Slug
Overview
Slug is a common name
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...

 that is normally applied to any gastropod mollusc
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

 that lacks a shell, has a very reduced shell, or has a small internal shell. (This is in contrast to the common name snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

, applied to gastropods that have a coiled shell large enough that the soft parts of the animal can retract fully into it.)

Slugs belong to several different lineages which also include snails that have shells.
Encyclopedia
Slug is a common name
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...

 that is normally applied to any gastropod mollusc
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

 that lacks a shell, has a very reduced shell, or has a small internal shell. (This is in contrast to the common name snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

, applied to gastropods that have a coiled shell large enough that the soft parts of the animal can retract fully into it.)

Slugs belong to several different lineages which also include snails that have shells. The various families of land slugs are not very closely related, despite a superficial similarity in the overall body form. The shell-less condition has arisen many times independently during the evolutionary past, and thus the category "slug" is emphatically a polyphyletic one.

As well as land slugs, there are also many marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 slugs and even one freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 slug genus (Acochlidium
Acochlidium
Acochlidium is a genus of freshwater slugs, an aquatic gastropod mollusks in the family Acochlidiidae.Acochlidium species have no shell.-Species:Species within the genus Acochlidium include:* Acochlidium amboinense Strubell, 1892...

), but the common name
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...

 "slug" is most frequently applied to air-breathing land slugs, while the marine forms are usually known as sea slug
Opisthobranchia
Opisthobranchs are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping...

s. Land gastropods with a shell that is not quite vestigial, but is too small to retract into (like many in the family Urocyclidae
Urocyclidae
Urocyclidae is a family of air-breathing land snails and semi-slugs, terrestrial pulmonate gastropod mollusks in the superfamily Helicarionoidea, ....

), are known as semislugs.

Slugs, like all other gastropods, undergo torsion
Torsion (gastropod)
Torsion is an anatomical event which takes place during the very early part of the life of snails and slugs of all kinds. In other words, torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, mantle and shell 180˚...

 (a 180° twisting of the internal organs) during development. Internally, slug anatomy clearly shows the effects of this rotation, but externally the bodies of slugs appear rather symmetrical, except for the positioning of the pneumostome
Pneumostome
The pneumostome is a feature of the external body anatomy of an air-breathing land slug or land snail. It is a part of the respiratory system of gastropods....

, which is on one side of the animal, normally the right hand side.

The soft, slimy bodies of slugs are prone to desiccation
Desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...

, so land-living slugs are confined to moist environments and must retreat to damp hiding places when the weather is dry.

Morphology and behavior

Like other pulmonate land snails, most slugs have two pairs of 'feelers' or tentacles on their head. The upper pair is light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 sensing
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...

 and has eyespots at the ends, while the lower pair provides the sense of smell
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...

. Both pairs are retractable, and can be regrown if lost.

On top of the slug, behind the head, is the saddle-shaped mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

,
and under this are the genital opening and anus. On one side (almost always the right hand side) of the mantle is a respiratory opening, which is easy to see when open, but difficult to see when closed. This opening is known as the pneumostome
Pneumostome
The pneumostome is a feature of the external body anatomy of an air-breathing land slug or land snail. It is a part of the respiratory system of gastropods....

. Within the mantle in some species is a very small, rather flat shell.

Like other snails, a slug moves by rhythmic waves of muscular contraction on the underside of its foot. It simultaneously secretes a layer of mucus on which it travels, which helps prevent damage to the foot tissues. Around the edge of the foot is the 'foot fringe'.

Some slug species hibernate underground during the winter in temperate climates, but in other species, the adults die in the autumn.

Vestigial shell

Most slugs retain a remnant of their shell, which is usually internalized. This organ generally serves as storage for calcium salts, often in conjunction with the digestive glands.

An internal shell is present in the Limacidae and Parmacellidae.

Adult Philomycidae, Onchidiidae and Veronicellidae lack shells.

