The word snail is a common name that can be used for almost all members of the molluscan class Gastropoda which have coiled shells in the adult stage. (Those snails which do not have a shell or only a very small shell are usually called slugs. Snails which have a broadly conical shell which is not coiled, or appears not to be coiled, are most often known as limpets.)
The class of Gastropoda (the snails and slugs) is second only to the insects in terms of total number of species. Snails are extraordinarily diverse in habitat, form, behavior, and anatomy, and therefore what is true of one snail species may not at all be true of another.
Snails can be found in a wide range of different environments from ditches to deserts to the abyssal depths of the sea. The great majority of snail species are marine. Many others are terrestrial, and numerous kinds can be found in fresh water, and even brackish water. Many snails are herbivorous, though a few land species and many marine species are omnivores or predatory carnivores.
Although the average person might perhaps be more familiar with terrestrial snails, land snails are in the minority. Marine snails have much greater diversity, and a greater biomass. Snails which respire using a lung belong to the group Pulmonata, while those with gills form a paraphyletic group, in other words, snails with gills are divided into a number of taxonomic groups that are not very closely related.
Snails with lungs and with gills have diversified widely enough over geological time that a few species with gills can be found on land, numerous species with a lung can be found in freshwater, and a few species with a lung can be found in the sea.
Although the word snail is often used for all shelled gastropods, the word "snail" can also be used in a much more limited sense, to mean various larger species of air-breathing (pulmonate) land snails. The majority of this article is about air-breathing land snails.
Whichever reasonably large land snail species is most commonly seen or most commonly eaten in a given area, that species will usually be referred to simply as "snails" by the local people. In many parts of the world, the fairly large edible species Helix aspersa or Cornu aspersum, has been introduced, and has become a pest in farms and gardens, so this is perhaps a good example of a species commonly known as "the snail."
Gastropod species which lack a conspicuous shell are commonly called slugs rather than snails, although, other than having a reduced shell or no shell at all, there are really no appreciable differences between a slug and a snail except in habitat and behavior. A shell-less animal is much more maneuverable, and thus even quite large land slugs can take advantage of habitats or retreats with very little space – places that would be totally inaccessible to a similar-sized snail, such as under loose bark on trees or under stone slabs, logs or wooden boards lying on the ground.
Taxonomic families of land slugs and sea slugs occur within numerous larger taxonomic groups of shelled species. In other words, the reduction or loss of the shell has evolved many times independently within several very different lineages of gastropods, thus the various families of slugs are very often not closely related to one another.
When retracted into their shells, many snails with gills (including many marine, some freshwater and some terrestrial species) are able to protect themselves with a door-like anatomical structure called an operculum. (The operculum of some sea snails has a pleasant scent when burned, so it is sometimes used as an ingredient in incense.)
Snails range greatly in size. The largest land snail is the Giant African Snail or Ghana Tiger Snail (Achatina achatina; Family Achatinidae), which can measure up to 30 cm. Pomacea maculata (Family Ampullariidae), or Giant Apple Snail is the largest freshwater snail, with a diameter of up to 15 cm and a mass of over 600 g. The biggest of all snails is Syrinx aruanus, an Australian marine species which can grow up to 77.2 cm (30 inches) in length and 18 kg (40lbs) in weight.
As the snail grows, so does its calcium carbonate shell. A snail's shell forms a logarithmic spiral. Most snail shells are right-handed, meaning that if you hold the shell with the apex (the tip, or the juvenile whorls) pointing towards you, then the spiral proceeds in a clockwise direction from the apex to the opening. When the animal reaches full adult size, many species of snails build a thickened lip around the opening of the shell. At this point the animal stops growing, and begins reproducing.
The shells of snails and other molluscs, and some snail egg casings, are primarily made up of calcium carbonate. Because of this, molluscs need calcium in their diet and environment to produce a strong shell. A lack of calcium, or low pH in their surroundings, can result in thin, cracked, or perforated shells. Usually a snail can repair damage to its shell over time if its living conditions improve, but severe damage can be fatal.
