Land snails, along with water snails, are number two on the list of the most species on earth, second only to insects. They’ve done well for themselves living quiet lives under cover of darkness. Seas snails are admired for their beautiful shells, but land snails, with their duller coloration, usually go unnoticed and undisturbed. Well, there are those pesky ones that gardeners despair of and do on-going battle with—the shell-less ones called slugs.
Most land snails, though, do little if any damage and are considered beneficial because they’re food for other wildlife, including mammals, birds, reptiles, and insects. Their shell is made of calcium, so it’s also a rich source of that for other animals. Snails are food for humans, too. Back in olden times, Roman soldiers carried snails with them for food. Today, some snails are gourmet delicacies on the menus of fine restaurants around the world and called escargot (ess-KAR-go).
Helix pomatia, called “escargot” on restaurant menus. (Waugsberg / Wiki; cc by-sa 3.0)
Background
Snails are related to other animals that have an obvious shell, such as clams, mussels, and oysters. They belong to a group 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 found terrestrial life to their liking and now live just about everywhere, from deserts to tropics, from sea level to mountaintops, and in all parts of the world except Antarctica. Including your yard, of course! Around the world, there are 35,000 species of land snails. Five hundred live on North American soil.
Size
A snail’s most noticeable feature is, of course, its shell. Its purpose is to protect from weather and predators and hold in moisture. A drawback is that it reduces a land snail’s mobility. It also limits its size. Unlike water snails which have buoyancy to lighten the load, land snails must move along hauling the full weight. And, what a difference it makes!
The largest sea snail, the giant Australian Trumpet, Syrinx aruanus, can be as long as 3 feet (91 cm). By comparison, the biggest land snail is thought to be the African Giant Snail, Achatina achatina, with a shell that measures only 7 inches (18 cm). The largest shell-less snail is probably the Pacific Banana Slug, Ariolimax columbianus, just under 10 inches (25 cm) in length.
At the other end of the scale is Acmella nana, the world’s smallest, which has a shell height of 0.027 inches (0.7 mm) and can pass through the eye of a needle! Another small snail worth noting is Partula rosea, no more than 0.5 inches (12.7 mm). Native to the Pacific Islands, this snail is now extinct in the wild. Half the world’s population—about 100—now live in a protective habitat in a British zoo.
The shell
Snail shells have personality! Some are sporty with bands of color. Some are more sedate with smooth, single-color shells, and still others are bumpy nonconformists. Cuban Painted Snails are show-offs and defy anyone to pass by without stopping to admire.
Variations of the Cuban Painted snail, Polymita picta. (© Mark Brandon Shutterstock)
Most snails stay quieter, camouflaging themselves in dull shades of white, gray, brown or amber to hide better in their terrestrial environment. There are other variables, too, among shells—height and width, number of whorls and ridges, and whether they’re thick or thin. Shells are usually hairless, but some species, mainly as juveniles, have some. It’s thought this helps them cling to wet leaves.
Most land snails are a dull color. (bangdoll / Flickr; cc by-sa 3.0
Slugs vary, too, but since they’re essentially shell-less it’s harder to discern their differences. It often takes an expert’s examination of the foot’s structure or other tiny features to distinguish one species from another. While most are very dull, there are a few that make a dramatic statement, such as the Banana Slug, Ariolimax columbianus.
Banana Slug. (Murray Foubister / Wiki; cc by-sa 2.0
The shell consists of three layers:
- Hypostracum (hi-POS-truh-cum), the innermost layer.
- Ostracum, the middle layer, consisting mostly of calcium carbonate.
- Periostracum, the skin, a mix of proteins that hold the shell’s color. After a snail dies, this layer erodes, exposing the white or gray color of underlying calcium carbonate.
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.
Torsion
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 the left gives shells a characteristic spiral and is specific to each species.
This snail has a right, or dextral, spiral. (Mhy – Pixabay; PD)
To determine whether a shell is coiling right or left, look at the apex, the center point where it begins swirling outward. Right (dextral) spirals will go in a clockwise direction toward the aperture. And conversely, left (sinistral) coiling spirals grow counterclockwise.
This snail has a left, or sinistral, spiral. (Soorelis / Pixabay; PD)
The wall of the shell thickens as it grows larger. (With many sea snails, it’s nearly unbreakable by the time they reach old age.) Calcium is so important for development that a diet deficient in it will produce a thin, cracked shell. If this persists, it can be fatal. (To prevent it, owners of pet snails provide calcium-rich cuttlebone for them to feed on.)
