Slug, orland slug, is acommon name for any apparently shell-lessterrestrialgastropodmollusc. The wordslug is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a small internal shell, particularlysea slugs andsemi-slugs (this is in contrast to the common namesnail, which applies to gastropods that have a coiled shell large enough that they can fully retract their soft parts into it).
Varioustaxonomic families of land slugs form part of several quite different evolutionary lineages, which also include snails. Thus, the various families of slugs are not closely related, despite the superficial similarity in overall body form. The shell-less condition has arisen many times independently as an example ofconvergent evolution, and thus the category "slug" ispolyphyletic.
Of the six orders ofPulmonata, two – theOnchidiacea andSoleolifera – solely comprise slugs. A third group, theSigmurethra, contains various clades of snails, semi-slugs (i.e. snails whose shells are too small for them to retract fully into), and slugs.[1] The taxonomy of this group is in the process of being revised in light of DNA sequencing.[2] Research suggests that pulmonates are paraphyletic and basal to theopisthobranchs, which are a terminal branch of the tree.[3] The familyEllobiidae are also polyphyletic.
The external anatomy of a slug includes the following:
Tentacles: Like other pulmonate land gastropods, the majority of land slugs have two pairs of 'feelers' ortentacles on their head. The upper pair is light-sensing and has eyespots at the ends, while the lower pair provides thesense of smell. Both pairs are retractable instylommatophoran slugs, but contractile inveronicellid slugs.
Mantle: On top of the slug, behind the head, is the saddle-shapedmantle. In stylommatophoran slugs, on the right-hand side of the mantle is arespiratory opening, thepneumostome, which is easier to see when open; also on the right side at the front are the genital opening and anus. Veronicellid slugs have a mantle covering the whole dorsal part of the body, they have no respiratory opening, and the anus opens posteriorly.
Tail: The part of a slug behind the mantle is called the 'tail'.
Keel: Some species of slugs, for exampleTandonia budapestensis, have a prominent ridge running over their back along the middle of the tail (sometimes along the whole tail, sometimes only the posterior part).
Foot: The bottom side of a slug, which is flat, is called the 'foot'. Like almost all gastropods, a slug moves by rhythmic waves ofmuscular contraction on the underside of its foot. It simultaneously secretes a layer ofmucus that it travels on, which helps prevent damage to the foot tissues.[4] Around the edge of the foot in some slugs is a structure called the 'foot fringe'.
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.[5] An internal shell is present in theLimacidae[6] and Parmacellidae.[7] AdultPhilomycidae,[6]Onchidiidae[8] andVeronicellidae[9] lack shells.
Slugs' bodies are made up mostly of water and, without a full-sized shell, their soft tissues are prone todesiccation. They must generate protective mucus to survive. Many species are most active following rainfall or during nighttime since there is increased moisture on the ground. In drier conditions, they hide in damp places such as undertree bark, fallenlogs, rocks and manmade structures, such asplanters, to help retain body moisture.[4] Like all other gastropods, they undergotorsion (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 more or less symmetrical, except thepneumostome, which is on one side of the animal, normally the right-hand side.
Slugs produce two types ofmucus: one is thin and watery, and the other thick and sticky. Both kinds arehygroscopic. 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 that coats the whole body of the animal.[4] The mucus secreted by the foot contains fibres that help prevent the slug from slipping down vertical surfaces.
The "slime trail" a slug leaves behind has some secondary effects: other slugs coming across a slime trail can recognise 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 behaviour of some carnivorous slugs.[4] 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, or the mucus itself can be distasteful.[10] Some slugs can also produce very sticky mucus which can incapacitate predators and can trap them within the secretion.[11] Some species of slug, such asLimax maximus, secrete slime cords to suspend a pair during copulation.
