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Snail

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Snail
Un amour d'escargots.jpg
Scientific Classification
Selected Snails
Image Description
Snail love.jpg

Snails are gastropods (meaning stomach-footed). “Snail” is just a common name for almost any gastropod that has a coiled shell once they mature (the only exceptions being slugs with non-existent or significantly small shells, and limpets, whose shells are not coiled). Snails are popular pets, and are a commonplace addition to salt-water aquariums. Despite that, they are often seen as pests, eating many garden plants and destroying crops. Some well-known species are the garden snail and apple snail. Snails are found in almost any environment. They can range in size from less than a millimeter in width (the narrow-mouthed whorl snail) [1], to over 30 inches. The largest terrestrial snail is the Giant African Snail (Achatina fulica), and the Giant Apple Snail (Pomacea maculata) is the largest freshwater snail. The largest snail in the world is a marine species known as Syrix aruanus [2], [3].

Contents

Anatomy

Shell

Snail Shell

All snails are univalves (they have a single shell used for protection and shelter). There are over 75,000 species of snails, so obviously the shapes of the shells are diverse. A strong muscle connected to the columella holds the snail’s shell onto the body. Snail shells are made almost completely of calcium carbonate (97.5 %). Calcium phosphate and silicate, magnesium carbonate and oxide, iron, manganese oxide and other organic substances make up the rest of the composition. Most shells have a conical or oval shape. All shells have twists known as whorls, which are widest at the base of the shell and get progressively tighter as they spiral towards the center. The newest whorl, also known as the body whorl, is at the base; any spirals above it are jointly known as the spire. The apex (tip) is at the center. All whorls are clearly separated by a groove dubbed the suture. An aperture (opening) in the body whorl has an inner-lip and outer-lip. New material added to the lips increases the size of the shell. Some snails appear to have ridges on their shells. These wrinkles result from the variation in growth rates; due to environmental conditions, the shell will grow thicker in some places (during warm months), and much thinner elsewhere (in colder months). The ridges are known as axial ribs. Even before the snail hatches, it has formed a single small whorl around itself known as a protoconch. As the snail grows, new material is secreted by the mantle. The mantle lines the inside of the bottom whorl. In order to form the spiral shape, it must have a central rod for support. Such a rod is called a columella; it goes all the way through the center of the shell.[4]. If the snail feels threatened, it can draw back into its shell for defense. They use a trap-door like plate called an operculum to seal off the opening. Snails also reside in their shells during overly-hot or -cool weather to keep from drying out[5].

Land Snails

Anatomy of a Snail

The snail has a few basic structures such as the foot, head, shell and visceral mass. The visceral mass includes internal organs housed within the shell. Internal organs include a lung, the digestive system, the kidney, liver and the reproductive system. Only the head and foot are visible. They also have a nervous system running throughout the body, but it is concentrated mostly in the head [6]. The snail has a mantle cavity that is supplied with many blood vessels that function as their lung [7]. Snails breathe by raising and lowering the floor of the pallial (mantle) cavity, much like a human diaphragm. Snails are able to breathe through their skin as well. The mantle cavity also holds the heart. A snail’s two-chambered heart (one ventricle and one atrium) is located near the respiratory organs. They have a single valve between the atrium and ventricle to prevent the back-flow of blood. Their circulatory system is mostly open; the only exceptions are the few blood vessels that bring blood to the foot or the visceral sac [8]. After tearing apart food with a chitinous tongue called the radula, their food travels down the gullet. Digestion actually starts with the saliva, which works by digesting carbohydrates and by softening the food. The food is pre-digested in the crop and digested in the three-chambered stomach. Symbiotic bacterium helps digest cellulose. Digestion is aided by a digestive gland called a hepatopancreas; it is often referred to as the liver, but has many more functions. The hepatopancreas helps break down proteins and lipids. Food then passes through the intestine where simple sugars are reabsorbed. The intestine opens to the anus, located near the respiratory opening [9]. Land-dwelling snails are uricotelic (they excrete mostly solid uric acid).

Close-up of a snail's eye
Depending on the species, a snail will have one to two retractable pairs of tentacles on its head. The eyes are always located on the longer of the two pairs [10]. The eyes are able to move about to get the best possible view. Snails have a four-part brain capable of learning. Scientific research has shown that snails are actually quite smart; they possess associative thinking based on experience and conditioning [11].
Underside of a snail's foot

Snails move by a kind of locomotion called reptation [12]. The snail’s foot helps it move by secreting mucus from a gland near the mouth. Without this mucus trail, the snails would risk injury of sharp objects and the environment would be too rough for their soft bodies to move across. But with the mucus, snails can crawl across things like knives and needles. Unlike slugs, the snail’s slime trail is spotty instead of continuous. In addition to the mucus, a snail’s foot is protected by the mantle. The mantle is also found around the shell. When snails move, they often seem overtaken by spasms. These tremors are actually caused by the contraction and expansion of the muscles in the foot. By creating a rippling motion along the foot, a snail can push and pull itself along any surface [13]. Despite their slowness, there is a recurring cadence when they move [14].

Gilled Snails

Sea snails breathe using gills, just like a fish. The gills, like land snail’s lungs, are located in the mantle cavity. Fresh-water snails may also have gills, but many have lungs and must surface to get oxygen. Unlike land-dwelling snails, a marine snail’s eyes are placed at the base of the tentacles and cannot move. [15]. Aquatic snails are not uricotelic, so they excrete ammonia.

