About land snails
Land snail (Helix aspersa) (Macrophile / Wik)
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We may not often notice the snails in our garden, but we know them on sight -- soft, featureless blobs with wiggling antennae and a shell on their back. They're like little stealth creatures, living around us unnoticed until they munch on a plant (always a favorite one).
Living quiet lives under cover of darkness, land snails have done well for themselves: Together 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.
It's no surprise that snails are related to other animals with an obvious shell, like clams, mussels and oysters, but they're also related to squid and octopuses. All belong to a group of highly varied animals called mollusks (phylum Mollusca). The first snail-like mollusk lived about 550 million years ago on the seafloor. Then, 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 the land snails discussed here (order Pulmonata) now live just about everywhere: from deserts to tropics, from sea level to mountaintops, and in all parts of the world except Antarctica.
There are still plenty of snails in the sea and many are admired for their beautiful shells. Land snails, on the other hand, aren't given much respect (especially by gardeners who despair of the damage done by the shell-less ones we call "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 to predators, because their shell is made of it. Back in olden times, Roman soldiers carried snails with them for food. Today, known by the French term escargot (esk-AR-go), some snails are gourmet delicacies on the menu of fine restaurants around the world (one such is Helix aspersa, an introduced species from Europe, who has become common in many parts of the U.S.)
A snail's most noticeable feature is his shell. It protects him from weather and certain predators. It also helps him to hold in moisture. On the other hand, it reduces his mobility on land. In water, snails have buoyancy to make their shells light, but on land they have to constantly lug around a heavy "home." This limits the overall size of land snails. So while a water snail might be up to three feet long, like the giant Australian Trumpet (Syrinx aruanus), the largest land snails are 10 to 12 inches. The largest U.S. land snail is probably the Pacific Banana Slug (Ariolimax columbianus), so named because of his habitat along the Pacific Coast, his color and shape. His body is just under 10 inches long. Among the smallest snails in the world is a European species, Ammonicera rota, only 0.04 to 0.06-inch long. Another small snail is Partula rosea. 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 a photo of this snail here.
The shell: Shells are wildly 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 various shades of white, gray, brown or amber to hide better in their environment.
The Atlantic Three-tooth Snail
(Triodopsis juxtidens) has dull
coloration like most land snails.
(David Kirsh / Jaxsnails.org)
Slugs are very different from each other, too, but since they are essentially shell-less, those differences are more subtle. Some species are very 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: 1) the hypostracum (hi-POS-truh-cum), the innermost layer; 2) the ostracum (OSS-truh-cum), the middle layer, consisting mostly of calcium carbonate; and 3) the shell skin or periostracum, which is made of 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, particularly as juveniles, have hairs. It's thought the hairy shell helps the snail 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 tell whether a shell is coiling right or left, look at the apex (the spot where the shell began growing outward). Right spirals will go in a clockwise direction from the the apex to the aperture.
As the shell grows larger, it's 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, snail pet owners provide calcium-rich cuttle-bone for their snails to feed on.
Snails emerge from their shell through the aperture. The aperture of most water snails has a hardened "lid," called an operculum, (o-PER-Q-lum). These snails can completely withdraw into their shell and close it. Land snails 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 completely hide in, 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.
This slug's mantle can easily be seen covering
the left half of his back. (Michal Zacharzewski)
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. 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 that protective covering.
All snails produce mucus -- lots and lots of it. It helps them move. It insulates their body and keeps away dirt and germs. It holds moisture so they don't desiccate. Mucus 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, mucus trail left by snails on sidewalks and flowerpots.
Body: A snail's body is soft and, like an earthworm's, lacks a spine or other bones. It's divided into three parts: the head, foot and what's called the visceral mass (the 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. In the case of slugs, they carry a tiny mantle on their back.
The snail's brain is very simple compared to other animals. Still, 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 shows marine snails are smart enough to identify and follow the mucus trail of other snails. This 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.
