| Land snail | |
|---|---|
 | |
| Helix pomatia, a species of air-breathing land snail used forescargot, is a little bit larger than the common garden snail. | |
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| Cornu aspersum (previouslyHelix aspersa) – the common garden snail – in Israel | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Gastropoda |
| Groups included | |
| |
Aland snail is any of the numerous species ofsnail that live on land, as opposed to thesea snails andfreshwater snails.Land snail is thecommon name forterrestrialgastropodmollusks that haveshells (those without shells are known asslugs). However, it is not always easy to say which species are terrestrial, because some are more or lessamphibious between land and fresh water, and others are relatively amphibious between land and salt water.
Land snails are apolyphyletic group comprising at least ten independent evolutionary transitions to terrestrial life (the last common ancestor of all gastropods was marine).[1][2][3][4][5] The majority of land snails arepulmonates that have a lung and breathe air. Most of the non-pulmonate land snails belong to lineages in theCaenogastropoda, and tend to have agill and anoperculum. The largest clade of non-pulmonate land snails is theCyclophoroidea, with more than 7,000 species.[6] Many of these operculate land snails live in habitats or microhabitats that are sometimes (or often) damp or wet, such as inmoss.
Land snails have a strong muscular foot; they usemucus to enable them to crawl over rough surfaces and to keep their soft bodies from drying out. Like other mollusks, land snails have amantle, and they have one or two pairs of tentacles on their head. Their internal anatomy includes aradula and a primitive brain.In terms of reproduction, manycaenogastropod land snails (e.g.,diplommatinids) aredioecious,[7][8] but pulmonate land snails arehermaphrodites (they 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. They are used as food by humans in various cultures worldwide, and are raised on farms in some areas for use as food.
Land snails move by gliding along on their muscularfoot, which is lubricated withmucus and covered withepithelialcilia.[9] 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.[10] Snails move at a proverbially lowspeed (1 mm/s is a typical speed foradultHelix lucorum[11]). Snails secrete mucus externally to keep their soft bodies from drying out. They also secrete mucus from the foot to aid inlocomotion by reducingfriction, 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.[12] The mucus that land snails secrete with the foot leaves aslime 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 amantle, 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 ashell which is part of their anatomy since the larval stage. When they are active, the organs such as thelung,heart,kidney, andintestines remain inside the shell; only the head and foot emerge. The shell 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 proteinconchiolin, most land snails need a good supply ofcalcium in their diet and environment to produce a strong shell. A lack of calcium, or lowpH 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 anoperculum.
Land snails range greatly in size. The largest living species is the giant African snail or Ghana Tiger Snail (Achatina achatina; FamilyAchatinidae), which can measure up to 30 cm.[13][14] The largest land snails of non-tropical Eurasia are endemic Caucasian snailsHelix buchi andHelix goderdziana from the south-easternBlack Sea area inGeorgia andTurkey; diameter of the shell of the latter may exceed 6 cm.[15] At the other end of the size spectrum isAngustopila psammion, a species with shell diameter of 0.60-0.68 mm.[16]
Most land snails bear one or two pairs oftentacles on their heads. In mostland snails the eyes are carried on the first (upper) set of tentacles (calledommatophores or more informally 'eye stalks') which are usually roughly 75% of the width of the eyes. The second (lower) set of tentacles act asolfactory organs. Both sets of tentacles are retractable inland snails.
A snail breaks up its food using theradula inside its mouth. The radula is achitinous ribbon-like structure containing rows of microscopic teeth. With this the snail scrapes at food, which is then transferred to thedigestive 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 cerebralganglia of the snail form a primitivebrain which is divided into four sections. This structure is very much simpler than the brains ofmammals,reptiles andbirds, but nonetheless, snails are capable ofassociative learning.[17]
Since snails in the genusHelix are terrestrial rather than freshwater or marine, they have developed a simple lung for respiration. (Most other snails and gastropods havegills, instead.)
Oxygen is carried by the blood pigmenthemocyanin. Both oxygen andcarbon dioxide diffuse in and out of blood through thecapillaries. Amuscular valve regulates the process of opening and closing the entrance of the lung. When the valve opens, the air can either enter or leave the lung. The valve plays an important role in reducing water loss and preventing drowning.
As the snail grows, so does itscalcium carbonate shell. The shell grows additively, by the addition of new calcium carbonate, which is secreted by glands located in the snail'smantle. 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 alogarithmic spiral. Most snail shells are right-handed ordextral 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 aclockwise direction from the apex to the opening.
