Cornu aspersum
Cornu aspersum, known by the common name garden snail, is a species of land snail in the family Helicidae, which includes some of the most familiar land snails. Of all terrestrial molluscs, this species may be the most widely known. It was classified under the name Helix aspersa for over two centuries, but the prevailing classification now places it in the genus Cornu.
The Garden Snail is relished as a food item in some areas, but it is also widely regarded as a pest in gardens and in agriculture, especially in regions where it has been introduced accidentally, and where snails are not usually considered to be a menu item.
Description
The adult bears a hard, thin calcareous shell in diameter and high, with four or five whorls. The shell is variable in coloring and shade of color, but generally it has a reticulated pattern of dark brown, brownish-golden, or chestnut with yellow stripes, flecks, or streaks. The aperture is large and characteristically oblique, its margin in adults is whitish and reflected.The body is soft and slimy, brownish-grey, and able to be retracted entirely into the shell, which the animal does when inactive or threatened. When injured or badly irritated the snail produces a defensive froth of mucus that might repel some enemies or overwhelm aggressive small ants and the like. It has no operculum; during dry or cold weather it seals the aperture of the shell with a thin membrane of dried mucus; the term for such a membrane is epiphragm. The epiphragm helps the snail retain moisture and protects it from small predators such as some ants.
The snail's quiescent periods during heat and drought are known as aestivation; its quiescence during winter is known as overwintering. When overwintering, Cornu aspersum avoids the formation of ice in its tissues by altering the osmotic components of its blood ; this permits it to survive temperatures as low as. During aestivation, the mantle collar has the ability to change its permeability to water. The snail also has an osmoregulatory mechanism that prevents excessive absorption of water during hibernation. These mechanisms allow Cornu aspersum to avoid either fatal desiccation or hydration during months of either kind of quiescence.
During times of activity the snail's head and "foot" emerge. The head bears four tentacles; the upper two are larger and bear eye-like light sensors, and the lower two are tactile and olfactory sense organs. The snail extends the tentacles by internal pressure of body fluids, and retracts all four tentacles into the head by invagination when threatened or otherwise retreating into its shell. The mouth is located beneath the tentacles, and contains a chitinous radula with which the snail scrapes and manipulates food particles.
The shell of Cornu aspersum is almost always right-coiled, but exceptional left-coiled specimens are also known; see Jeremy for an example.
Taxonomy
The accepted name of the species was long considered to be Helix aspersa, a member of the genus Helix, like the Roman snail Helix pomatia. However, in a number of publications since 1990, it has instead been placed in various genera previously considered as subgenera of Helix. One such genus is Cornu, which is appropriate if the species is considered as congeneric with the species previously known as Helix aperta. Then the name would be Cornu aspersum. Previously there was debate whether Cornu was a valid generic name, but a 2015 ruling has confirmed that it is so. Until this was established, Italian research teams and others used the generic name Cantareus instead. Other workers, including Ukrainian and Russian research teams, who regard H. aspersa and H. aperta as being in different genera, call the former Cryptomphalus aspersus.Analyses based on DNA sequences have now established that C. aspersum and C. aperta share a clade with snails in the genera Otala and Eobania, distinct from the clade containing Helix, so it is no longer tenable to consider them as species of Helix.
Many subspecific varieties have been described on the basis of shell characters. The most prominent example nowadays is the subspecies Cornu aspersum maximum, originally described as a large shelled form from Algeria. In the recent scientific literature the name has been applied both to large Algerian snails and to a large form found in snail farms. Some Algerian forms are indeed genetically quite distant from the usual, most widespread form, but the large form in snail farms is different again. It is also problematic that there was a prior use of the name Helix aspersa maxima unassociated with Algeria. The subspecies maximum is formally considered by some authorities as a junior synonym of Cornu aspersum.
Life cycle
Like other Pulmonata, individuals are hermaphrodites, producing both male and female gametes. Reproduction is predominantly, and probably exclusively, by outcrossing. During a mating session of several hours, two snails exchange sperm reciprocally. H. aspersa snails stab a calcite spine, known as a love dart, into their partner. The mucus coating the love dart contains a chemical that diverts sperm away from being digested. This is important for sperm competition because individuals mate repeatedly and the donated sperm can remain viable for 4 years. About 10 days after fertilisation, the snail lays a batch of on average 50 spherical, pearly-white eggs into crevices in the topsoil, or sheltered under stones. In a year it may lay approximately six batches of eggs. The size of the egg is 3 mm.After snails hatch from the egg, they mature in one or more years. Maturity takes two years in Southern California, while it takes only 10 months in South Africa. In captivity snails can become sexually mature within 3.5 months of hatching, before they stop growing. The lifespan of snails in the wild is typically 2–3 years.
