Slug

For other uses, see Slug (disambiguation).

Land slugs
Swann Slug, Arion rufus - red color form
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
Subclass:	Orthogastropoda
Superorder:	Heterobranchia
Order:	Pulmonata
Suborder:	Eupulmonata
Infraorder:	Stylommatophora

Slugs are gastropod molluscs without shells or with very small internal shells, in contrast to snails, which have a prominent coiled shell. The loss or reduction of the shell is a derived character, and the same basic body design has independently evolved several times, making slugs a polyphyletic group. Although they undergo torsion (180 degree twisting of internal organs) during development, their bodies are streamlined and worm-like, and so show little external evidence of it. Slugs include both marine and terrestrial species. The main group of marine or sea slugs are the nudibranchs. However, the ecological information in the article below applies mainly to land slugs.

The soft, slimy bodies of slugs are prone to desiccation, so land-living slugs are confined to moist environments.

Morphology

Slugs macerate food using their radula, a rough, tongue-like organ with many tiny tooth-like denticles.

Like snails, slugs have two pairs of 'feelers' or tentacles on their head. The upper pair--optical tentacles--are light sensors; 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. The mantle also has a hole, the pneumostome, for respiration. The slug moves by rhythmic muscular action of its foot.

Taxonomy

Sea slugs belong to the suborder Nudibranchia (order Opisthobranchia).

Leopard Slug, Somerville, MA

Land slugs mostly belong to the suborder Stylommatophora (order Pulmonata).

Among the various species of land slug are the grey field slug, Deroceras reticulatus; the garden slug Arion hortensis; the leopard slug, Limax maximus; and the banana slug, Ariolimax columbianus.

Ecology and behaviour

Most slugs eat leaves, fungus, and decaying vegetable material, but some are predators and most also eat carrion including dead of their own kind. Frogs, toads, snakes, hedgehogs, and some birds and beetles are natural slug predators. Slugs, when attacked, can contract their body, making themselves harder and more compact and thus more difficult for many animals to get a hold of. The unpleasant taste of the mucus is also a deterrent.

Anatomy of a slug

File:IMG 0883.JPG

A slug from North Bend, WA

Mucus

Slugs produce two types of mucus: one which is thin and watery, and another which is thick and sticky. Both are hygroscopic. The thin mucus is spread out from the centre of the foot to the edges. The thick mucus spreads out from front to back.

Mucus is very important to slugs as it helps them move around, and contains fibres which prevent the slug from sliding down vertical surfaces. Mucus also provides protection against predators and helps retain moisture. Some species use slime cords to lower themselves on to the ground, or suspend from them during copulation.

Reproduction

Slugs are hermaphroditic: having both female and male reproductive organs. Once a slug has located a mate they encircle each other and sperm is exchanged through their protruding genitalia. A few days later around 30 eggs are laid into a hole in the ground. Although some species hibernate underground during the winter in temperate climates, in other species, the adults die in the autumn.

A commonly seen practice among many slugs is apophallation, when one or both of the slugs chew off the other's penis. The penis of these species is curled like a cork-screw and often becomes entangled in their mate's genitalia in the process of exchanging sperm. Apophallation allows the slugs to separate themselves. Once the penis has been removed, the slug remains female for the rest of its life.

Various species of slug can also reproduce via tiny "darts" of sperm which they fling in the direction of their mate's genitalia.

A slug from the Western Ghats

Classification

• Subinfraorder Orthurethra
  • Superfamily Achatinelloidea Gulick, 1873
  • Superfamily Cochlicopoidea Pilsbry, 1900
  • Superfamily Partuloidea Pilsbry, 1900
  • Superfamily Pupilloidea Turton, 1831
• Subinfraorder Sigmurethra
  • Superfamily Acavoidea Pilsbry, 1895
  • Superfamily Achatinoidea Swainson, 1840
  • Superfamily Aillyoidea Baker, 1960
  • Superfamily Arionoidea J.E. Gray in Turnton, 1840
  • Superfamily Buliminoidea Clessin, 1879
  • Superfamily Camaenoidea Pilsbry, 1895
  • Superfamily Clausilioidea Mörch, 1864
  • Superfamily Dyakioidea Gude & Woodward, 1921
  • Superfamily Gastrodontoidea Tryon, 1866
  • Superfamily Helicoidea Rafinesque, 1815
  • Superfamily Helixarionoidea Bourguignat, 1877
  • Superfamily Limacoidea Rafinesque, 1815
  • Superfamily Oleacinoidea H. & A. Adams, 1855
  • Superfamily Orthalicoidea Albers-Martens, 1860
  • Superfamily Plectopylidoidea Moellendorf, 1900
  • Superfamily Polygyroidea Pilsbry, 1894
  • Superfamily Punctoidea Morse, 1864
  • Superfamily Rhytidoidea Pilsbry, 1893
    • Family Rhytididae
  • Superfamily Sagdidoidera Pilsbry, 1895
  • Superfamily Staffordioidea Thiele, 1931
  • Superfamily Streptaxoidea J.E. Gray, 1806
  • Superfamily Strophocheiloidea Thiele, 1926
  • Superfamily Trigonochlamydoidea Hese, 1882
  • Superfamily Zonitoidea Mörch, 1864

See also

• Apophallation
• Banana slug
• Black slug
• Great Grey Slug
• Shelled Slug
• Spanish slug
• Yellow Slug

External links

• Biology of Slugs
• Ohio State University slug fact sheet
• Leopard Slug Info
• Pancake Slug (Veronicella sloanei) Info