Snail anatomy (Jeff Dahl / Wiki)
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 his mouth. They do the same 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, like our fingernails do. Snails don't bite, but if you allow one to crawl on your hand, he might "taste" you with his tongue and you'll feel his raspy teeth. It's painless, feeling 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 of these appendages at the head end of their body. One pair sits higher than the other and is longer. This pair contains smelling sensors, as well as 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 on 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 are equipped with chemoreceptors which can taste and smell. They're also used for feeling around in their environment. At a hint of danger, both pairs of tentacles are quickly withdrawn. Muscles are used to withdraw them, but blood pressure extends them.
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 breathes, it's kept closed to hold moisture in.
Underside of a snail
climbing a blade of
grass. The breathing
hole can be easily seen.
Click to enlarge. (Sean
Mack / Wiki)
Movement: Snails move by using a large, muscular "foot" located on the underside of their body. Flat and smooth, very maneuverable, the foot pushes against a surface by producing 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 probably Helix aspersa. At his speediest on a nice, smooth surface, he travels about 6 or 7 inches per minute.
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 between a surface and the snail's fragile body. It's so effective they can climb over sharp surfaces without harm. Here's a series of photos taken by snail expert Robert Nordsieck showing a snail safely climbing over a knife's sharp edge.
Snails are surprisingly strong for their size. An experiment with a Helix aspersa who weighed 1/4-ounce showed he could vertically drag 2-1/2 ounces. Another snail, weighing 1/3-ounce, pulled 17 ounces horizontally -- more than 50 times his own weight.
Land snails are usually active at night when humidity is high, but they may come out in daytime when it rains to do some foraging. When moisture conditions are unsuitable, they estivate (see Habitat, below). Many species hibernate in winter. Either way, they seal the aperture with dried mucus, called an epiphragm. The epiphragm is usually transparent and it sometimes "glues" the snail to a surface, like a shady wall, rock or tree branch. When hibernating, the heart slows down from about 36 beats per minute to only three or four and oxygen use is reduced to 1/50th of normal.
Snails aren't 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 carnivorous ones, to find prey. Some species have a well-developed homing behavior.
Snails live in such places as marshes, woodlands, pond margins and flower and vegetable gardens. They stay 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 that others can't. All snails live where there's moisture and darkness. Well, almost all: There are some hardy species who live in semi-arid regions and survive by sleeping 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. They sleep in a state of reduced metabolic activity. Called estivation, it's similar to hibernation, except the sleep-state isn't as deep. The snails stay that way until it rains. When it does, there's a rush of activity -- they must eat, mate and lay eggs before the environment dries up again.
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 down streams. 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, or omnivorous, eating both plants and meat. There are a few predator species who eat other snails and nematodes (tiny worms). Snails will also eat empty snail shells, sap, animal droppings, even inorganic stuff, like limestone and cement (for the calcium content).
Early summer is courtship time for snails. Most water snail species have separate sexes, but land snails are hermaphrodites (her-MOFF-row-dytes), meaning their body contains all the sex organs of both male and female. It would seem they'd hardly need a mate, but they do. Two snails mate by aligning their bodies so the penis of each is inserted into the vagina of the other. Mating may go on for several hours. They exchange sperm, then each stores the sperm in a special storage pouch and uses it to fertilize their own 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.
However, with some snail species, when they're ready to mate, there's a rather shocking turn of events: Each pierces the body of the other with a long, sharp spear, called a "love dart." Ouch! This, 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 they'll exchange are internally digested before finding their way to the safety of the storage pouch. This, needless to say, reduces the number of fertilized eggs in a major way. The love dart is nature's rather extreme solution: It transfers a mucus that seems to prevent a snail's body from digesting so 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 being harpooned? Yes, actually, they do. There's research indicating that snails frequently jostle in an effort to stab, but not be stabbed.
Snails may lay their eggs singly or in clusters of dozens, depending on the species. They may bury their eggs in soft, moist soil by digging down with their foot and their "tail," or some hide them in moist, 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 wait to hatch until conditions are right.) As soon as they hatch, the hungry snails begin feeding on their egg shell, so rich in calcium. They may also consume any other shells they find, even if the egg is still occupied.
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. Their life span 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 who make it can 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.