Re: How do snails breathe onland? not in water

Hi Laura,

Most snails, as you know, live in water and cannot survive on land. They have either one or two gills called ctenidia that are the gas exchange organs. Some snails, called pulmonates, can breathe on land. Pulmonate snails and slugs do not have ctenidia, which would not work on land because ctenidia (like all animal gills) must be constantly surrounded by water to function and there isn't a lot of water sitting around on land. Instead, pulmonates have an internal space in the body called a mantle cavity that is well supplied with blood vessels and acts as the animal's lung. In fact, the word "pulmonate" comes from the same Latin root that gives us the word "pulmonary", which means "lung." So you see, land snails have a lung that works almost exactly the same way that our lungs work.

The lung is open to the outside by a hole called a pneumostome on the right side of the animal's body. Terrestrial snails and slugs can open and close the pneumostome at will, opening it to inhale or exhale and closing it to keep from drying out. Oxygen diffuses from the air inside the mantle cavity into the animal's bloodstream and carbon dioxide diffuses from the bood into the mantle cavity, to be released to the outside environment when the animal exhales.

Some pulmonate snails live in freshwater lakes, ponds, and rivers, and others on land. Only a few species live in the ocean. All of them, even the aquatic ones, must come to the air to breathe. Most aquatic pulmonates can survive for short periods by breathing through their skins, but still rely on the occasional gulp of air to obtain oxygen. And terrestrial snails and slugs will drown if completely submerged for very long. This is why some gardeners put out cups of beer to trap snail pests: the beer isn't poisonous to the snail, but entices the snails to fall into the cups and drown.

If you'd like to learn more about pulmonate snails, a Google search for "pulmonate" should turn up a lot of information. Many of the sites are written by aquarists who keep the snails as pets and lawnmowers in aquaria; I have hundreds of them in my freshwater planted tanks and they do a great job eating algae. You'll also find sites with photos and information on terrestrial snails and slugs.

I hope this helps!

Allison J. Gong
Mad Scientist