Waiting out the drought

Estivation is dormancy during heat and drought, as hibernation is dormancy during cold. A land snail in estivation looks dead enough to alarm people. The body is drawn up into the shell. A film covers the opening, clear or white, sometimes in several layers if the dry spell has lasted. The shell is stuck to a wall, a stem, or the underside of a board. Touch it and the animal does not answer quickly.

The film is an epiphragm. The snail secretes mucus across the aperture and may reinforce it with calcium. It is not a permanent lid, and it is not the operculum that some other snail groups grow as a horny or shelly door attached to the foot. Most familiar garden snails have no operculum. They make a temporary seal when they need one, and they break it or resorb it when the air is wet again. A fresh epiphragm can look like a healing wound. It is architecture.

Inside, metabolism drops. Water loss slows without quite stopping. Snails survive weeks of drought this way, and some arid-land species survive years. Garden snails are not desert specialists. A long hot week on a south-facing wall kills many of them, especially the small ones. The animals you find sealed under a dense shrub or deep in a stone pile have reached, or stumbled into, a better pocket of air. The difference between those two sites can be the whole population.

Do not soak a sealed snail to wake it for a photograph. If the epiphragm is intact and the shell is attached, the animal is doing the correct thing. Rain is the signal it is waiting for. A splash from a hose is a crude substitute, and it can wash the snail off its hold onto dry paving, which is a worse place than the one it chose.