Snails can’t release themselves entirely from their shell, but they can move in and out of it through their aperture. Water sails have a hardened “lid,” and can completely close themselves inside. Land snails, don’t have one. Instead, some seal the aperture with a covering of mucus (called an epiphragm) and others with part of their foot.
Snails enclose themselves 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 long periods, if necessary.
The size of a shell doesn’t necessarily reflect its occupant. Some species have one that’s large enough to hide entirely within; others have a shell too small for that.
Slugs
As for slugs, depending on the species, they either have no shell, have a minuscule one at the tail-end of their body, or a tiny internal one. There are essentially no other differences between a slug and snail with a shell.
Slugs are long, muscular, and slimy. They’re commonly black or dark brown and 0.5–2.0 inches long (13 to 51 mm). They’re more prone to desiccation because they lack the moisture retention a shell would provide.
Arion vulgaris slugs, Netherlands species. (Apdency / Wiki; PD)
Slime
Snails produce slime, which is a mucus that has different purposes. They use it for movement, to isolate the body from dirt and germs, and for moisture so they don’t desiccate. It’s made of a gel that can change its density from a solid to near-liquid. So, it can be thin for easy gliding across a smooth surface or thick to protect from a rough one. Some slugs can produce a cord of mucus on which to lower themselves. 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:
- Head
- Foot
- Visceral mass (digestive, excretory and reproductive organs mixed with no divider between them).
- Head
Head
The brain is surprising. Although it holds only nerve cells and ganglia for transporting signals, it’s capable of associative learning and can form long-term memories. In fact, according to surprised neuroscientists studying long-term memory, the only thing limiting a snail’s learning ability is the limited number of neurons in their brain. Otherwise, their cellular and molecular processes are almost exactly like that of humans. A 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.
(A12, JeffDahl / Wiki; cc by-sa 3.0)
The mouth contains a unique tongue, called the radula (RAD-joo-luh), which has rows of hard, course “teeth” on it made of chitin. To eat, the 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 the 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
The tentacles are extremely important. Most land snails have two pairs.
Eyes at the tip of the upper tentacles. (Gemma Stiles / Flickr; cc by 2.0)
One pair, which sits higher than the other, is longer and has eyes that look like nothing more than shiny black dots. One per tentacle, they’re located at the tip (at the base in sea snails). They can be moved back and forth and up and down to get a better view. For them, though, the view is limited because they’re unable to focus, so it’s fuzzy and mainly distinguishes between light and dark.
The shorter tentacles hold chemoreceptors that can taste and smell. They’re usually held lower and used for feeling around the immediate environment. At a hint of danger, snails quickly withdraw all four tentacles. Muscles do the work for that, but blood pressure is what extends them.
Visceral mass (internal organs)
The visceral mass (all the internal organs) is covered by the “mantle,” a muscular, skin-like organ that lines the inside of the shell and secretes calcium carbonate for shell-building.
Land snails need only one lung for breathing. Muscles in the mantle expand and compress the lung, drawing air in and driving carbon out through an opening called the pneumostome. Located on the right side of the body, it can be opened and closed at will. Between breaths, it’s kept closed to hold moisture in.
The foot of Garden Snail, Cornu aspersum, clinging to a plant stem. Note the breathing hole in the pneumostome. (Sean Mack / Wiki; cc by 3.0)
Foot, movement
A muscular “foot” located on the underside of the body moves a snail forward only, never backward. Large, flat, smooth and very maneuverable, it pushes against a surface with a wave-like motion. You may be wondering if snails ever move faster than a “snail’s pace?” [perfectpullquote align=”right” bordertop=”false” cite=”” link=”” color=”” class=”” size=””]“Will you walk a little faster? said a whiting to the snail, there’s a porpoise close behind us and he’s treading on my tail!” — Alice’s Adventures in Wonderland, ch.10, The Lobster-Quadrille,’ by Lewis Carroll, 1865[/perfectpullquote]
Well, it’s all relative, but some of the larger land snails virtually gallop by creating huge waves in their foot. The fastest in the world 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 or 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 them cling to things, even upside down. If you’ve ever tried to pick one up, you’ve experienced it. Mucus also provides a protective layer when needed. It’s so effective that snails can climb over sharp surfaces, including a razor’s edge, without harm.