Some snailshibernate during the winter (typically October through April in the Northern Hemisphere). They may alsoestivate in the summer indrought conditions. To stay moist during hibernation, a snail seals its shell opening with a dry layer of mucus called anepiphragm.
The great majority of land snails arehermaphrodites with a full set of reproductive organs of both sexes, able to produce bothspermatozoa andova. A few groups of land snails such as thePomatiidae, which are distantly related toperiwinkles, have separate sexes: male and female. The age of sexual maturity varies depending on species of snail, ranging from as little as 6 weeks[18] to 5 years.[19] Adverse environmental conditions may delay sexual maturity in some snail species.[20]
Mostpulmonate air-breathing land snails performcourtship 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 morelove darts are fired into the body of the partner.
Pulmonate land snails are prolific breeders andinseminate each other in pairs tointernally 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 sperm can be exchanged. Fertilization then occurs and the eggs develop. Each brood may consist of up to 100eggs.
Garden snails bury their eggs in shallowtopsoil 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 thegrove 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.
There have beenhybridizations of snail species; although these do not occur commonly in the wild, in captivity they can be coaxed into doing so.
Parthenogenesis has been reported only in one species of slug,[21] but many species can self-fertilise.[22]
C. obtusus is a prominent endemicsnail species of theEastern Alps. There is strong evidence for selfing (self-fertilization) in the easternmost snail populations as indicated bymicrosatellite data.[23] Compared to western populations, in the eastern population mucous gland structures employed in sexual reproduction are highly variable and deformed suggesting that in selfing organisms these structures have reduced function.[23]
Most species of land snail are annual, others are known to live 2 or 3 years,[24][25] but some of the larger species may live over 10 years in the wild.[26] For instance, 10-year old individuals of the Roman snailHelix pomatia are probably not uncommon in natural populations.[27] Populations of some threatened species may be dependent on a pool of such long-lived adults.[28] In captivity, the lifespan of snails can be much longer than in the wild, for instance up to 25 years inH. pomatia.[29]
In the wild, snails eat a variety of different foods. Terrestrial snails are usually herbivorous, however some species are predatory carnivores or omnivores, including the genusPowelliphanta, which includes the largest carnivorous snails in the world, native to New Zealand.[30] Prominent predatory snail families include theSpiraxidae,Haplotrematidae andRhytididae. The diet of most land snails can include leaves, stems, soft bark, fruit, vegetables,fungi andalgae. They may have a specialized crop of symbioticbacteria that aid in digestion, especially with the breakdown of the polysaccharidecellulose into simple sugars. Some species can cause damage toagricultural crops andgarden plants, and are therefore often regarded aspests.
Many predators, both specialist and generalist, feed on snails. Some animals, such as thesong thrush, break the shell of the snail by hammering it against a hard object, such as stone, to expose its edible insides. Other predators, such as some species offrogs, circumvent the need to break snail shells by simply swallowing the snail whole, shell and all.
Some carnivorous species of snails, such as thedecollate snail and therosy wolf snail, also prey on other land snails. Such carnivorous snails are commercially grown and sold to combat pest snail species. Many of these also escape into the wild, where they prey on indigenous snails, such as the Cuban land snails of the genusPolymita, and the indigenous snails ofHawaii.
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 landvertebrate groups, three examples beingthrushes,hedgehogs andPareas snakes. Invertebrate predators includedecollate snails,ground beetles,leeches, certain landflatworms such asPlatydemus manokwari[31] and even the predatory caterpillarHyposmocoma molluscivora.
In the case of the marsh snailSuccinea putris, the snails can beparasitized by a microscopic flatworm of the speciesLeucochloridium paradoxum, which then reproduces within the snail's body. The flatworms invade the snail's eye stalks, causing them to become enlarged. Birds are attracted to and consume these eye stalks, consuming the flatworms in the process and becoming the finalhosts of the flatworm.[32]
Human activity poses great dangers to snails in the wild. Pollution andhabitat destruction have caused the extinction of a considerable number of snail species in recent years.[33][34]
Snails easily suffer moisture loss. Snails are most active at night and afterrainfall. During unfavourable conditions, a snail remains inside its shell, usually under rocks or other hiding places, to avoid being discovered bypredators. In dry climates, snails naturally congregate near water sources, including artificial sources such as wastewater outlets ofair conditioners.