Most snails bear one or two pairs of tentacles on their heads. In most land snails the eyes are carried on the tips of the first (upper) set of tentacles (called ommatophores or more informally 'eye stalks') which are usually roughly 75% of the width of the eyes. The second (lower) set of tentacles act as olfactory organs. Both sets of tentacles are retractable in land snails. The eyes of most marine and freshwater snails are found at the base of the first set of tentacles.
A snail breaks up its food using the radula, which is a chitinous structure containing microscopic hooks called cuticulae. With this the snail scrapes at food, which is then transferred to the digestive tract. This is why, in a quiet setting, a large land snail can be heard 'crunching' its food: the radula is tearing away at what it is eating.
The cerebral ganglia of the snail form a primitive brain divided into four sections. This structure is very much simpler than the brains of mammals, reptiles and birds, but nonetheless, snails are capable of associative learning.
Some freshwater snails such as apple snails have gills and a "door" or operculum to close the shell when they withdraw. This structure functions as protection from predators as well as protecting the soft tissues from desiccation when an aquatic habitat dries out temporarily.
All land snails are hermaphrodites, producing both spermatozoa and ova. Some freshwater snails, such as Apple Snails, and marine species, such as periwinkles, have separate sexes; they are male and female. Most snails can mate when they are around 1 year old.
Prior to reproduction, most land snails perform a ritual courtship before mating. This may last anywhere between two and twelve hours. Prolific breeders, pulmonate land snails inseminate each other in pairs to internally fertilize their ova. Each brood may consist of up to 100 eggs.
Pulmonate land snails and slugs have a reproductive opening on one side of the body, near the front, through which the outer reproductive organs are extruded so that exchange of sperm can take place. After this, fertilization occurs and the eggs develop.
Garden snails bury their eggs in shallow topsoil primarily while the weather is warm and damp, usually 5 to 10 cm down, digging with their foot. Egg sizes differ between species, from a 3 mm diameter in the grove snail to a 6 cm diameter in the Giant African Land Snail. After 2 to 4 weeks of favorable weather, these eggs hatch and the young emerge. Snails may lay eggs as often as once a month.
The snail's shell develops while it is still an embryo; it is, however, very weak, and needs an immediate supply of calcium. Newly hatched snails obtain this by eating the egg from which they hatched. Baby snails cannibalizing other eggs, even unhatched ones, has been recorded. Promptly after they are finished ingesting their egg casings, they crawl upwards through the small tunnel remaining from when their parent dug their nest. At this stage, the young are almost completely transparent and colorless. Their shell is usually slightly smaller than the egg they hatched from, but their body length when out of their shell is slightly greater than the egg diameter. After a few weeks, the snails will begin to show their first tinge of color, usually slightly blue, before they turn their adult color. Roughly three months after they have hatched, they will look like miniature versions of their mature kin. They will continue to grow, usually for two to three years, until they reach adult size, although there have been confirmed recordings of snails growing amazingly fast - becoming even bigger than their parents in little more than a month. Irrespective of their rate of growth, however, it will still take at least 1 year before they are sexually mature.
There have been hybridizations of snails; although these do not occur commonly in the wild, in captivity they can be coaxed into doing so.
Parthenogenesis has also been noted in certain species.
Freshwater pond snails do not lay their eggs in the ground, but instead they attach them to something solid, or in some genera they carry the eggs internally until they hatch, a form of vivipary.
In the pulmonate marsh snail, Succinea putris, there is a parasitic flatworm, Leucochloridium paradoxum, which prevents the snail from retracting its enlarged and parasitized eye stalk, which thus makes the snail much more likely to be eaten by a bird, its final host.
Humans also pose great dangers to snails in the wild. Pollution and the destruction of habitats has caused the extinction of a number of snail species in recent years.