Deroceras juvenile with eggs of another (larger) slug species, most likely ArionidaePair of mating slugs, suspended bymucous thread and pad on wallA pair of mating slugs
Slugs arehermaphrodites, having both female and male reproductive organs.[12] Once a slug has located a mate, they encircle each other and sperm is exchanged through their protruded genitalia.
Apophallation has been reported only in some species of banana slug (Ariolimax) and one species ofDeroceras. In the banana slugs, the penis sometimes becomes trapped inside the body of the partner. Apophallation allows the slugs to separate themselves by one or both of the slugs chewing off the other's or its own penis. Once the penis has been discarded, banana slugs are still able to mate using only the female parts of the reproductive system.[12][13][14]
In a temperate climate, slugs usually live one year outdoors. In greenhouses, many adult slugs may live for more than one year.[15]
Slugs play an important role in theecosystem by eating decaying plant material andfungi.[16] Most carnivorous slugs on occasion also eat dead specimens of their own kind.
Slugs are preyed upon by variousvertebrates andinvertebrates. The predation of slugs has been the subject of studies for at least a century. Because some species of slugs are consideredagricultural pests, research investments have been made to discover and investigate potential predators in order to establish biological control strategies.[21]
Slugs are preyed upon by virtually every major vertebrate group. With many examples amongreptiles,birds,mammals,amphibians andfish, vertebrates can occasionally feed on, or be specialised predators of, slugs.[21] Fish that feed on slugs include the brown trout (Salmo trutta), which occasionally feeds onArion circumscriptus, anarionid slug.[21] Similarly, the shortjaw kokopu (Galaxias postvectis) includes slugs in its diet.[22] Amphibians such as frogs and toads have long been regarded as important predators of slugs. Among them are species in the genusBufo,Rhinella andCeratophrys.[21]
Beetles in the familyCarabidae, such asCarabus violaceus andPterostichus melanarius, are known to feed on slugs.[29][30] Ants are a common predator of slugs; some ant species are deterred by the slug's mucus coating, while others such asdriver ants will roll the slug in dirt to absorb its mucus.
A slug retracts its head and contracts its body after being touchedThe alarm response posture of theKerry slug, which is found only in this species
When attacked, slugs can contract their body, making themselves harder and more compact and more still and round. By doing this, they become firmly attached to thesubstrate. This, combined with the slippery mucus they produce, makes slugs more difficult for predators to grasp. The unpleasant taste of the mucus is also a deterrent.[10] Slugs can also incapacitate predators through the production of a highly sticky and elastic mucus which can trap predators in the secretion.[11]
Some species present different response behaviors when attacked, such as theKerry slug. In contrast to the general behavioral pattern, the Kerry slug retracts its head, lets go of the substrate, rolls up completely, and stays contracted in a ball-like shape.[38] This is a unique feature among all theArionidae,[39] and among most other slugs.[38] Some slugs can self-amputate (autotomy) a portion of their tail to help the slug escape from a predator.[40] Some slug specieshibernate underground during the winter in temperate climates, but in other species, the adults die in the autumn.[20]
Intra- and inter-specificagonistic behavior is documented, but varies greatly among slug species. Slugs often resort to aggression, attacking bothconspecifics and individuals from other species when competing for resources. This aggressiveness is also influenced byseasonality, because the availability of resources such as shelter and food may be compromised due toclimatic conditions. Slugs are prone to attack during the summer, when the availability of resources is reduced. During winter, the aggressive responses are substituted by agregarious behavior.[41]
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 can make the crop more vulnerable to rot and disease.[42] Excessive buildup of slugs within some wastewater treatment plants with inadequate screening have been found to cause process issues resulting in increased energy and chemical use.[43]
In a few rare cases, humans have developedAngiostrongylus cantonensis-inducedmeningitis from eating raw slugs.[44] Live slugs that are accidentally eaten with improperly cleaned vegetables (such aslettuce), or improperly cooked slugs (for use in recipes requiring larger slugs such asbanana slugs), can act as a vector for a parasitic infection in humans.[34][45]
As control measures, baits are commonly used in both agriculture and the garden. In recent years,iron phosphate baits have emerged and are preferred over the more toxicmetaldehyde, 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 baits.[46] Methiocarb baits are no longer widely used. Parasitic nematodes (Phasmarhabditis hermaphrodita) are a commercially available biological control method that are effective against a wide range of common slug species. The nematodes are applied in water and actively seek out slugs in the soil and infect them, leading to the death of the slug. This control method is suitable for use in organic growing systems.