Mucus

Slugs' bodies are made up mostly of water, and without a full-sized shell, their soft tissues are prone to desiccation
Desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...

. They must generate protective mucus to survive. Many species are most active just after rain because of the moist ground. In drier conditions, they hide in damp places such as under tree bark, fallen logs, rocks, and man-made structures, such as planters, to help retain body moisture.

Slugs produce two types of mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

: one which is thin and watery, and another which is thick and sticky. Both kinds of mucus are hygroscopic
Hygroscopy
Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically 'changed,' somewhat, by an increase in volume, stickiness, or other...

. The thin mucus spreads from the foot's centre to its edges, whereas the thick mucus spreads from front to back. Slugs also produce thick mucus which coats the whole body of the animal.

The mucus secreted by the foot contains fibres which help prevent the slug from slipping down vertical surfaces. The "slime trail" that a slug leaves behind has some secondary effects: other slugs coming across a slime trail can recognize the slime trail as produced by one of the same species, which is useful in finding a mate. Following a slime trail is also part of the hunting behavior of some carnivorous slugs.

Body mucus provides some protection against predators, as it can make the slug hard to pick up and hold by a bird's beak, for example, and the mucus itself can be distasteful.

Some species of slug secrete slime cords to lower themselves onto the ground, or to suspend a pair of slugs during copulation.

Reproduction

Slugs are hermaphrodites, having both female and male reproductive organs.

Once a slug has located a mate, they encircle each other and sperm is exchanged through their protruded genitalia. A few days later the slugs lay around 30 eggs in a hole in the ground, or beneath the cover of an object such as a fallen log.

Apophallation
Apophallation
Apophallation is a technique resorted to by some species of air-breathing land slugs such as Limax maximus and Ariolimax spp. In these species of hermaphroditic terrestrial gastropod mollusks, after mating, if the slugs cannot successfully separate, a deliberate amputating of the penis takes...

 is a commonly seen practice among many slugs. In apophallating species, the penis curls like a corkscrew
Corkscrew
A corkscrew is a kitchen tool for drawing stopping corks from wine bottles. Generally, a corkscrew consists of a pointed metallic helix attached to a handle. The user grips the handle and screws the metal point into the cork, until the helix is firmly embedded, then a vertical pull on the...

 and during mating often becomes entangled in the mate's genitalia. Apophallation allows the slugs to separate themselves by one or both of the slugs chewing off the other's penis. Once its penis has been removed, the slug is still able to mate using only the female parts of its reproductive system.

Ecology

Many slug species play an important ecosystem role by eating dead leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

, fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

, and decaying vegetable material. Other species eat parts of living plants.

Some slugs are predators and eat other slugs and snails, or earthworm
Earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...

s.

Most carnivorous slugs on occasion also eat carrion
Carrion
Carrion refers to the carcass of a dead animal. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters include vultures, hawks, eagles, hyenas, Virginia Opossum, Tasmanian Devils, coyotes, Komodo dragons, and burying beetles...

, including dead of their own kind.

Predators

Frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

s, toad
Toad
A toad is any of a number of species of amphibians in the order Anura characterized by dry, leathery skin , short legs, and snoat-like parotoid glands...

s, snakes, hedgehog
Hedgehog
A hedgehog is any of the spiny mammals of the subfamily Erinaceinae and the order Erinaceomorpha. There are 17 species of hedgehog in five genera, found through parts of Europe, Asia, Africa, and New Zealand . There are no hedgehogs native to Australia, and no living species native to the Americas...

s, Salamander
Salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

s, eastern box turtles, rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

s, Caecilian
Caecilian
The caecilians are an order of amphibians that superficially resemble earthworms or snakes. They mostly live hidden in the ground, making them the least familiar order of amphibians. All extant caecilians and their closest fossil relatives are grouped as the clade Apoda. They are mostly...

s and also some birds and beetles are slug predators.

