A Snail Is Born
Oregon forestsnail. Photo copyright William Leonard, from the CalPhotos database.
DB: This is Earth and Sky with a question from David Dixon of Houston, Texas. “Do snails come from underground? And are they born with their shells?”
JB: David, when you think of snails, you might imagine them somewhere moist – in garden soil or under a stump. That’s because all land snails are descendants of snails that live in the ocean. And some of the land snails’ most important adaptations are geared toward keeping them in a moist environment reminiscent of the sea.
DB: Land snails exist because they evolved lungs – in contrast to their marine ancestors and all modern marine snails, which breathe with gills. Early land snails had lungs encased inside their shells – their lungs were housed in a small, wet cavity . . .
JB: And moisture is still the order of the day for snails. In fact, because it’s so important for developing land snails to stay moist, they grow inside protective eggs, which adult snails often lay in the soil. When they emerge from the ground, the young are tiny versions of adults – complete with their own shells.
DB: But picture a bucket filled with seashells, and you’ll realize that marine snails are much more diverse than land snails. Some do release young larvae into the water without eggs or shells. The larvae might grow their shell later – as they mature.
JB: David, thanks for your question. Special thanks to the U.S. Forest Service and to the National Fish and Wildlife Foundation. We’re Block and Byrd for Earth and Sky.
The following individual was interviewed for today’s show. Our thanks to:
Isidro Bosch
Associate Professor Department of Biology
SUNY
Geneseo, NY
Web sites:
Snails and Shells Links at Suite 101.com (Suite101.com)
If you’re interested in reproductive biology, check out this article on “snail love darts (McGill University)
Notes:
- Land snails are descended from marine snails. They have important adaptations to survive out of seawater. Marine snails use gills to breath underwater. Land snails breath with lungs-which they must keep moist. Their lungs are encased inside a moist cavity within their shell, and there is generally a small respiratory pore in the shell to allow fresh air in for the snail to breath.
*Land snails are called Pulmonates-lunged snails
*Marine snails (including the ancestors of the Pulmonates) are called Prosobranchs
*In both the ocean and on land there are groups of snails who have lost their shells. These are known as the sea slugs, and land slugs.
*Primitive marine snails and most land snails have a twisted body. During development the body undergoes “torsion” and the entire body mass twists around so that the anus ends up facing forward, over the area of what would be the animal’s right shoulder. No one really knows why this is true.
*Marine slugs (snails without shells) undergo torsion during development, but then straighten out again.
*Land snails grow their young inside eggs, to help protect them from drying out-another adaptation to living on land. The adult lays an egg case, usually in soil or underground, the young develop in the eggs, and they emerge as full formed, but small, adults-complete with shell.
- Marine snails may lay egg cases, or they may propel young shell-less larvae directly into the water column where they grow, and develop into adults.
*All snails-whether they are land, sea, or slug-are known as Gastropods.
*One special feature of this group is the “radula.” This is a sort of raspy tongue, which many snails use to scrape plant materials (notice the trails in the algae left by the snails in your fish tank). Some snails have radulas that are adapted to drill into the shells of prey. These carnivorous snails can drill into a clamshell and suck out the contents.
*Snails have unusual mating biology. They are complex hermaphrodites-snails can behave as both males and females (see link to article above).
Additional Teacher Resources
Geocities: Snail Facts
This site provides a good resource for all students learning introductory lessons concerning snails. This fact sheet covers snail facts, snail anatomy, snail life cycle, snails as pets, snail tales, and links to more information.
Kiddy House, All About Snails for Kids and Teachers: All About Snails
This site recommended by the U.S. Environmental Protection Agency, provides a great resource for younger students as an introduction to snails. It presents a variety of subjects in a way younger children can understand, such as snail characteristics, what a snail eats, how big can a snail grow, life history, how snails breathe, and related facts and activities.
