What is a slug?

PHOTO

UPPER:
Pupa sulcata, an opisthobranch sea slug, New Caledonia, October 1993.
LOWER RIGHT: Limax maximus, a pulmonate land slug, Sydney garden.
LOWER LEFT: Scutus antipodes, the "prosobranch" elephant slug, Sydney.
PHOTOS: Bill Rudman.

RELATED TOPIC

Detorsion - how snails became sea slugs

All these animals are called slugs. The word "slug" describes a body shape rather than a group of species with a common ancestor. Essentially, "slugs" are "snails" that have either lost their shells or are in the process of losing their shells. This has happened many times in snail evolution. The most obvious "slug groups" are the the sea slugs and the land slugs.

The sea slugs (ORDER Opisthobranchia) contain not only the spectacular nudibranchs, which have all lost their shells, but also primitive groups such as the Cephalaspidea or Bubble Shells which have snail-like members with heavy external shells (Pupa sulcata) as well as members with brightly coloured bodies and no shell (Sagaminopteron ornatum ).

The land snails (ORDER Pulmonata) have on a number of separate occasions evolved "slugs". One common northern hemisphere family, the Limacidae, have cursed most of the temperate world with pests such as Limax maximus. The name "pulmonata" refers to their development of the mantle cavity into a lung for airbreathing. The large hole visible in the accompanying photo below, is the pore (pneumostome) through which air moves in and out of the lung.

The marine snails (previously ORDER Prosobranchia)are no longer considered to be a single group with a common phylogenetic ancestry. Amongst these animals, slugs have evolved many times. The Australian Elephant Slug, Scutus, in fact retains a shield like shell (white streak in photo) which is hidden beneath flaps of skin.

LOWER RIGHT: Limax maximus, a pulmonate land slug, Sydney garden.
LOWER LEFT: Scutus antipodes, the "prosobranch" elephant slug, Sydney.

Authorship details
Rudman, W.B., 1998 (October 26) What is a slug?. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/nonslug

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