Reproduction

Neritina snail eggs

Snails are hermaphroditic (contain both male and female reproductive organs). Even so, most snails mate in order to produce eggs. Since a snail’s lifespan does not usually last more than five to seven years, snails are considered sexually mature once they are one year old [16]. Mating takes place during late spring to early summer; occasionally there will be a second mating during summer. Depending on the climate, snails may mate as late as October and a second time two weeks later. Snails can act as one gender in a season and the other gender next season. They can also play both roles at once and fertilize each other at the same time[17]. Before mating, snails put on a mating ritual which can be two to twelve hours long. After this ritual, the snails will fertilize each other’s eggs. Snails can store the sperm for up to a year, but typically lay eggs within a couple of weeks. There can be up to 100 eggs. After the eggs are fertilized, they are buried in the soil (or beside a hard area, like a rock, for marine species) and begin to develop inside the embryo. Within two to four weeks, the tiny snails have hatched. Their shells are still very weak, so in order to obtain calcium, they eat their own egg – or even another snail’s egg. They stay within the nest for a few months until they develop fully [18]. A few species of snails (mostly fresh-water) actually give birth to live young, but the majority lay eggs. Other snails also reproduce by broadcast-spawning (releasing the sex cells into the water). If the cells are released close enough to another cell, the sperm fertilizes the egg and embryonic development begins [19].

Ecology

Land Snails

Terrestrial snails can live in virtually any habitat from harsh deserts and mountain climates, to swamps and gardens.
Snail on a chain-link fence
Some species are known as anthropophilic (they are mostly found around human-inhabited areas).
Estivating snails

Snails that live in the desert or arid climates can stay in their shells for up to two years. This is a state called estivation; it's like hibernation, but it is triggered by dry and hot conditions instead of cold. They will remain sealed in their shell until the water is replenished. From October through April, some snails will hibernate. A snail’s diet includes such things as leaves, plants, fruit, mushrooms, stems, soft bark, etc [20]. Though it is rare, some snails have been found to be carnivorous or omnivorous [21].Snails have many predators in the wild. They are in danger of attack from many vertebrates (salamanders, shrews, etc.), beetles, leeches, and even predatory snails and caterpillars. Firefly larvae prey upon snails. Birds like the thrush, grouse and other ground foragers eat terrestrial snails. Parasites are also a big problem for snails. Snails are very important in the cycling of calcium. All snails need calcium carbonate which they get from plants and, when they are eaten, pass it along the food chain. But calcium isn't the only thing snails pass along. Snails are intermediate hosts of all sorts of parasites, including the sheep liver fluke (Porch, pg. 458). Snails play a small role in decomposition, though not as much as other animals. Snails even have medicinal uses. Anti-agglutinin is made from the albumin glands of Helix aspersa. Every gardener knows the problem posed by snails. Crops and agriculture are both greatly affected by snails. Non-native snails introduced to the U.S. have become a big problem and are in the process of being eliminated.

Aquatic Snails

Snail in an Aquarium

The majority of snails are actually aquatic (live in water). They can thrive in any kind of water, from freshwater to oceans and everything in between. Marine and fresh-water snails like to eat foods such as plankton, algae, and other organisms that live in the sand or on the shoreline. Animals like the Botia family of fresh-water fish, parasites and birds pose a threat to aquatic snails [22].

Lifespans of the snails vary with the species; they can live from one (apple snails) to seven (Achatinidae snails) years in the wild. Parasites and predators are the leading causes of death. But the destruction of the snail’s habitat, along with pollution, are pushing some species to extinction. When kept as pets, snails can live for around ten to fifteen years. Some snails have even been reported to live up to thirty years. Since snails don't move very far during their short life, they are excellent signals of a site’s conditions. Because of their need for calcium, they are very perceptive of the calcium in plants and soil. Things like pollution, drought and fires affect snail population. Snails have even been used by the European archaeologists to deduce the past environments. Scientists have used snails for biological control. Both terrestrial and aquatic snails are popular in cuisine around the world [23].

Escargots

Escargot

Escargot is just a general name for any edible snail. Snails have been eaten for thousands of years, being traced all the way back to the Pleistocene era and the Roman Empire. Snails are eaten most often in European countries. In France they are known as escargots; in Portugal, caracóis; in Sicily, babbaluci and in Maltese, bebbux. They are called caracoles in Spain and are also quite popular in Greece. Snails are viewed as everything from delicacies in France, to cheap bar snacks in Portugal. Lleida even has a snail festival called Aplec del cargol. There are three commonly eaten species: Helix pomatia is typically prepared in its shell; Helix aspersa, also known as the European brown snail, prepared in various ways depending on the area’s tradition; and Helix aspersa maxima. Snails are typically boiled/steamed, then served on a special plate that has little hollows for the shell to fit in. Small tongs and two-tined forks are used to pull the flesh from the shell. The flesh is then dipped in the provided sauce, which is usually garlic and butter. Snails are either collected from the wild or harvested from farms. Heliciculture is the harvesting of snails. Since the contents of snail’s stomachs can be very toxic, farmed snails are put through a process of purging. They are either starved, or fed with a new diet, usually of cereals, greens or other dried foods. Snails shouldn’t be fed for three days before being prepared as food [24], [25].

Gallery

References

Thomas E. Porch and Brad R. Bratorf, BIOLOGY Third Edition with Laboratory Exercises, Greenville, South Carolina: BJU PRESS, 2005

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