Slug and its mucus trail. (Luis Parravicini / Flickr; cc by-nc-sa 2.0)
Snails are surprisingly strong for their size. An experiment with a Cornu aspersum that weighed 0.25 ounces (7.1 g) showed it could vertically drag 2.5 ounces (70.9 g). Another snail, weighing 0.33 ounces (9.4 g), pulled 17 ounces (0.5 kg) horizontally—more than fifty times its weight!
Reproduction
Early summer is courtship time. Land snails are hermaphrodites (her-MOFF-row-dytes), which means their body contains both male and female sex organs.
Snails mating. (Linda / Flickr; cc by 2.0)
They mate by aligning their bodies so that the penis (yes, they have one!) of each is inserted into the vagina (that, too) of the other. Mating may go on for several hours. After they exchange sperm, each stores it in a special pouch and uses it to fertilize its eggs, sometimes over the course of several months. Before all this, though, there’s 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 and romantic.
But, not so fast! With a few species, there’s a shocking twist: Each pierces the body of the other with a long, sharp spear, called a “love dart.” Whoa! That sure changes the mood, and begs the question, “Why?” Despite our shocked reaction to this, theirs is to get it on! Apparently, it stimulates them.
So, do they mind getting harpooned? Well, yes; yes they do! It hurts, and there’s research indicating they frequently jostle to stab, but not be stabbed. For a relieved 30 percent of them, the other’s love dart misses or fails to penetrate.
Love dart
A love dart is more than sadomasochistic foreplay, however. It prevents a catastrophe: It happens that more than ninety-nine percent of the sperm exchanged by the snails gets digested internally before finding its way to the safety of the storage pouch. That reduces the number of fertilized eggs significantly. So, the love dart, as it happens, is nature’s extreme solution: It transfers a mucus that seems to prevent the snail’s body from digesting too much semen.
Snail eggs. (Chai / Wiki; cc by 2.0)
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.
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, so rich in calcium. They may also consume any other shells they find, even if the egg is still unhatched! At this stage, shells are transparent and have only one whorl, but over the next few weeks, they slowly take on color. In about three months they’ll have adult coloration. Snails reach adult size and sexual maturity in two to three years.
Behavior
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 (a stage of “sleep” that’s not quite as deep as hibernation) and stay that way until it rains. In winter, many species hibernate, in which their heart slows down from about thirty-six beats per minute to only three or four and oxygen use is reduced to one-fiftieth of normal.
Scientists are theorizing that land snails might also just sleep, as in a regular sort of way. You know, a little nap from time to time. Research on pond snails has shown they sleep for two or three days at a time, so why not land snails?
Epiphragm sealing the opening of a hibernating Roman Snail, Helix pomatia. (Hannes Grobe / Wiki; cc by-sa 2.5)
Whether they’re estivating or hibernating, or maybe just “sleeping,” snails seal their aperture with an epiphragm, a layer 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. In temperate climates, some slugs hibernate underground in winter, but adults of other species die.
Snails spend their active time searching for food and eating, and 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 species, to find prey.
Habitat
Typically snails live where they can find moisture and darkness. Well, most of them: 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.
The rest live in such places as marshes, woodlands, pond margins, flower and vegetable gardens, under leaves, mulch, rocks, logs, in cracks and crevices, and flowerpots and other yard fixtures. Slugs, unencumbered by a shell, can squeeze into places 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 can’t do it quickly, they may not escape a dangerous change. Mostly, snails are moved through flooding and streams. Also, humans distribute them in soil or 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 swept up by the wind.
Food sources
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 other snails and nematodes (tiny worms).
Pet snail eating a carrot. (twistypigeon / Flickr; cc by 2.0)
Snails also eat empty snail shells, sap, animal droppings, and even inorganic stuff, such as limestone and cement (for the calcium content).
Lifespan
The lifespan of land snails depends on the species. Heavy predation by beetles, birds and other animals means most don’t make it through their first year. Many are eaten as eggs. Those that do make it live around two or three years. Captive snails have lived ten to fifteen years or more.
Predators
Predators take a huge toll. They include mammals, such as rats, moles, badgers, and humans, as well as birds, toads, frogs, crabs, turtles, beetles and ants. A species goes extinct; “Lonely George,” the last of his kind
*Top photo: Mrooczek262 / Pixabay; PD)
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