Living quiet lives under cover of darkness, land snails have done very well for themselves: Along with water snails, they’re number two on the list of most species on earth, second only to insects. There are 43,000 snail species living in the sea, in freshwater or on land. Five hundred land species live on North American soil alone.
There are plenty of snails in the sea; many admired for their beautiful shells. Land snails, on the other hand, don’t get much respect, especially by gardeners who despair of the damage done by the shell-less snails, called slugs. In reality, most land snails are beneficial and do little, if any, damage.
Snails are important as food for other wildlife, including mammals, birds, reptiles and insects. They’re also a source of calcium for predators because their shell is made of it. Snails are food for humans, too. Back in olden times, Roman soldiers carried snails with them for food.
Today, known by the French term escargot (ess-KAR-go), some snails are gourmet delicacies on the menus of fine restaurants around the world.
Snails are related to other animals with an obvious shell, such as clams, mussels and oysters. They belong to a class (Gastropoda) of highly varied animals called mollusks (phylum Mollusca). The first snail-like mollusk lived on the seafloor during the late Cambrian Period about 550 million years ago. During the Middle Permian Period, around 286 million years ago, some moved onto land and began breathing with lungs, instead of gills. They obviously found terrestrial life to their liking, because land snails now live just about everywhere, from deserts to tropics, from sea level to mountaintops, and in all parts of the world except Antarctica.
A snail’s most noticeable feature is its shell, which protects it from weather and predators. It also holds in moisture, which snails need. A drawback is that the shell reduces a land snail’s mobility. Water snails have the buoyancy to make their shells lightweight, but land snails have to lug around a heavy “home,” which limits their overall size.
Just compare: The largest water snail, the giant Australian Trumpet (Syrinx aruanus), grows up to three feet long (0.9 m). The largest land snail with a shell is thought to be the African Giant Snail (Achatina achatina), which has a shell that measures only 7.1 inches long (18 cm). The largest shell-less snail is probably the Pacific Banana Slug (Ariolimax columbianus), just under 10 inches long (25.4 cm).
Among the smallest snails in the world is a European species, Ammonicera rota, only 0.04 to 0.06-inch-long (1.02 to 1.5 mm). Another small snail worth noting is Partula rosea, no more than 1/2-inch-long (12.7 mm). Native to the Pacific Islands, this snail is on the verge of extinction: Half the world’s population of them — about 100 — now live in a special room in a British zoo. You can see how small this snail is here and good photos here.
The shell: Shells are highly variable in appearance. They come in different shapes, colors and patterning. They vary, too, by the number and color of bands, height and width, number of whorls and ridges, and whether they’re smooth or bumpy, or thick or thin.
Water snails get most of the fashion glory, as they often sport shells with intricate shapes and bright patterns of red, orange or yellow. Most land snails must make do with camouflage colors, usually in shades of white, gray, brown or amber to hide them better in their environment.
Slugs differ from each other, too, but since they’re essentially shell-less, their differences are more subtle. Some species are brightly colored, but most slugs are dull. It often takes an expert’s examination of the structure of the foot or other tiny features to distinguish one species from another.
A snail’s shell consists of three layers:
- The hypostracum (hi-POS-truh-cum), the innermost layer.
- The ostracum (OSS-truh-cum), the middle layer, consisting mostly of calcium carbonate.
- The periostracum, the shell’s skin, which is a mix of proteins that hold the shell’s color. After a snail dies, this layer erodes away, exposing the white or gray color of underlying calcium carbonate
Shells are usually hairless, but some species, mainly as juveniles, have hairs. It’s thought the hairy shell helps the snails cling to wet leaves.
The shell has its start during embryonic development, but it isn’t a living thing. It grows layer-by-layer as cells located on the lip of the aperture (the shell’s opening) release a calcium carbonate material. A liquid at first, it gradually hardens.
During the early stages, the shell undergoes a complicated action, known as torsion, that twists its position from the rear to forward-facing. Torsion to the right (most common) or to the left gives shells a characteristic spiral and is specific to each species.
To determine whether a shell is coiling right or left, look at the apex, the center spot where the shell began growing outward. Right spirals will go in a clockwise direction toward the aperture. And, of course, left coiling spirals grow counterclockwise.
As the shell grows larger, its walls grow thicker. (With many sea snails, the shell is nearly unbreakable by the time they reach old age.) Calcium is so important for development that a diet deficient in calcium will produce a thin, cracked shell. If this condition persists, it can be fatal. (To prevent this, owners of pet snails provide calcium-rich cuttlebone for their snails to feed on.)
Snails move in and out of their shell through the aperture. Water sails have a hardened “lid,” and can completely close themselves inside. Land snails, however, don’t have one. Instead, some species seal the aperture with a covering of mucus (called an epiphragm) and others with part of their “foot.”
Snails close themselves up for several reasons: for protection from a predator, to escape inclement weather — too hot, too cold, too dry — or to take a rest. They can stay within the shell for lengthy periods, if necessary.