Slugs, when attacked, can contract their body, making themselves harder and more compact, and combined with the slippery mucus is more difficult for many animals to grasp. The unpleasant taste of the mucus is also a deterrent.

Some slugs can self-amputate (autotomy
Autotomy
Autotomy or self amputation is the act whereby an animal severs one or more of its own appendages, usually as a self-defense mechanism designed to elude a predator's grasp...

) a portion of their tail to help the slug escape from a predator.

Human relevance

The great majority of slug species are harmless to humans and to their interests, but a small number of species are serious pests of agriculture and horticulture. They can destroy foliage faster than plants can grow, thus killing even fairly large plants. They also feed on fruits and vegetables prior to harvest, making holes in the crop, which can make individual items unsuitable to sell for aesthetic reasons, and which can make the crop more vulnerable to rot and disease.

As control measures, baits are the norm in both agriculture and the garden. In recent years iron phosphate baits have emerged and are preferred over the toxic metaldehyde
Metaldehyde
Metaldehyde is an organic compound with the formula 4. It is commonly used as a pesticide against slugs, snails, and other gastropods. It is the cyclic tetramer of acetaldehyde.-Production and properties:...

, especially because domestic or wild animals may be exposed to the bait. The environmentally safer iron phosphate has been shown to be at least as effective as poisonous baits. Methiocarb baits are no longer widely used.

Other slug control methods are generally ineffective, but can be somewhat useful in small gardens. These include beer traps, diatomaceous earth
Diatomaceous earth
Diatomaceous earth also known as diatomite or kieselgur/kieselguhr, is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 1 micrometre to more than 1 millimetre, but typically 10 to...

, crushed eggshells, coffee grounds, and copper.

It is of scientific interest that salt kills slugs by causing water to leave its body owing to osmosis
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

  but this is not used for agricultural control as soil salinity is detrimental to crops.

In a few rare cases, humans have contracted parasite-induced meningitis
Meningitis
Meningitis is inflammation of the protective membranes covering the brain and spinal cord, known collectively as the meninges. The inflammation may be caused by infection with viruses, bacteria, or other microorganisms, and less commonly by certain drugs...

 from eating raw slugs.

A banana slug
Banana slug
Banana slug is a common name for three species of air-breathing land slug in the genus Ariolimax. These slugs are often yellow in color and are sometimes spotted with brown like a ripe banana....

, Ariolimax dolichophallus, named "Sammy" is the mascot of the University of California at Santa Cruz.

In rural southern Italy, the garden slug Arion hortensis was used to treat gastritis, stomach ulcers or peptic ulcers by swallowing it whole and alive. Given that it is now known that most peptic ulcers are caused by Helicobacter pylori
Helicobacter pylori
Helicobacter pylori , previously named Campylobacter pyloridis, is a Gram-negative, microaerophilic bacterium found in the stomach. It was identified in 1982 by Barry Marshall and Robin Warren, who found that it was present in patients with chronic gastritis and gastric ulcers, conditions that were...

, the merit of swallowing a live slug is questionable. A clear mucus produced by the slug is also used to treat various skin conditions including dermatitis, warts, inflammations, calluses, acne and wounds.

Taxonomy

Of the six orders of Pulmonata
Pulmonata
The Pulmonata, or "pulmonates", are an informal group of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills...

, two – the Onchidiacea and Soleolifera – solely comprise slugs. A third family, the Sigmurethra
Sigmurethra
Sigmurethra is a taxonomic category of air-breathing land snails and slugs, terrestrial pulmonate gastropod molluscs. This is an Informal Group which includes the majority of land snails and slugs....

, contains various clades of snails, semi-slugs (i.e. snails whose shells are too small for them to retract fully into) and slugs.

