Slug

Source: Wikipedia, the free encyclopedia.
For marine slugs, see Sea slug. For snails, see Snail. For other uses, see Slug (disambiguation).

Slug
Various species of British land slugs, including (from the top) the larger drawings: Arion ater, Kerry slug, Limax maximus and Limax flavus
Various species of
Limax flavus
Arion sp., from Vancouver, BC
Arion sp., from Vancouver, BC
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Groups included
A slug on a wall in Kanagawa, Japan.

Slug, or land slug, is a

mollusc. The word slug 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, particularly sea slugs and semi-slugs (this is in contrast to the common name snail
, which applies to gastropods that have a coiled shell large enough that they can fully retract their soft parts into it).

Various taxonomic families of land slugs form part of several quite different evolutionary lineages, which also include snails. Thus, the various families of slugs are closely related, because of the superficial similarity in the overall body form. The shell-less condition has arisen many times independently as an example of convergent evolution, and thus the category "slug" is polyphyletic.

Taxonomy

Of the six orders of

opisthobranchs, which are a terminal branch of the tree. The family Ellobiidae
are also polyphyletic.

Description

Drawing of slug with labels for the foot (bottom side), the foot fringe that surrounds it, the mantle behind the head, the pneumostome for breathing, and the optical and sensory tentacles
The external anatomy of a slug

The external anatomy of a slug includes the following:

Physiology

An active Ambigolimax slug in Fremont, California

Slugs' bodies are made up mostly of water and, without a full-sized shell, their soft tissues are prone to

tree bark, fallen logs, rocks and manmade structures, such as planters, to help retain body moisture.[3] Like all other gastropods, they undergo torsion (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 the pneumostome
, which is on one side of the animal, normally the right-hand side.

Slugs produce two types of mucus: one is thin and watery, and the other thick and sticky. Both kinds are hygroscopic. 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.[3] 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.[3] 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.[9] Some slugs can also produce very sticky mucus which can incapacitate predators and can trap them within the secretion.[10] Some species of slug, such as Limax maximus, secrete slime cords to suspend a pair during copulation.

Reproduction

Deroceras juvenile with eggs of another (larger) slug species, most likely Arionidae
mucous
thread and pad on wall
A pair of mating slugs

Slugs are hermaphrodites, having both female and male reproductive organs.[11] 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 approximately thirty eggs in a hole in the ground, or beneath the cover of an object such as a fallen log.

Ariolimax) and one species of Deroceras. 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.[11][12][13]

In a temperate climate, slugs usually live one year outdoors. In greenhouses, many adult slugs may live for more than one year.[14]

Ecology

Slugs play an important role in the

fungi.[15]
Most carnivorous slugs on occasion also eat dead specimens of their own kind.

Feeding habits

Manali, India

Most species of slugs are generalists, feeding on a broad spectrum of organic materials, including leaves from living plants,

predators and eat other slugs and snails, or earthworms.[15][17]

Lehmannia sp. feeding on a small fruit in Mexico City

Slugs can feed on a wide variety of

strawberries.[18] They also feed on carrots, peas, apples, and cabbage that are offered as a sole food source.[16]

Slugs from different families are

ontogenetic stages.[16]

Predators

Slugs are preyed upon by various

agricultural pests, research investments have been made to discover and investigate potential predators in order to establish biological control strategies.[19]

Vertebrates

Slugs are preyed upon by virtually every major vertebrate group. With many examples among

Galaxias postvectis) includes slugs in its diet.[20] Amphibians such as frogs and toads have long been regarded as important predators of slugs. Among them are species in the genus Bufo, Rhinella and Ceratophrys.[19]

Reptiles that feed on slugs include mainly

Cyclodomorphus casuarinae) and the common lizard (Zootoca vivipara).[19][23][24]

Birds that prey upon slugs include

charadriiform birds as slug predators.[19]

Mammals that eat slugs include foxes, badgers and hedgehogs.[25][26]

Invertebrates

Beetles in the family

driver ants
will roll the slug in dirt to absorb its mucus.

Parasites and parasitoids

Slugs are

Arion ater, Arion hortensis, Limax maximus, Tandonia budapestensis, Milax gagates, and Tandonia sowerbyi.[29][30]
R. limacum can often be seen swarming about their host's body, and live in its respiratory cavity.

Several species of nematodes are known to parasitise slugs. The nematode worms

salivary glands and rectum of Limax maximus.[33] Species of widely known medical importance pertaining to the genus Angiostrongylus are also parasites of slugs. Both Angiostrongylus costaricensis and Angiostrongylus cantonensis, a meningitis-causing nematode, have larval stages that can only live in molluscs, including slugs, such as Limax maximus.[31]

Insects such as

dipterans are known parasitoids of molluscs. To complete their development, many dipterans use slugs as hosts during their ontogeny. Some species of blow-flies (Calliphoridae) in the genus Melinda are known parasitoids of Arionidae, Limacidae and Philomycidae. Flies in the family Phoridae, specially those in the genus Megaselia, are parasitoids of Agriolimacidae, including many species of Deroceras.[34] House flies in the family Muscidae, mainly those in the genus Sarcophaga, are facultative parasitoids of Arionidae.[35]

Behavior

Slug contracts itself and retracts its tentacles when attacked
A brown and yellow spotted slug curled up into a tight ball so that its head is withdrawn completely, its mantle edge and tail are nearly touching, and none of its foot surface is exposed
The alarm response posture of the Kerry 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 the substrate. 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.[9] Slugs can also incapacitate predators through the production of a highly sticky and elastic mucus which can trap predators in the secretion.[10]

Some species present different response behaviors when attacked, such as the

hibernate underground during the winter in temperate climates, but in other species, the adults die in the autumn.[18]

Intra- and inter-specific

gregarious behavior.[39]

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 can make the crop more vulnerable to rot and disease.[40] 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.[41]

In a few rare cases, humans have developed Angiostrongylus cantonensis-induced meningitis from eating raw slugs.[42] Live slugs that are accidentally eaten with improperly cleaned vegetables (such as lettuce), or improperly cooked slugs (for use in recipes requiring larger slugs such as banana slugs), can act as a vector for a parasitic infection in humans.[32][43]

Prevention

Main article: Molluscicide

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 toxic metaldehyde, 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.[44] 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 include

Conservation tillage worsens slug infestations. Hammond et al. 1999 find maize/corn and soybean in the US to be more severely affected under low till because this increases organic matter, thus providing food and shelter.[48]

Gallery

See also

References

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  6. ^ Alonso, M. R.; Ibañe, M. (1981). "Estudio de Parmacella valenciannesii Webb & Van Beneden, 1836, y consideraciones sobre la posicion sistematica de la familia Parmacellidae (Mollusca, Pulmonata, Stylommatophora)". Boletín de la Sociedad de Historia Natural de les Baleares. 25: 103–124.
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  41. ^ Thompson, M. (2018) 'Evaluating Opportunities and Barriers to Improving the Energy Efficiency of Small Nebraska Wastewater Treatment Plants', pp.83 Archived 6 June 2022 at the Wayback Machine
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Further reading

External links

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