The size of a shell doesn’t necessarily reflect its occupant. Some species have a shell that’s large enough for them to hide within completely, others have a shell too small for that. The shell of slugs is so tiny it either sits on the tail-end of their body or is internal. Sometimes it’s non-existent.
Slugs and slime: Slugs have a long, muscular, slimy body. They lack an obvious shell, but, otherwise, there’s basically no difference between slugs and snails. The most common garden slugs are black or dark-brown and 1/2 to 2 inches long (12.7 to 50.8 mm). In temperate climates, some slugs hibernate underground in winter. The adults of other species die in winter. Slugs are more prone to desiccation than snails with shells because they lack their protective covering.
All snails produce a kind of mucus — lots and lots of it. They need it to move. It also insulates their body and keeps away dirt and germs. It holds moisture, too, so they don’t desiccate. Mucus, or slime, can be thin for easy gliding across a smooth surface or thick to protect them from rough surfaces. When snails go into a sleep-state, mucus plugs the shell opening to keep them moist and safe. Some slugs can produce a cord of mucus to lower themselves on. You may have noticed the shiny slime trail left by snails on sidewalks and flowerpots.
Body: A snail’s body is soft and, like that of an earthworm, lacks a spine or other bones. It’s divided into three parts:
- Visceral mass (digestive, excretory and reproductive organs)
The visceral mass is covered by the “mantle.” The mantle is a skin-like organ that lines the inside of the shell and secretes calcium carbonate for shell-building.
Brain: Yes, they have a brain! And, it’s surprising. It’s simple compared to other animals, but they’re capable of associative learning and can form long-term memories. In fact, according to surprised neuroscientists studying long-term memory, the only thing limiting snails’ learning ability is the limited number of neurons in their brain. Otherwise, their cellular and molecular processes are almost exactly like that of humans. Another study, of water snails (which also use mucus), showed they’re smart enough to identify and follow the mucus trail of other snails. That saved them time and effort by not having to lay down as much mucus of their own. The researchers believe this probably applies to all snail species.
Mouth: A snail’s mouth contains a unique tongue, called the radula (RAD-joo-luh). The radula has rows of hard, course “teeth” on it made of chitin. To eat, a snail draws this rasp-like tongue over food, scraping bits of it into its mouth. They do the same thing with soft stone, such as limestone, which provides the calcium they need for building their shell. All this scraping wears the tip of the tongue away, not surprisingly. But the radula grows from the base throughout the life of a snail, as our fingernails do. Snails don’t bite, but if you allow one to crawl on your hand, it might “taste” you with its tongue, and you’ll feel its raspy teeth. It’s painless and feels like a cat’s tongue.
Tentacles and eyes: A snail’s tentacles are extremely important. All snails have at least one pair, and most land snails have two pairs on their head.
One pair sits higher than the other and is longer. This pair contains smelling sensors and the eyes, which look like nothing more than shiny black dots. One eye per tentacle, they’re located at the tip (or at the base of marine species). Snails can move their tentacles back and forth and up and down to get a better “view.” The view is very limited, however — the eyes are well developed, but they aren’t able to focus. As a consequence, snail vision is so fuzzy it’s limited to the distinctions between light and dark.
The snail’s shorter tentacles have chemoreceptors that can taste and smell. Snails also use them to feel around their environment. At a hint of danger, snails quickly withdraw both pairs of tentacles. Muscles are used to withdraw them, but blood pressure extends them.
Breathing: Snails breathe with only one lung. Muscles in the mantle expand and compress the lung. Air enters and carbon dioxide exits through an opening called the pneumostome, located on the right side of the snail’s body. The pneumostome can be opened and closed at will. Between breaths, it’s kept closed to hold moisture in.
Movement: Snails move by using a muscular “foot” located on the underside of their body. Large, flat, smooth and very maneuverable, the foot pushes against a surface with a wave-like motion. You may be wondering if snails ever move faster than a “snail’s pace.” Well, it’s all relative, but some of the larger land snails can “gallop” by creating very large waves in their foot. The world’s fastest land snail is thought to be the Garden Snail, Cornu aspersum (formerly Helix aspersa). At its speediest on a nice, smooth surface, it travels about 6 or 7 inches per minute (1.8 to 2.1 m).
To help them move, snails prepare the surface by secreting a thin layer of mucus from a gland at the front of their foot. The mucus reduces friction, but also produces a suction that helps snails cling to things, even upside down. If you’ve ever tried to pick one up, you’ve experienced the suction. The mucus also provides a protective layer when needed. It’s so effective snails can climb over sharp surfaces without harm. Here’s a video showing a snail climbing safely over a knife’s blade — and a razor’s edge!
Snails are surprisingly strong for their size. An experiment with a Cornu aspersum that weighed 1/4-ounce (7.1 g) showed it could vertically drag 2-1/2 ounces (70.9 g). Another snail, weighing 1/3-ounce (9.4 g), pulled 17 ounces (0.5 kg) horizontally — more than 50 times its weight!
