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Dormancy Phases in Snails

 

Snails are very leisurely animals - a fact few would doubt. They mainly enter resting phases when environmental conditions are unfavorable. Two environmental factors are especially influential: dryness and cold. Most native snail species are active at night, in the early morning, or in the evening, when it is still cool and moist. Also, most snails are mostly active after rain. Usually that is the only time they can be found easily without deliberately searching for them.

Aestivation

 
Common door snail (Alinda biplicata): Lambrecht, Rhineland-
Palatinate, Germany. Photo: Matthias Buck (iNaturalist).		  
Final shell whorls of Alinda biplicata, laid open. The shell spire
(Columella), as well as the Clausilium, are visible.
Photo: Mathijs Zonneveld (iNaturalist). Enlarge!
The heat of the day and the dry midday hours are spent in a resting phase known as estivation. How this is carried out differs greatly from species to species. What land snails have in common during aestivation, however, is that they seal the shell aperture with a mucus membrane after retreating to a resting place. An exception are the door snails (Clausiliidae): they do not need a mucus membrane. As their name suggests, they possess a closing mechanism in which a small calcareous plate, the clausilium, seals the shell aperture when the snail withdraws. Thanks to their narrow, spindle-shaped shell, door snails can also retreat into protective crevices in trees and walls, where they forage otherwise.


Eastern heath snail (Xerolenta obvia) on a mesh-wire fence in
Vienna. Photo: Robert Nordsieck.		  
Other snails do not have this possibility and rely on the protective effect of their shell. Snails spending their aestivation outdoors are often recognizable by their light-colored shells, which reflect sunlight and reduce evaporation. This is why banded snails (e.g., the white-lipped snail, Cepaea hortensis) in open habitats are usually yellow.

Some species are even better adapted to sunlight and drought: heat-loving (xerophilic) snails of dry habitats, such as heath snails (e.g., Xerolenta obvia) or the zebra snail (Zebrina detrita, Enidae), have white shells, often with dark stripes for camouflage. On warm days, entire roadside bushes can be seen covered with small heath snails resting there in aestivation.

Other gastropods aestivate underground. Slugs, for example, cannot protect themselves with a shell and therefore retreat to sheltered places, often found under boards in gardens. Many slugs also burrow into the soil during dry periods.

 
A zebra snail (Zebrina detrita) aestivating on a
grass leaf. Photo: Robert Nordsieck.
Unlike their small relatives, the banded snails, Roman snails (Helix pomatia) also burrow into the ground or withdraw deep into the vegetation layer, where they are protected against drought. The Corsican snail (Helix ceratina) can burrow as deep as 60 cm into the soil. The Sicilian snail (Cornu mazzullii), on the other hand, can acidify its mucus to etch holes into limestone rock, into which it retreats during heat and drought - at the same time providing protection against predators.

Aestivation of the Roman Snail.

Hibernation


The hibernation lid (epiphragm) not only pro-
tects a Roman snail against frost, but also a-
gainst desiccation. Photo: Monika Samland.		  
The opposite extreme is winter cold, which most snails cannot easily withstand. Many slugs, such as the large round-back slugs (Arionidae), die in autumn after laying their eggs. However, many snails are capable of hibernating and can live for several years. Especially advanced is the hibernation behavior of the Roman snail (Helix pomatia). It seals its shell aperture with a calcareous epiphragm, retreats deep into its shell, and creates an insulating air pocket as additional protection against frost.

They spend the winter in a self-dug burrow, often lined with moss. If the Roman snail also expels excess water and breaks down large unneeded molecules in its bloodstream into smaller components, it can survive frost down to –40 °C without freezing. During hibernation, activity is reduced to the absolute minimum, and even the heartbeat slows drastically.

The epiphragm used for overwintering is probably only secondarily an adaptation to cold, although it does protect against penetrating ice. Roman snails also produce an epiphragm under dry conditions, provided that enough calcium is available.

Their Mediterranean relatives, such as the brown garden snail (Cornu aspersum) or the green snail (Cantareus apertus), form an epiphragm only for protection against dryness, since they do not need to undergo true hibernation in winter in their native home.

A snail goup that benefits from the hibernation of many other snails are the glass snails (Vitrinidae). Those small predatory snails feed on other snails in hibernation. Frosty temperatures do not seem to bother them - they even crawl over snow while hunting.

Hibernation of the Roman Snail.

Do snails sleep?

 
Mountain glass snail (Semilimax kotulae). Photo: Gianbattista Nardi.
Naturally, it is difficult to determine whether such a slow-moving animal as a snail is asleep or awake when it is inactive. Recent research has shown, however, that snails also have resting phases even when they are not protecting themselves from dryness or cold. These studies were conducted on aquatic snails (Great pond snail, Lymnaea stagnalis).

Researchers first observed snail behavior to identify sleep-like states. They found that snails repeatedly entered resting phases of about 20 minutes, during which they were relaxed and showed a markedly reduced response to stimuli—both touch and food. Interestingly, the pond snails did not even retract into their shells during these rest phases.

Land snails also show resting phases that may indicate sleep, though they usually spend them inside their shell to protect themselves against dryness and predators.

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