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Terrestrial Snails

Banded snail (Cepaea hortensis), though without bands … [RN]

Ellobium aurismidae. From the exhibition "Hard shell, soft bo- dy" in the Schönbrunn House of the Desert in Vienna. [RN]

At different times during the evolution of snails (Gastropoda), some members of otherwise marine snail groups independently adapted to life on land. This sometimes took place passing fresh water habitats, such as rivers and lakes), and sometimes the direct way passing the coastal habitats, that share characters of sea and land.

The different evolutionary origins of terrestrial snails make their systematic biology a rather difficult subject. According to different characters, terrestrial snails can be divided into two different groups: the pulmonary snails (Pulmonata), i.e. snails breathing with lungs, and the descendants of prosobranch snails (Prosobranchia), originally breathing with gills.

Following the recent development of scientific knowledge on gastropod systematics, especially the discovery of unknown gastropod groups during the 1970s, the Prosobranchia group has been divided into several independent subgroups, some of which are more closely related to other outside groups than formerly prosobranch groups. So the former common character, the pulmonary cavity in front of the visceral hump, is only a common, but not an exclusive, character of those groups.

Mainly in juvenile snails, the lung and other organs can easily be seen through the translucent shell wall. [RN]

So today several independent groups of prosobranch snails are placed facing the pulmonary snails (Pulmonata) and the opisthobranch snails (Opisthobranchia), whose gills are placed in the rear part of the body.

The antiquated tripartition into Pulmonata, Prosobranchia and Opisthobranchia, however, is still found in many school and text books.

Their great adaptability has made terrestrial snails one of the most successful animal groups on the earth: Falkner (1990) states a number of around 25,000 species worldwide. Numerous special adaptations have made this possible for snails, to which the dry land as a matter of fact is really dry. Consequently, most adaptations mainly help in saving water and in breathing dry air. Further developments, for example, helped adapt the sensitive organs, reproduction and development.

The original breathing organ of snails is the comb gill (Ctenidium). As this type of gill is only able to acquire oxygen from water, all terrestrial and numerous limnic snail species have reduced it. Oxygen from dry air now had to be acquired directly trough the tissues of the pallial cavity wall. Among the pulmonary gastropods (Pulmonata) a network of blood vessels there makes an efficient means of oxygen acquisition.

Mantle and respiratory hole are well visible in this Roman snail (Helix pomatia). [RN]

The loss of water by exhaling, to snails presents a major danger which has to be reacted to in several adaptations. As the mantle, apart from generally protecting the body, now additionally has the task of protecting it against evaporation, it grows to an especially thick tissue fold in terrestrial snails. The flow of air into the mantle cavity now happens through the respiratory hole (pneumostome). To open or close it, the snail can use a ring muscle and thus control the flow of air and the loss of water.

A snail's slime gives additionally protects it against evaporation. It is hygroscopic, which means, it attracts water rather than releasing it. So even millions of years after their ancestors left the sea, terrestrial snails still wear a coat of water around it.

White lipped banded snail (Cepaea hortensis) aestivating on a tree. [RN]

A further adaptation to dryness can be found in snails' behaviour. If the weather becomes to dry, they will look for a suitable hiding place to endure such times – to aestivate.

To do so, some snail species will dig into the ground. Others will crawl up plant stems and remain there. To aestivate, they close the shell mouth with a dried mucus membrane, which also sticks them to their place.

Small snails and larger ones, whose shell form allows them to do so – such as the lapidary snail (Helicigona lapicida) or the spindle-like forms of door snails – will hide in crevices of tree bark, rocks and walls. Door snails (Clausiliidae), especially common on the Balkan Peninsula, as their name indicates, are further protected by a special closing apparatus.

"Jumping" Cornu aspersum (see text). [RN]

Another part of snail life, which had to adapt to life on dry land, is locomotion. When a snail crawls over the ground, it loses water over the foot sole. So terrestrial snails are able to move by crawling only on parts of their foot sole, thus minimizing the loss of water there. This type of locomotion is called "jumping", as the snail's slime trail is discontinuous, as can be seen in the picture on the right. The shell of the brown garden snail (Cornu aspersum) in the picture also shows the admirable ability of snails to repair their shells, although this one has been quite severely damaged.

The Roman snail's shell.

