Online Field Guides

See also Forumites' slug and snail photos

Slugs and snails belong to the class Gastropoda in the phylum Mollusca. Gastropods characteristically have a single shell, which is usually coiled. The visceral mass (containing the internal organs) has also undergone torsion through 180^o so the openings of the digestive, reproductive and excretory tracts are at the front of the animal.

Apart from land snails and slugs, Gastropoda also includes freshwater snails, sea snails (including limpets) and sea slugs.

The snail shell is constructed from calcium carbonate laid down in a protein matrix. In many land snails, the shell is covered in a protein coat called the periostracum. In chloritid snails (family Camaenidae), the periostracum is extended into fine hairs.

The shell has been reduced or lost in some species and these animals are called slugs. This shell loss has occurred in several lineages.

In Helicarionidae, Vitrinidae and some other snail families, the shell is reduced, so the animal cannot withdraw into it completely. These animals are often referred to as semi-slugs. The shells of semi-slugs are usually thin and fragile, with little calcium carbonate.

The mantle is a flap of tissue that encloses the mantle cavity, into which the main organ systems open. It also secretes the shell and periostracum. In land snails and slugs, the edge of the mantle is almost completely fused to the rest of the body. An oval opening, the pneumostome, allows gas exchange in the mantle cavity.

The only species of native slug commonly encountered in gardens is the Red Triangle Slug, Triboniophorus graeffei. It is found along the east coast from Wollongong to north of Brisbane. Isolated occurrences occur in rainforest blocks in North Queensland and on Mount Kaputar, New South Wales. Although the slugs may be difficult to find, they leave characteristic zigzag feeding trails on tree trunks.

Triboniophorus is related to native slugs in New Caledonia, New Zealand and sub-Antarctic islands. The triangle on the back is a reduced mantle.

(Picture of Triboniophorus graeffei from webpage of forum regular davidavid. See more of davidavid’s photographs)

The Leopard Slug, Limax maximus, is one of the largest and most conspicuous slugs in gardens and parks.

It is difficult to distinguish between the small arionid slugs (Arion hortensis, Arion intermedius and others). They are usually 2-3 cm long and are often marked with longitudinal stripes in shades of grey. The colour of the mucus varies between species (colourless, white or yellow).

Much easier to identify is the Black Slug, Arion ater, which has been recorded from a few localities in SE Australia. The Black Slug is very large. When irritated it retracts into a hemispherical shape. Mucus from this slug is extraordinarily sticky and difficult to remove from skin.

Field slugs, Deroceras spp., are small, pink or grey slugs that occur in a range of habitats, including cultivated land where they can be a pest of crops.

In warmer areas, veronicellid slugs are common in soil, under logs and under flower pots. The most frequently encountered veronicellid is Laevicaulis alte. Another species, Vaginulus plebeius, has also been introduced into Australia. These species are characterised by the flattened form and an expanded mantle, which extends over the upper surface of the body.

Other introduced slugs:

• Lehmannia nyctelia
• Lehmannia valentiana
• Limacus flavus

(Some of the links in this section are to plates from Gary Barker’s book on snails and slugs introduced to New Zealand. This is an excellent book that covers many of the introduced species found in southern Australia)

Coming soon