Snails have been eaten for thousands of years, beginning in the Pleistocene. They are especially abundant in Capsian sites in North Africa but are also found throughout the Mediterranean region in archaeological sites dating between 12,000 and 6,000 years ago. However, it should be noted that wild-caught land snails that are undercooked can harbor a parasite that may cause a rare kind of meningitis. Specialized snail caviar is also growing in popularity in European cuisine.
Snails are a delicacy in French cuisine, where they are called escargot. In an English-language menu, escargot is generally reserved for snails prepared with traditional French recipes (served in the shell with a garlic and parsley butter).
Snails are also popular in Portuguese cuisine (although not in the north of the country) where they are called in Portuguese caracóis, and served in cheap snack houses and taverns, usually stewed (with different mixtures of white wine, garlic, piri piri, oregano, coriander or parsley, and sometimes chouriço). Bigger varieties, called caracoletas, are generally grilled and served with a butter sauce, but other dishes also exist such as feijoada de caracóis. Overall, Portugal consumes about 4,000 tonnes of snails each year.
Traditional Spanish cuisine also uses snails ("caracoles"), consuming several species such as Helix aspersa, Helix punctata, Helix pisana or Helix alonensis among others. Small to medium-size varieties are usually cooked in several spicy sauces or even in soups, while the bigger ones may be reserved for other dishes such as the "arroz con conejo y caracoles" (a paella-style rice with snails and rabbit meat, very popular in the inner regions of south-eastern Spain). Snails are very popular in Catalonia, where they are called "caragols" or "cargols." In fact, a snail celebration, the "Aplec del cargol," takes place in Lleida each May, drawing more than 200,000 visitors from abroad. Popular Catalonian recipes for snails are a la llauna, grilled inside their own shells and then eaten after dipping them in garlic mayonnaise, or a la gormanda, boiled in tomato and onion sauce.
In Greece, snails are popular in the island of Crete, but are also eaten in other parts of the country and can even be found in supermarkets, sometimes placed alive near partly refrigerated vegetables. In this regard, snails are one of the few live organisms sold at supermarkets as food. They are eaten either boiled with vinegar added, or sometimes cooked alive in a casserole with tomato, potatoes and squashes. Another cooking method is the Kohli Bourbouristi (χοχλιοί μπου(ρ)μπουριστοί) traditional Cretan dish, which consists of fried snails in olive oil with lemon.
In Sicily, snails (or babbaluci as they are commonly called in Sicilian) are a very popular dish as well. They are usually boiled with salt first, then served with tomato sauce or bare with oil, garlic and parsley. Snails are similarly appreciated in other Italian regions, such as Sardinia.
In southwestern Germany there is a regional specialty of soup with snails and herbs, called "Black Forest Snail Chowder" (Badener Schneckensuepple).
Heliciculture is the farming of snails. "They are protected in the wild almost everywhere (at least, the Roman Snail must not be collected any more), but the Roman Snail and the Garden Snail (Cornu aspersum) especially are cultivated on snail farms."
Achatina fulica, the Giant East African Snail, is sliced and canned and sold to consumers as escargot. In parts of West Africa, specifically Ghana, snails are served as a delicacy. Achatina achatina, Ghana tiger snails, are also known as some of the largest snails in the world.
Various snail species are eaten in Asian cuisines as well.
Snails were widely noted and used in divination. The Greek poet Hesiod wrote that snails signified the time to harvest by climbing the stalks, while the Aztec moon god Tecciztecatl bore a snail shell on his back. This symbolised rebirth; the snail's penchant appearing and disappearing was analogised with the moon. More recently, Carl Jung noted that the snail was representative of the self in dreams. In psychology, the soft insides are analogous to the subconscious, as the shell is the conscious.
In contemporary speech, the expression "a snail's pace" is often used to describe a slow, inefficient process.
The phrase "snail mail" is used to mean regular postal service delivery of paper messages as opposed to the delivery of E-mail or electronic mail, which is virtually instantaneous.