Land snail

Helix pomatia, a species of air-breathing land snail

Colonies of snails in Sicily.

Cornu Aspersum (previously known as Helix Aspersa) - the common garden snail

A land snail is a common name for any of the numerous species of snail that live on land, as opposed to those that live in salt water and fresh water. Land snails are terrestrial gastropod mollusks that have shells (those without shells are known as slugs). In reality however, it is not always easy to say which species are terrestrial, because some are more or less amphibious between land and freshwater, and others are relatively amphibious between land and saltwater.

The majority of land snails are pulmonates, i.e. they have a lung and breathe air. A minority however belong to much more ancient lineages where their anatomy includes a gill and an operculum. Many of these operculate land snails live in habitats or microhabitats that are sometimes (or often) damp or wet, such as for example in moss.

Land snails have a strong muscular foot; they use mucus to enable them to crawl over rough surfaces, and in order to keep their soft bodies from drying out. Like other mollusks, land snails have a mantle, and they have one or two pairs of tentacles on their head. Their internal anatomy includes a radula and a primitive brain. In terms of reproduction, the majority of land snails are hermaphrodite (have a full set of organs of both sexes) and most lay clutches of eggs in the soil. Tiny snails hatch out of the egg with a small shell in place, and the shell grows spirally as the soft parts gradually increase in size. Most land snails have shells that are right-handed in their coiling.

A wide range of different vertebrate and invertebrate animals prey on land snails, and they are used as food by humans in various cultures worldwide, and are even raised on farms as food in some areas.

Biology

Physical characteristics

Sphincterochila boissieri in Hamakhtesh Hagadol, northern Negev. Diameter is 2.1 cm.

Underside of a snail climbing a blade of grass, showing the muscular foot and the pneumostome or respiratory pore on the animal's right side

Land snails move by gliding along on their muscular foot, which is lubricated with mucus and covered with epithelial cilia.^[1] This motion is powered by succeeding waves of muscular contractions that move down the ventral of the foot. This muscular action is clearly visible when a snail is crawling on the glass of a window or aquarium.^[2] Snails move at a proverbially low speed (1 mm/s is a typical speed for adult Helix lucorum^[3]). Snails secrete mucus externally to keep their soft bodies from drying out. They also secrete mucus from the foot to aid in locomotion by reducing friction, and to help reduce the risk of mechanical injury from sharp objects, meaning they can crawl over a sharp edge like a straight razor and not be injured.^[4] The mucus that land snails secrete with the foot leaves a slime trail behind them, which is often visible for some hours afterwards as a shiny "path" on the surface over which they have crawled.

Snails (like all molluscs) also have a mantle, a specialized layer of tissue which covers all of the internal organs as they are grouped together in the visceral mass. The mantle also extends outward in flaps which reach to the edge of the shell and in some cases can cover the shell, and which are partially retractable. The mantle is attached to the shell, and creates the shell and makes shell growth possible by secretion.

Most molluscs, including land snails, have a shell which is part of their anatomy since the larval stage, and which grows with them in size by the process of secreting calcium carbonate along the open edge and on the inner side for extra strength. Although some land snails create shells that are almost entirely formed from the protein conchiolin, most land snails need a good supply of 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. When retracted into their shells, many snails with gills (including some terrestrial species) are able to protect themselves with a door-like anatomical structure called an operculum.

Land snails range greatly in size. The largest living species is the Giant African Snail or Ghana Tiger Snail (Achatina achatina; Family Achatinidae), which can measure up to 30 cm.^[5][6] The largest land snails of non-tropical Eurasia are endemic Caucasian snails Helix buchi and Helix goderdziana from the south-eastern Black Sea area in Georgia and Turkey; diameter of the shell of the latter may exceed 6 cm ^[7]

Most land snails bear one or two pairs of tentacles on their heads. In most land snails the eyes are carried on 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.

Internal anatomy

Light micrograph of a section through a snail's eye (Helix pomatia). 1 anterior chamber, 2 lens in the posterior chamber, 3 retina, 4 optic nerve

The anatomy of a common snail

Garden Snail, Helix aspersa defecating

Alimentary system. A snail breaks up its food using the radula inside its mouth. The radula is a chitinous ribbon-like structure containing rows of microscopic teeth. With this the snail scrapes at food, which is then transferred to the digestive tract. In a very quiet setting, a large land snail can be heard 'crunching' its food: the radula is tearing away at the surface of the food that the snail is eating.