The taxonomy of this group is in the process of being revised in the light of DNA sequencing. It appears that pulmonates are paraphyletic and basal to the opisthobranchs which are a terminal branch of the tree. The family Ellobiidae
Ellobiidae
Ellobiidae is a family of small air-breathing land snails, terrestrial pulmonate gastropod mollusks in the clade Eupulmonata. This is the only family in the superfamily Ellobioidea, according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005)....

 are also polyphyletic.
  • Subinfraorder Orthurethra
    Orthurethra
    Orthurethra is a clade of terrestrial pulmonate gastropods .In the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005, Orthurethra is treated as an informal group and subclade of Stylommatophora.- Taxonomy :...

    • Superfamily Achatinelloidea Gulick, 1873
    • Superfamily Cochlicopoidea
      Cochlicopoidea
      Cochlicopoidea is a superfamily of air-breathing land snails, terrestrial gastropod mollusks in the informal group Orthurethra....

       Pilsbry, 1900
    • Superfamily Partuloidea
      Partuloidea
      Partuloidea is a a superfamily of air-breathing land snails.According to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005, this superfamily of terrestrial pulmonate gastropod mollusks is in the clade Stylommatophora and the informal group Orthurethra.- Families:Families within the...

       Pilsbry, 1900
    • Superfamily Pupilloidea
      Pupilloidea
      The Pupilloidea are a superfamily of air-breathing land snails, terrestrial gastropod mollusks in the informal group Orthurethra.-Taxonomy:This superfamily contains the following families:*Family Pupillidae*Family Argnidae*Family Chondrinidae...

       Turton, 1831
  • Subinfraorder Sigmurethra
    Sigmurethra
    Sigmurethra is a taxonomic category of air-breathing land snails and slugs, terrestrial pulmonate gastropod molluscs. This is an Informal Group which includes the majority of land snails and slugs....

    • Superfamily Acavoidea
      Acavoidea
      The Acavoidea are a taxonomic superfamily of air-breathing land snails and slugs, terrestrial pulmonate gastropod mollusks in the informal group Sigmurethra.This taxonomy was based on the study by Nordsieck, published in 1986...

       Pilsbry, 1895
    • Superfamily Achatinoidea
      Achatinoidea
      The Achatinoidea are a superfamily of air-breathing land snails and slugs, terrestrial gastropod mollusks in the informal group Sigmurethra.-Taxonomy:...

       Swainson, 1840
    • Superfamily Aillyoidea Baker, 1960
    • Superfamily Arionoidea
      Arionoidea
      Arionoidea is a taxonomic group, a superfamily of air-breathing land slugs, shell-less terrestrial pulmonate gastropod mollusks.- Families :Families within the superfamily Arionoidea include:* Arionidae* Anadenidae* Ariolimacidae* Binneyidae...

       J.E. Gray
      John Edward Gray
      John Edward Gray, FRS was a British zoologist. He was the elder brother of George Robert Gray and son of the pharmacologist and botanist Samuel Frederick Gray ....

       in Turnton, 1840
    • Superfamily Athoracophoroidea
      • Family Athoracophoridae
        Athoracophoridae
        Athoracophoridae, common name the leaf-veined slugs, are a family of air-breathing land slugs, terrestrial pulmonate gastropod mollusks in the infraorder Stylommatophora, the stalk-eyed snails and slugs...

    • Superfamily Orthalicoidea
      Orthalicoidea
      The Orthalicoidea are a superfamily of air-breathing land snails, terrestrial gastropod mollusks in the informal group Sigmurethra.- 2005 taxonomy :...

      • Subfamily Bulimulinae
    • Superfamily Camaenoidea Pilsbry, 1895
    • Superfamily Clausilioidea
      Clausilioidea
      The Clausilioidea are a superfamily of air-breathing land snails and slugs, terrestrial gastropod mollusks in the informal group Sigmurethra.This classification was based on different studies dealing with the Clausiliidae, published by H...

       Mörch
      Otto Andreas Lowson Mörch
      Otto Andreas Lowson Mörch was a biologist, specifically a malacologist. He lived in Sweden, in Denmark, and in France.- Taxa described :Bibliography and taxa described by Otto Andreas Lowson Mörch include:...