In contrary to their prosobranch relatives, pulmonary snails are exclusively hermaphrodites. Together with losing the shell lid (operculum) this is a character pulmonary snails have in common with the opisthobranch snails (Opisthobranchia). Both evolved from common ancestors, only later their evolution went separate ways.

Because it has four tentacles instead of two, this Roman snail (Helix pomatia) has a much larger field of vision to better ob- serve its environment. [RN]

To be a hermaphrodite doubles an animal's chance of reproduction in regions and times with a low density of individuals. In contrary to marine snails' development past different planctontic larval stages, terrestrial snails' development takes place almost exclusively within the egg, off which in the end, complete young snails hatch, which resemble the adults of their kind in the most important characters.

Reproduction of the Roman snail (Including a description of the genital apparatus).

Other than among water-living snails, most terrestrial snails' field of vision is strongly enhanced by placing the eyes on top of special eye stalks, which make a second pair of tentacles. All four tentacles can be withdrawn to protect those important sensors. Observing a snail touching something with its tentacles, one can easily see how fast they can be withdrawn inside the head. The terrestrial group of pulmonary snails and the richest in species of all snails is consequently called stalk-eye snails (Stylommatophora).

The lips below the tentacles mainly provide the snail with chemical information on the ground it is currently crawling over. Some carnivorous snails, such as the rosy wolf snail (Euglandina rosea) find their prey, other terrestrial snails, by following their slime trail. They consequently have very large lips, almost appearing to be a third pair of tentacles.

Pursuing its prey, a wolf snail also will sometimes crawl under water. But not only carnivorous snails, also some terrestrial slugs, such as the Arion genus, may crawl under water unharmed, for example to eat the water plants in a garden pond.

On the other hand, the terrestrial members of different prosobranch groups can clearly be distinguished from the Stylommatophora. A round-mouthed snail (Pomatias elegans), for example, only has one pair of tentacles at its disposition, its eyes are placed at the base of those tentacles, which also cannot be withdrawn.

Round-mouthed snail (Pomatias elegans), with a visible shell lid (operculum). Picture: Michael Stemmer.

At its foot's end (its tail, so to speak) the round-mouthed snail also wears an operculum to close the shell mouth, as is typical for a prosobranch snail.

A round-mouthed snails' head extends to form a proboscis. The mantle cavity is not closed like that of a pulmonate, so this snail does not possess a clearly visible respiratory hole. Compared to terrestrial pulmonary snails, there is a much smaller number of species of terrestrial prosobranch snails, many of which live in the humid tropics, in the temperate zone under vegetation, or even underground.

Among terrestrial snails, like among opisthobranchs (see the nudibranch sea slugs) there is a tendency to shell reduction, especially as an adaptation to subterraneous or carnivorous way of life.

Arion slugs basically are omnivores; they eat mushrooms and carrion, as well as plant matter. [RN]

Apart from slugs, which have no external shell left (like Arion and Limax slugs), there are also different grades of shell reduction among so called semi-slugs (generally also referred to as slugs, just like to complete kind), such as among glass snails (Vitrinidae and Zonitidae), the least grade being a glassy translucent shell, due to the groups' name. Further grades of shell reduction can be seen in carnivorous semi-slug families, such as Testacellidae and Daudebardiidae.

Other than in the sea, habitats on land tend to change within relatively small distances. The ecological specialisation of species supported that way, it leads towards the evolution of new species by speciation.

As they necessarily have to camouflage to hide from predators, terrestrial snails are usually not as colourful as their marine relatives. However, collections of snail shells have a large importance for the understanding of a biotope's composition.

Shell characters usually at least roughly allow the determination of a snail's species. A more accurate determination sometimes makes necessary a scientific anatomical examination of the snail's body by a specialist. Especially the characters of its genital apparatus are helpful in determination purposes, as this part of the body is different between all snail species.

Outside in the country, a determination key of externally visible shell and body characters (the latter especially for slugs) may be helpful for an approximate determination of terrestrial snails. Shell characters to examine may be colour, pattern, form and dimensions of the shell.

This may, however, become rather difficult among snail species such as Merdigera obscura (the genus name fittingly meaning "excrement-bearer"), which hides under a camouflage from dirt and excrement, so it usually is only recognisable as a snail to a person who knows what to look for.