Roman snails (Helix pomatia) aestivating at the former railway
station of Breitenlee near Vienna. [RN]
The most important way of terrestrial snails to protect themselves against desiccation and evaporation is the layer of mucus covering all of their body. A snail's slime is hygroscopic - it accumulates water rather than releasing it. So the snail, a creature most suited to humid air, is coated in a natural layer of water.
A membrane from dried mucus protects this snail's shell mouth.
Especially in dry weather it may be that the snail's slime will not be able to protect it sufficiently against desiccation. It is then that the snail withdraws into it's shell and begins to cover the shell aperture in mucus, which will then dry out to form a membrane called a diaphragm.
Subsequently, the snail reduces all vital functions to a necessary basic level and waits in this dryness sleep or aestivation, until the surrounding air is sufficiently humid again to allow for the snail not to die from desiccation.
In summer, Roman snails remain in aestivation during the heat of the day and will not come out but later in the evening to look for food.
During dry periods, Roman snails dig themselves into the ground or hide under roots and leaves near the ground. In shadowy, protected places, Roman snails can often be found in numbers.
Some snails also climb plant stems and trees to reach cooler layers of air. Others, like door snails (Clausiliidae) and the lapidary snail (Helicigona lapicida) hide in wall crevices and in the undergrowth.
A zebra snail (Zebrina detrita) aestivating on a
plant stem. [RN]
Terrestrial snails specially suited to dryness (xerophilous snails), like heath snails (Helicella, Xerolenta), sand hill snails (Theba pisana) and zebra snails (Zebrina detrita) stay in bushes in large numbers to endure times of dryness. Their whitish shells are also specially designed to protect them from desiccation by reflecting the sunlight. The aperture sticking to the place the snail is hiding, their shell will protect them optimally against predators seeking advantage from the snail's inability to move.
A "jumping" brown garden snail (Cornu aspersum), tail region
Even aestivation may be insufficient to protect the snail during hot summers. Compared to hibernation in winter, when a calcareous lid protects the shell aperture, snails lose more than double the amount of water during aestivation.
So especially in regions with an abundance of limestone, Roman snails may form a calcareous aperture lid also in summer. This hints to the winter lid of Roman snails basically being a dryness adaptation from their original area in the Mediterranean.
Some snail species, such as the green snail (Cantareus apertus) from the Mediterranean, even close their apertures with a calcareous lid exclusively during summer, they never hibernate.
Some snails also adapted their method of locomotion to dryness: So the brown garden snail (Cornu aspersum) tries to reduce desiccation on a dry ground by "jumping": They only touch the ground with part of their foot leaving behind a typically discontinuous slime thread.
Dormancy Phases in Snails.