The mucus seal snails produce during hibernation is called an epiphragm. It forms across the shell opening, or aperture, and helps prevent the snail’s soft body from drying out.
Most epiphragms are made from dried mucus secreted around the shell’s rim. The membrane may be transparent or translucent, and it can attach the snail to a rock, wall, tree branch, or plant stem. By closing the aperture, it sharply reduces water loss while the snail remains inactive.
Some species produce a more specialized structure before long winter dormancy. The Roman snail, Helix pomatia, can form a hard, calcareous epiphragm reinforced with calcium carbonate. This stronger seal is better suited to months spent buried in soil and may include a tiny opening that permits oxygen exchange.
An epiphragm is temporary and should not be confused with an operculum. An operculum is a more permanent, door-like structure found in other gastropods. Snails may also form epiphragms during short dry-weather rests, not only during winter hibernation.