Gastropods fact sheet

What are gastropods?

Gastropods are soft-bodied animals without body segmentation. Some species have an external shell made of calcareous material for protection.

Gastropods belong to the mollusc group. Molluscs include snails, slugs, octopus, shellfish and other species.

Molluscs are the second largest group of invertebrates after arthropods with around 85,000 species.

Garden snail with a coiled shell, soft body and four eye stalks extended. Gastropods are characterised by having three main body areas – a head, mantle and tail. Snails have an additional area called a visceral mass contained in the shell.

Fast facts – Gastropods

What are they – Gastropods belong to the mollusc group. Molluscs include snails, slugs, octopus, shellfish and other species.

Body features – Gastropods are soft-bodied animals without body segmentation. Some have an external shell made of calcareous material for protection. Gastropods have three main body areas – a head, mantle and tail. Snails have an additional area called the visceral mass, which is contained inside the shell.

Examples – Molluscs are the second largest group of invertebrates after arthropods, with around 85,000 species.

Where they live – Found all around the world in oceans, freshwater, brackish water and soil.

Oval cockle shells lying face down on a rock, showing the hard outer shell that protects the mollusc. Cockles are a type of mollusc. They are protected by a hard outer shell.

Case study – red triangle slug

Appearance

The red triangle slug is the largest native land slug in Australia. It has a breathing hole in its distinctive red triangle on its back and eyes on stalks.

Red triangle slugs move by gliding along a surface of mucus that is produced from glands on the foot. It can grow up to 14 centimetres long.

Pale pink red triangle slug with a red border along its base, a red triangle on its back containing a breathing hole, and two eye stalks. Most native Australian slugs only have two eye stalks compared to four on introduced species.

Habitat

Red triangle slugs are found in damp environments in various habitats including city gardens, forests and woodlands.

Bushland with gum trees, shrubs, grassy understorey and leaf litter where red triangle slugs hide and feed. Red triangle slugs hide in the leaf litter or under bark. They will emerge after rain looking for food.

Diet

The red triangle slug feeds on microscopic algae growing on tree trunks and rocks. It will even eat algae growing on fences, pavers and household walls.

Red triangle slug feeding trails on a surface, forming lines of small overlapping ring shapes like chain links. Red triangle slugs create distinctive 'chain-link' patterns as they feed on algae on tree trunks.

Role in the ecosystem

Red triangle slugs are an important food source for birds, bats and lizards.

Two blue-tongued lizards on the ground among tall grasses and mulch, predators of red triangle slugs. Blue tongue lizards are one of the animals that prey on red triangle slugs.

Red triangle slug anatomy

Slugs are hermaphrodites. This means they are both male and female.

Head and mantle

Red triangle slugs have a mouth with thousands of tiny teeth called radula. They use their radula to scrape algae off smooth surfaces.

They have two optical tentacles which are retractable with light sensitive eye spots. They are also used for smelling and feeling the surrounding environment.

The respiratory pore, or pneumostome, is used for breathing. It is located in the red triangle mark on their back. This is part of the slug's mantle.

The mantle is made of a thicker flesh than the head. When frightened or not active the slug retracts its head toward the mantle for protection.

The red triangle on the slug is designed to warn off and deter predators.

Tail and foot

The tail of the slug starts at the end of its mantle and continues along its body.

The underside of a slug is called the foot. Slugs move by muscular contraction of the foot.

Red triangle slug showing the breathing hole inside its red triangle marking, which warns and deters predators. The breathing hole of the red triangle slug is visible in its red triangle. The red triangle is designed to deter predators.

More invertebrate fact sheets

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