Distribution
Cornu aspersum is native to the Mediterranean region and its present range stretches from northwest Africa and Iberia, eastwards to Asia Minor and Egypt, and northwards to Britain.File:Escargot sur une feuille.jpg|thumb|Cornu on a white mulberry leaf in Johannesburg, South Africa
Cornu aspersum is a typically anthropochorous species; it has been spread to many geographical regions by humans, either deliberately or accidentally. Nowadays, it is cosmopolitan in temperate zones, and has become naturalised in regions with climates that differ from the mediterranean climate in which it evolved. Its passive anthropochory is the likeliest explanation for genetic resemblances between allopatric populations. Its anthropochorous spread may have started as early as during the Neolithic Revolution some 8500 BP. Such anthropochory continues, sometimes resulting in locally catastrophic destruction of habitat or crops.
Its increasing non-native distribution includes parts of Europe, such as Bohemia in the Czech Republic since 2008. It is present in Australia, New Zealand, North America, Costa Rica and southern South America. It was introduced to Southern Africa as a food animal by Huguenots in the 18th century, and into California as a food animal in the 1850s; it is now a notorious agricultural pest in both regions, especially in citrus groves and vineyards. Many jurisdictions have quarantines for preventing the importation of the snail in plant matter.
A number of North African endemic forms and subspecies have been described on the basis of shell characters. Cornu aspersum aspersum, in French commonly called the "petit gris", is native to the Mediterranean area and Western Europe, but has been spread widely elsewhere. The name Cornu aspersum maximum has been applied to a large form kept in heliciculture, but this is genetically distinct from large Algerian forms earlier given this name.
Ecology
Cornu aspersum is a primarily a herbivore. It feeds on numerous types of fruit trees, vegetable crops, rose bushes, garden flowers, and cereals. It also is an omnivorous scavenger that will feed on rotting plant material and on occasion scavenge animal matter, such as crushed snails and worms. Cornu aspersum can obtain the calcium required to build its shell by consuming soil. In turn it is a food source for many other animals, including small mammals, some bird species, lizards, frogs, centipedes, predatory insects such as glowworms in the family Lampyridae, and predatory terrestrial snails. The species may be of use as an indicator of environmental pollution, because it deposits heavy metals, such as lead, in its shell.Parasites
Parasites of Cornu aspersum include a number of nematodes. Metacercariae of various species of the digenean genus Brachylaima have also been reported, and those have potential for being harmful to people because the adults can infect humans. However, the snails are capable of trapping cercariae in their shell, thus possibly reducing the intensity of infestation by parasites.Behavior
The snail secretes thixotropic adhesive mucus that permits locomotion by rhythmic waves of contraction passing forward within its muscular foot. Starting from the rear, the contraction of the longitudinal muscle fibres above a small area of the film of mucus causes shear that liquefies the mucus, permitting the tip of the tail to move forward. The contracted muscle relaxes while its immediately anteriad transverse band of longitudinal fibres contract in their turn, repeating the process, which continues forward until it reaches the head. At that point the whole animal has moved forward by the length of the contraction of one of the bands of contraction. However, depending on the length of the animal, several bands of contraction can be in progress simultaneously, so that the resultant speed amounts to the speed imparted by a single wave, multiplied by the number of individual waves passing along simultaneously.A separate type of wave motion that may be visible from the side enables the snail to conserve mucus when moving over a dry surface. It lifts its belly skin clear of the ground in arches, contacting only one to two thirds of the area it passes over. With suitable lighting the lifting may be seen from the side as illustrated, and the percentage of saving of mucus may be estimated from the area of wet mucus trail dabs that it leaves behind. This type of wave passes backwards at the speed of the snail's forward motion, therefore having a zero velocity with respect to the ground.
An estimate from 1974 for a top speed of 0.03 mph has become popular. However, this estimate has been questioned since in competitions between snails only speeds of 2.4 mm/s have been achieved.
Cornu aspersum has a strong homing instinct, readily returning to a regular hibernation site.