Early summer is courtship time for snails. While most water snail species have separate sexes, land snails do not. They’re hermaphrodites (her-MOFF-row-dytes), which means their body contains both male and female sex organs. That being the case, it would seem they’d hardly need a mate, but they do!
Snails mate by aligning their bodies so that the penis of each is inserted into the vagina of the other. Mating may go on for several hours. After they exchange sperm, each stores the sperm in a special pouch and uses it to fertilize its eggs, sometimes over the course of several months. Before all this, though, they perform a courtship ritual. They caress each other with their tentacles, nibble at lips, and rock their bodies back and forth. It goes on for hours before mating commences. It seems, well, rather sweet.
But, not so fast! With a few species, there’s a shocking turn of events: Each pierces the body of the other with a long, sharp spear, called a “love dart.” Whoa! That, of course, begs the question, “Why?” Apparently, it sexually stimulates them or at least serves to announce it’s time to mate.
More importantly, it prevents a minor catastrophe: It happens that more than 99 percent of the millions of sperm the snails will exchange get digested internally before finding their way to the safety of the storage pouch. That reduces the number of fertilized eggs significantly. So, the love dart is nature’s rather extreme solution: It transfers a mucus that seems to prevent a snail’s body from digesting too much semen.
Most snails don’t see all that well, so one-third of love darts miss their target (or fail to penetrate.) Do the snails seem to mind getting harpooned? Yes, they do! There’s research indicating that snails frequently jostle to stab, but not be stabbed.
Land snails may lay their eggs singly or in clusters of dozens, depending on the species. They might bury their eggs in soft, moist soil by digging down with their foot or hide them in damp, protected places like leaf litter and under logs.
The eggs hatch in about two to four weeks, depending on the species and favorable weather (they don’t hatch until conditions are right.) As soon as they hatch, the hungry snails begin feeding on their eggshell, which is rich in calcium. They may also consume any other shells they find, even if the egg is still unhatched! At this stage, the tiny snails are transparent. Over the next few weeks, they slowly take on color. In about three months they’ll have adult coloration. They reach adult size and sexual maturity in two to three years.
Land snails are usually active at night when humidity is high, but they may come out in the daytime when it rains to do some foraging. If conditions get too dry, they estivate and stay that way until it rains. In winter, many species hibernate. Their heart slows down from about 36 beats per minute to only three or four and oxygen use is reduced to 1/50th of normal.
Whether they’re estivating or hibernating, snails seal their aperture with an epiphragm, which is made of dried mucus. The epiphragm is usually transparent and sometimes “glues” the snail to a surface, such as a shady wall, rock or tree branch.
All snails live where they can typically find moisture and darkness. Well, almost all: There are a few hardy species that live in semi-arid regions and survive by estivating when weather conditions are dry, which is most of the time. These snails bury themselves and withdraw into their shell, plugging the opening to conserve moisture. When it rains, there’s a rush of activity — they must eat, mate and lay eggs before the environment dries up again.
It’s possible that land snails sleep in a way that’s different from estivation or hibernation, a “regular” sort of sleep, as it were. Research on pond snails has shown they sleep for two to three day periods at a time, so why not land snails?
Snails spend their active time searching for food and eating. Also, to find mates, but they’re otherwise not social. They may be seen hiding in groups, but they don’t communicate other than to follow snail trails to find a mate or, for the few carnivorous ones, to find prey.
Snails live in such places as marshes, woodlands, pond margins, flower and vegetable gardens, under leaves, mulch, rocks, logs, in cracks and crevices, and under flower pots and other fixtures. Slugs, unencumbered by a shell, can squeeze into spots the others can’t.
Typically, snails stay within a small range, but they can easily be affected by disturbances in their environment. They’ll disperse to new areas, if possible, but since they’re unable to do it quickly, they may not escape a dangerous change. Mostly, snails are dispersed through flooding and streams. Also, humans disperse them in soil or in pots of flowers purchased at a garden center. Some have been found attached to the fur of an animal. There are also claims that tiny snail eggs can be picked up and dispersed by wind.
Land snails take advantage of whatever food they find within crawling distance and there’s a lot of variety in what they eat. The majority are herbivorous and graze on plants, fungi and algae. A few New Zealand species are carnivorous and feed on snails and nematodes (tiny worms). Snails also eat empty snail shells, sap, animal droppings, and even inorganic stuff, such as limestone and cement (for the calcium content).
The lifespan of land snails depends on the species. Heavy predation by beetles, birds and other animals means most snails don’t make it through their first year. Many, in fact, are eaten as eggs. Those that do make it live around two or three years. Captive snails have lived 10 to 15 years or more.
Predators include mammals, such as rats, moles, badgers and humans, as well as birds, toads, frogs, crabs, turtles, beetles and ants.
*Top photo: Mrooczek262 / Pixabay; PD)