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Slug is a common word, normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell, in contrast to snails, which are gastropods with coiled shells that are big enough to retract into. All slugs are descended from snails that gradually lost or reduced their shells over time. This shell-less condition has arisen many times in the evolutionary past, so the various taxonomic families of slugs are polyphyletic: they are not closely related, despite a superficial similarity in the overall body form. There are marine and terrestrial forms, but the common name "slug" is most frequently applied to air-breathing land species, while the marine forms are known as sea slugs. Land gastropods with a shell that is not quite vestigial, but is too small to retract into (like many in the family Urocyclidae), are known as semislugs.



Land slugs

Land slugs, like all other slow-moving gastropods, undergo torsion (a 180º 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, which is on one side of the animal, normally the right hand side.

The soft, slimy bodies of slugs are prone to desiccation, so land-living slugs are confined to moist environments and must retreat to damp hiding places when the weather is dry.

Morphology and behavior

Anatomy of a slug


Like other snails, slugs macerate food using their radula, a rough, tongue-like organ with many tiny tooth-like denticles.

Like other pulmonate land snails, most slugs have two pairs of 'feelers' or tentacles on their head. The upper pair are light sensors, while the lower pair provides the sense of smell. Both pairs are retractable and can be regrown if lost.

On top of the slug, behind the head, is the saddle-shaped mantle, 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. 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. Some slug species hibernate underground during the winter in temperate climates, but in other species, the adults die in the autumn.

Mucus

Slugs' bodies are mostly water, and without a full-sized shell, their soft tissues are prone to desiccation. They must generate protective mucus to survive. Many species are most active just after rain. In drier conditions they hide in damp places under tree bark, fallen logs, rocks, and man-made structures such as planters and so forth, to help retain body moisture.

Slugs produce two types of mucus: one which is thin and watery, and another which is thick and sticky. Both kinds of mucus 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 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 it has some secondary effects: other slugs coming across a slime trail can recognize others of the same species, which is useful in preparation to mating. 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, for example in a bird's beak.

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

The mucus of some species is mildly poisonous.

Reproduction

Slug eggs and baby
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 into a hole in the ground, or beneath the cover of an object such as a fallen log.

Apophallation is a commonly seen practice among many slugs. In apophallating species, the penis curls like a cork-screw 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, fungus, and decaying vegetable material. Other species eat parts of living plants.

Some slugs are predators, eating other slugs and snails, or earthworms.

Most carnivorous slugs on occasion also eat carrion, including dead of their own kind.

Predators

Frogs, toads, snakes, hedgehogs, Salamanders, eastern box turtles, humans 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) a portion of their tail to help to the slug escape from predator.

Human relevance

Most slugs are harmless to humans and to their interests, but a small number of species are great 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, poisoned baits are the norm in both agriculture and the garden. The preferred and most effective baits include the poison metaldehyde. In recent years iron phosphate baits have emerged and are sometimes preferred where pets may get into the bait. 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, crushed eggshells, coffee grounds, and copper. Salt can dehydrate and kill slugs by causing water to leave the slug's body owing to osmosis.

In a few rare cases, humans have contracted parasite-induced meningitis from eating raw slugs .

A banana slug, Ariolimax dolichophallus, named "Sammy" is the mascot of the University of California at Santa Cruzmarker.

In rural southern Italy, the garden slug Arion hortensis is used to treat gastritis or stomach ulcers by swallowing it whole and alive. A clear mucus produced by the slug can treat various skin conditions including dermatitis, warts, inflammations, calluses, acne and wounds..

Subinfraorders, superfamilies, and families



Sea slugs

The word "slug" or "sea slug" is also used for many marine species, almost all of which have gills. The largest group of marine shell-less gastropods or sea slugs are the nudibranchs. There are many other groups of sea slug such as the heterobranch sea butterflies, sea angels, and sea hares, as well as the only very distantly related, pelagic, caenogastropod sea slugs, which are within the superfamily Carinarioidea. There is even an air-breathing sea slug, Onchidella.

References

  1. Pekarinen E. (1994) "Autotomy in arionid and limacid slugs". Journal of Molluscan Studies 60(1): 19-23. abstract
  2. Slugs and Osmosis


Further reading

  • Burton D. W. (January 1982) "How to be sluggish". Tuatara 25(2): 48-63.


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