      , 1864
    • Superfamily Dyakioidea Gude & Woodward, 1921
    • Superfamily Gastrodontoidea
      Gastrodontoidea
      Gastrodontoidea is a taxonomic superfamily of air-breathing land snails, terrestrial pulmonate gastropod mollusks in the limacoid clade.- Taxonomy:According to the taxonomy of the Gastropoda , families in this superfamily include:...

       Tryon, 1866
    • Superfamily Helicoidea
      Helicoidea
      Helicoidea is a taxonomic superfamily of air-breathing land snails, terrestrial pulmonate gastropod mollusks in the clade Stylommatophora.-Families:There are 19 families within the superfamily Helicoidea according to Bouchet & Rocroi .* Helicidae...

       Rafinesque, 1815
    • Superfamily Helixarionoidea Bourguignat, 1877
    • Superfamily Limacoidea
      Limacoidea
      Limacoidea is a taxonomic superfamily of medium-sized to large, air-breathing land slugs and snails. They are terrestrial pulmonate gastropod mollusks in the infraorder Stylommatophora ....

       Rafinesque, 1815
    • Superfamily Oleacinoidea H. & A. Adams, 1855
    • Superfamily Orthalicoidea
      Orthalicoidea
      The Orthalicoidea are a superfamily of air-breathing land snails, terrestrial gastropod mollusks in the informal group Sigmurethra.- 2005 taxonomy :...

       Albers-Martens, 1860
    • Superfamily Plectopylidoidea Moellendorf, 1900
    • Superfamily Polygyroidea Pilsbry, 1894
    • Superfamily Punctoidea
      Punctoidea
      Punctoidea is a superfamily of air-breathing land snails and slugs, terrestrial pulmonate gastropod mollusks in the informal group Sigmurethra....

       Morse, 1864
    • Superfamily Rhytidoidea
      Rhytidoidea
      The Rhytidoidea are a superfamily of air-breathing land snails and slugs, terrestrial gastropod mollusks in the informal group Sigmurethra.This taxonomy was based on the study by Nordsieck, published in 1986...

       Pilsbry, 1893
      • Family Rhytididae
        Rhytididae
        Rhytididae is a taxonomic family of medium-sized predatory air-breathing land snails, carnivorous terrestrial pulmonate gastropod molluscs in the superfamily Rhytidoidea.This family has no subfamilies.-Anatomy:...

    • Superfamily Sagdidoidera Pilsbry, 1895
    • Superfamily Staffordioidea Thiele
      Johannes Thiele
      Johannes Thiele, full name Karl Hermann Johannes Thiele was a German zoologist specialized in malacology. His Handbuch der systematischen Weichtierkunde is a standard work...

      , 1931
    • Superfamily Streptaxoidea
      Streptaxoidea
      Streptaxoidea is a superfamily of air-breathing land snails, terrestrial pulmonate gastropod mollusks in the Stylommatophora.- 2005 taxonomy :...

       J.E. Gray
      John Edward Gray
      John Edward Gray, FRS was a British zoologist. He was the elder brother of George Robert Gray and son of the pharmacologist and botanist Samuel Frederick Gray ....

      , 1806
    • Superfamily Strophocheiloidea Thiele
      Johannes Thiele
      Johannes Thiele, full name Karl Hermann Johannes Thiele was a German zoologist specialized in malacology. His Handbuch der systematischen Weichtierkunde is a standard work...

      , 1926
    • Superfamily Parmacelloidea
      Parmacelloidea
      Parmacelloidea is a superfamily of air-breathing land slugs, terrestrial pulmonate gastropod mollusks in the clade Stylommatophora and the informal group Pulmonata...

    • Superfamily Zonitoidea Mörch
      Otto Andreas Lowson Mörch
      Otto Andreas Lowson Mörch was a biologist, specifically a malacologist. He lived in Sweden, in Denmark, and in France.- Taxa described :Bibliography and taxa described by Otto Andreas Lowson Mörch include:...

      , 1864

External links

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