The cerebral ganglia of the snail form a primitive brain which is 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.^[8]

Growth of the shell

As the snail grows, so does its calcium carbonate shell. The shell grows additively, by the addition of new calcium carbonate, which is secreted by glands located in the snail's mantle. The new material is added to the edge of the shell aperture (the opening of the shell). Therefore the centre of the shell's spiral was made when the snail was younger, and the outer part when the snail was older. When the snail reaches full adult size, it may build a thickened lip around the shell aperture. At this point the snail stops growing, and begins reproducing.

A snail's shell forms a logarithmic spiral. Most snail shells are right-handed or dextral in coiling, meaning that if the shell is held with the apex (the tip, or the juvenile whorls) pointing towards the observer, the spiral proceeds in a clockwise direction from the apex to the opening.

• 

  Snail with a rightwardly spiraling shell

• 

  Sinistral (left-handed) species of snail from western India

Hibernation and estivation

Some snails hibernate during the winter (typically October through April in the Northern Hemisphere). They may also estivate in the summer in drought conditions. If snails are not able to hibernate, the snail can die or its species may even go extinct like the Aldabra banded snail. To stay moist during hibernation, a snail seals its shell opening with a dry layer of mucus called an epiphragm.

Reproduction

Two Helicid snails make contact prior to mating.

Two Helix aspersa garden snails mating

The great majority of land snails are hermaphrodites with a full set of reproductive organs of both sexes, able to produce both spermatozoa and ova. A few groups of land snails such as the Pomatiidae which are distantly related to periwinkles, have separate sexes; they are male and female. The age of sexual maturity is variable depending on species of snail, ranging from as little as 6 weeks ^[9] to 5 years.^[10] Adverse environmental conditions may delay the onset of sexual maturity in some snails.^[11]

Prior to reproduction, most pulmonate air breathing land snails perform courtship behaviors before mating. The courtship may last anywhere between two and twelve hours. In a number of different families of land snails and slugs, prior to mating one or more love darts are fired into the body of the partner.

Prolific breeders, pulmonate land snails inseminate each other in pairs to internally fertilize their ova via 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. Fertilization then occurs and the eggs develop. Each brood may consist of up to 100 eggs.

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 mm 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. The cannibalization by baby snails of other eggs, even unhatched ones, has been recorded. Promptly after they are finished ingesting their egg casings, they crawl upwards through the small tunnel in order to digest the egg. 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 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.

There have been hybridizations of snail species; although these do not occur commonly in the wild, in captivity they can be coaxed into doing so.

Parthenogenesis has also been observed in certain species.^[12][13]

Lifespan

The lifespan of land snails varies from species to species. In the wild, Achatinidae snails live around 5 to 7 years and Helix snails live about 2 to 3 years. Many deaths are due to predators or parasites. In captivity, the lifespan of snails is much longer than in the wild, ranging from 10 to 15 years for many species. On occasion, snails have lived beyond this lifespan, up to 25 years.^[14]

Diet

In the wild, snails eat a variety of different foods. Terrestrial snails are usually herbivorous, however there are some species that are predatory carnivores or omnivores. The diet of most land snails can include leaves, stems, soft bark, fruit, vegetables, fungi and algae. Some species can cause damage to agricultural crops and garden plants, and are therefore often regarded as pests.

Predators

The larva of a glowworm (Lampyris noctiluca) attacking and eating a land snail

In an attempt to protect themselves against predators, land snails retract their soft parts into their shell when they are resting; some bury themselves. Land snails have many natural predators, including members of all the land vertebrate groups, three examples being thrushes, hedgehogs and Pareas snakes. Invertebrate predators include decollate snails, ground beetles, leeches, certain land flatworms such as Platydemus manokwari^[15] and even the predatory caterpillar Hyposmocoma molluscivora.

In the pulmonate marsh snail, Succinea putris, the parasitic flatworm Leucochloridium paradoxum prevents the snail from retracting its enlarged and parasitized eye stalk, causing birds, the final host of the flatworm, to be attracted to and consume these eye stalks.^[16]

Humans also pose great dangers to snails in the wild. Pollution and habitat destruction have caused the extinction of a considerable number of snail species in recent years.^[17][18]

Snails as human food

A snail farm in Provence

Land snails have been eaten for thousands of years, going back at least as far as the Pleistocene. Archaeological evidence of snail consumption is especially abundant in Capsian sites in North Africa, but is also found throughout the Mediterranean region in archaeological sites dating between 12,000 and 6,000 years ago.^[19][20] However, wild-caught land snails which are prepared for the table but are not thoroughly cooked, can harbor a parasite (Angiostrongylus cantonensis) that can cause a rare kind of meningitis.^[21]

Snail eggs, sold as snail caviar, are a specialty food that is growing in popularity in European cuisine.^[22]