Land snails have been eaten for thousands of years, going back at least as far as thePleistocene. Archaeological evidence of snail consumption is especially abundant inCapsian sites inNorth Africa, but is also found throughout theMediterranean region in archaeological sites dating between 12,000 and 6,000 years ago.[35][36] Snail eggs, sold as snailcaviar, are a specialty food that is growing in popularity in European cuisine.[37] Snails contain manynutrients. They are rich incalcium and also containvitamin B1 andE. They contain various essentialamino acids, and are low incalories andfat.[citation needed] 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 ofmeningitis.[38] The process of snail farming is calledheliciculture. The establishment of snail farms outside of Europe has introduced several species to North America, South America, and Africa, where some escapees have established themselves asinvasive species.[39][40][41][42]
In parts ofWest Africa, specificallyGhana, snails are served as a delicacy.[43]Achatina achatina, Ghana tiger snails, are also known as some of the largest snails in the world.Snail, called "igbin" in Yoruba language is a delicacy, widely eaten in Nigeria, especially among the Yorubas and Igbos. In Igbo language, snails are called "Ejuna" or "Eju".InCameroon, 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 NorthMorocco, small snails are eaten as snacks in spicy soup. The recipe is identical to this prepared inAndalusia (South Spain), showing the close cultural relationship between both kinds of cuisine.
Snails are eaten in severalEuropean countries, as they were in the past in theRoman Empire. Mainly three species, all from the familyHelicidae, are ordinarily eaten:
Snails are a delicacy inFrench cuisine, where they are calledescargots. 191 farms produced escargots in France as of 2014.[44] In anEnglish-language menu,escargot is generally reserved[citation needed] for snails prepared with traditional French recipes (served in the shell with agarlic 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.
Snails are also popular inPortuguese cuisine where they are called inPortuguesecaracóis, and served in cheapsnack houses and taverns, usuallystewed (with different mixtures ofwhite wine, garlic,piri piri,oregano,coriander or parsley, and sometimeschouriço). Bigger varieties, calledcaracoletas (especially,Cornu aspersum), are generally grilled and served with a butter sauce, but other dishes also exist such asfeijoada de caracóis. Overall,Portugal consumes about 4,000 tonnes of snails each year.[45]
TraditionalSpanish cuisine also uses snails ("caracoles" in Spanish; "caragols" or "cargols" in Catalan), consuming several species such asCornu aspersum,Otala lactea,Otala punctata andTheba pisana. Snails are very popular inAndalusia,Valencia andCatalonia. There are even snail celebrations, such as the "L'Aplec del Caragol", which takes place inLleida each May and draws more than 200,000 visitors from abroad.
Small to medium-sized varieties are usually cooked in one of several spicysauces or even in soups, and eaten as an appetizer. The bigger ones may be reserved for more elaborate dishes, such as the "arroz con conejo y caracoles" (apaella-style rice with snails andrabbit meat, from the inner regions of south-easternSpain), "cabrillas" (snails in spicy tomato sauce, typical of western Andalusia) and the Catalancaragols a la llauna (grilled inside their own shells and then eaten after dipping them in garlicmayonnaise) anda la gormanda (boiled in tomato and onion sauce).
InGreece, snails are popular in the island ofCrete, but are also eaten in many parts of the country and can even be found insupermarkets, sometimes placed alive near partly refrigeratedvegetables. In this regard, snails are one of the few live organisms sold at supermarkets as food. They are eaten either boiled withvinegar added, or sometimes cooked alive in acasserole with tomato,potatoes andsquashes. Limpets andsea snails also find their way to the Greek table around the country. Another snail cooking method is theKohli Bourbouristi (κοχλιοί μπου(ρ)μπουριστοί),[46] a traditionalCretan dish, which consists of fried snails inolive oil with salt, vinegar and rosemary.
They often feature onCyprus taverna menus, in themeze section,[47] under the namekaraoloi (καράολοι).[48]
InSicily, snails (orbabbaluci as they are commonly called inSicilian) are a popular dish. 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 asPiedmont where inCherasco there is the Italian National Institute of Heliculture.
Snails (orbebbux as they are called inMaltese) are a dish on the Mediterranean island ofMalta, generally prepared and served in the Sicilian manner.
In southwesternGermany 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 theRoman 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.[29]
Although there is not usually considered to be a tradition of snail eating inGreat Britain, common garden snailsCornu aspersum were eaten in theSouthwick area ofSunderland inNorth 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 byFrench immigrant glass workers.[49] "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.
InNew Caledonia,Placostylus fibratus (French:bulime) is considered a highly prized delicacy and is locally farmed to ensure supplies.[50] It is often served by restaurants prepared in the French style with garlic butter.
Metaldehyde andiron phosphate can be used to exterminate snails.[51] Sincecopper generates electric shocks that make it difficult for snails to move, it makes a great barrier material for them.[52]