Other slug control methods are generally ineffective on a large scale, but can be somewhat useful in small gardens. These includebeer traps [de],[47][48]diatomaceous earth,[49] crushed eggshells,coffee grounds, and copper.[50] Salt kills slugs by causing water to leave the body owing toosmosis[51] but this is not used for agricultural control as highsoil salinity is detrimental to crops.[citation needed][52]Conservation tillage worsens slug infestations. Hammond et al. 1999 findmaize/corn andsoybean in the US to be more severely affected under low till because this increases organic matter, thus providing food and shelter.[53]
^Loest, R. A. (1979). "Ammonia Volatilization and Absorption by Terrestrial Gastropods_ a Comparison between Shelled and Shell-Less Species".Physiological Zoology.52 (4):461–469.doi:10.1086/physzool.52.4.30155937.JSTOR30155937.S2CID87142440.
^abBranson, B. A (1980). "The recent Gastropoda of Oklahoma, Part VIII. The slug families Limacidae, Arionidae, Veronicellidae, and Philomycidae".Proceedings of the Oklahoma Academy of Science.60:29–35.
^Dayrat, B. (2009). "Review of the current knowledge of the systematics of Onchidiidae (Mollusca: Gastropoda: Pulmonata) with a checklist of nominal species".Zootaxa.2068:1–26.doi:10.11646/zootaxa.2068.1.1.S2CID4821033.
^Schilthuizen, M.; Thome, J. W. (2008). "Valiguna flava (Heynemann, 1885) from Indonesia and Malaysia: Redescription and Comparison with Valiguna siamensis (Martens, 1867)(Gastropoda: Soleolifera: Veronicellidae)".Veliger.50 (3):163–170.
^abNixon, P."Slugs".Home, Yard & Garden Pest Newsletter. College of Agricultural, Consumer and Environmental Sciences, University of Illinois. Retrieved14 December 2012.
^Avery, R. A. (1966). "Food and feeding habits of the Common lizard (Lacerta vivipara) in the west of England".Journal of Zoology.149 (2):115–121.doi:10.1111/j.1469-7998.1966.tb03886.x.
^Symondson, W. O. C.; Glen, D. M.; Erickson, M. L.; Liddell, J. E.; Langdon, C. J. (2000). "Do earthworms help to sustain the slug predator Pterostichus melanarius (Coleoptera: Carabidae) within crops? Investigations using monoclonal antibodies".Molecular Ecology.9 (9):1279–1292.Bibcode:2000MolEc...9.1279S.doi:10.1046/j.1365-294x.2000.01006.x.PMID10972768.S2CID23921693.
^Robinson, W. H.; Foote, B. A. (1968). "Biology and Immature Stages ofMegaselia aequalis, a Phorid Predator of Slug Eggs".Annals of the Entomological Society of America.61 (6):1587–1594.doi:10.1093/aesa/61.6.1587.
^Kerney, M. P; Cameron, A. D. & Jungbluth, J. H. (1983).Die Landschnecken Nord- und Mitteleuropas (in German). Hamburg and Berlin: Verlag Paul Parey. p. 138.ISBN978-3-490-17918-0.
^Rollo, C. D.; Wellington, W. G. (1979). "Intra- and inter-specific agonistic behavior among terrestrial slugs (Pulmonata: Stylommatophora)".Canadian Journal of Zoology.57 (4):846–855.doi:10.1139/z79-104.
.jpg)
.jpg)