Europe

Snails are eaten in several European countries, as they were in the past in the Roman Empire. Mainly three species, all from the genus Helix, are ordinarily eaten:

• Helix pomatia, or edible snail, generally prepared in its shell, with parsley butter (size: 40 to 55 mm for an adult weight of 25 to 45 g.; typically found in Burgundy, France; known as l'Escargot de Bourgogne).
• Helix lucorum, found throughout the Eastern Mediterranean region are commonly eaten in Greece and in some rural communities (ethnic Greeks and Georgian Catholics) in Georgia.
• Helix aspersa:
  • Helix aspersa also known as the European brown snail, is cooked in many different ways, according to different local traditions (size: 28 to 35 mm for an adult weight of 7 to 15 g.; typically found in the Mediterranean countries of Europe and North Africa and the French Atlantic coast; Helix aspersa aspersa known as le Petit-gris).
  • Helix aspersa maxima (size 40 to 45 mm for an average weight of 20 to 30 g.; typically found in North Africa).

Snails are a delicacy in French cuisine, where they are called escargots. 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). Before preparing snails to eat, the snails should be fasting for three days with only water available. After three days of fasting, the snails should be fed flour and offered water for at least a week. This process is thought to cleanse the snails.

Portuguese caracóis snack, species Theba pisana.

Snails are also popular in Portuguese cuisine 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.^[23]

Cooked French escargots, species Helix pomatia

Cooked Spanish "caracoles a la madrileña", species Helix aspersa

Traditional Spanish cuisine also uses snails ("caracoles" in Spanish; "caragols" or "cargols" in Catalan), consuming several species such as Helix aspersa, Otala lactea, Otala punctata or Theba pisana among others. Snails are very popular in Andalusia, Valencia and Catalonia. There are even snails celebration, as the "L’Aplec del Caragol", which takes place in Lleida each May and draws more than 200,000 visitors from abroad.

Small to medium-size varieties are usually cooked in several spicy sauces or even in soups and eaten as appetizer. The bigger ones may be reserved for other more elaborated dishes, such as the "arroz con conejo y caracoles" (a paella-style rice with snails and rabbit meat, from the inner regions of south-eastern Spain), "cabrillas" (snails in spicy tomato sauce, typical from western Andalusia) or the Catalonian caragols à la llauna (grilled inside their own shells and then eaten after dipping them in garlic mayonnaise) and à la gormanda (boiled in tomato and onion sauce).

In Greece, snails are especially popular in the island of Crete, but are also eaten in many 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. Limpets and sea snails also find their way to the Greek table around the country. Another snail cooking method is the Kohli Bourbouristi (κοχλιοί μπου(ρ)μπουριστοί)^[24] a traditional Cretan dish, which consists of fried snails in olive oil with salt, vinegar and rosemary.

They feature often in Cyprus taverna menus, under the mezes.^[25]

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.

Snails (or bebbux as they are called in Maltese) are a dish on the Mediterranean island of Malta, generally prepared and served in the Sicilian manner.

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. Some species such as the Roman Snail are protected in the wild in several European countries and must not be collected, but the Roman Snail and the Garden Snail (Cornu aspersum) are cultivated on snail farms.^[14]

Although there is not usually considered to be a tradition of snail eating in Britain, common garden snails Helix aspersa were eaten in the Southwick area of Sunderland in North East England. They were collected from quarries and along the stone walls of railway embankments during the winter when the snails were hibernating and had voided the contents of their guts. Gibson writes that this tradition was introduced in the 19th century by French immigrant glass workers.^[26] "Snail suppers" were a feature of local pubs and Southwick working men were collecting and eating snails as late as the 1970s, though the tradition may now have died out.

Africa

Achatina fulica, the giant east African snail, is canned and sliced and sold to consumers as escargot.

In parts of West Africa, specifically Ghana, snails are served as a delicacy. ^[27] Achatina achatina, Ghana tiger snails, are also known as some of the largest snails in the world.

In Cameroon, snails, usually called 'nyamangoro' and 'slow boys' are a delicacy especially to natives of the South West region of Cameroon. The snails are either eaten cooked and spiced or with a favourite dish called 'eru'.

In North Morocco, small snails are eaten as snacks in spicy soup. The recipe is identical to this prepared in Andalusia (South Spain), showing the close cultural relationship between both kinds of cuisine.

See also

• Freshwater snail
• Sea slug
• Sea snail
• Slug
• Cretan diet

References

External links

• Recipes for snails
• Predatory snails on the UF / IFAS Featured Creatures Web site.
• Site with many photographs