow over them and withdrawing into their protective shell fast.

Terrestrail Snails: Orientation.

Soil Composition: On land, snails mainly occur on soil rich in limestome. There, the snail has the ability to obtain calcium carbonate (CaCO₃) to build and maintain its shell, either directly from the soil or from its food. Also, the weathering of limestone results in a thin but fertile layer of soil with a thick cover of vegetation: This means a nutrition source for snails, as well as protection from predators as well as from desiccation by sunlight.

Terrestrial Snails: Soil Conditions.

Camouflage: Terrestrial snail generally are noticeably better camouflaged than their marine relatives, since their shells have much thinner walls and thus are much less well armoured. Some snail species can camouflage their shells actively or passively, since their shells' surfaces are covered with hairs. Those are useful for binding faeces and earth particles to camouflage the shell surface. Also, they probably reduce desiccation. Some snail groups' shell polymorphism, such as in banded snails (Cepaea) and Cuban tree snails (Polymita picta), serves to always protect one part of a species population against predators hunting by sight, such as birds, in front of a given background.

Terrestrial Snails: Camouflage.

Slugs - Life (Almost) Without A Shell

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Common or Spanish roundback slug (Arion vulgaris).
Photo: Robert Nordsieck.

Yellow-grey shell slug (Testacella haliotidea): Auckland, New
Zealand. Photo: Joseph Knight (iNaturalist).
Many different terrestrial snail groups have reduced their shells with the advantage of larger mobility. The gradual reduction of the shell in terrestrial gastropods, as well seen in the glass snail family (Vitrinidae) is also referred to as vitrinisation.

Apart from actual slugs lacking any form of external shell, there are also different forms of so called semi-slugs, e.g. shell slugs (Testacellidae) and Daudebardias (Daudebardiinae: Oxychilidae) bearing a small rudimentary shell at the dorsal end of their foot but unable to use it for protection.

Most slugs' best protection against predators and desiccation is the thick viscous slime or mucus covering their entire body. Apart from that, most of them are usually active during the dark hours and rainy weather. Well-known slug groups are, for example, the round-back slugs (Arionidae) and keel-back slugs (Limacidae) domestic to Central Europe.

Terrestrial Snails: Slugs - Life (Almost) Without A Shell.

Identification of Terrestrial Snails

The large species diversity of even domestic terrestrial snails makes it difficult for laymen to properly identify them. Very helpful may be field guide books (cf. Literature), identification pages in the internet (cf. Links) or so-called identification keys. More often than not, the proper doubtless identification of a snail may necessitate an anatomical examination, e.g. of the genital apparatus, undertaken by a specialist.

Terrestrial Snails: Identification.

Links

In German:

Literature

Kerney, M. P.; Cameron, R. A. D. (1979): "A Field Guide to the Land Snails of Britain and North-West Europe". Collins ( Description, ConchBooks).
Falkner, G.; Fechter, R. (1990): "Weichtiere". Mosaik-Verlag, Munich (In German).
Pfleger, V. (1984): "Schnecken und Muscheln Europas". Kosmos, Francksche Verlagshandlung Stuttgart (In German).
Wiese, V. (2024): Die Landschnecken Deutschlands" (3. Ed.). Quelle & Meyer Verlag, Wiebelsheim (In German).
Geyer, D. (1927): "Unsere Land- und Süßwassermollusken" (3. Ed.). K. G. Lutz Verlag, Stuttgart (In German).

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