Gastropods

Overview

Gastropods form the largest class of molluscs and include many well-known groups such as cowries, cone snails, tritons, whelks, nudibranchs and land snails. In general gastropods are characterised by a head (bearing a pair of eyes and a pair of tentacles), a large (creeping) foot and a visceral mass usually housed in a spiral, unchambered shell. To date approximately 1,000 species of marine gastropods have been recorded from Moreton Bay in south-east Queensland.

The class Gastropoda also includes many forms in which the adult shell is cap-shaped (true limpets, false limpets) or reduced, internalised or missing (semi-slugs, sea hares and nudibranchs). In most gastropods with a well-developed shell, the aperture of the shell is usually sealed by a trapdoor-like structure (operculum) when the animal retracts. In cowries, the shell aperture is reduced to a slit guarded by several tooth-shaped projections, thus eliminating the need for an operculum. 

Major groups include:

Marine Snails

Marine snails form the dominant component of molluscs throughout the world’s oceans. Although families such as the cowries, cone snails and murex snails may be the best known due to their attractive shells and often bright colour patterns, large numbers of ecologically important species are either drab, or small to microscopic in size. Shell shape and structure in marine snails can vary considerably. Cowries and olive snails possess glossy, naturally polished exteriors; murex snails are often adorned with elaborate spines; and limpet groups generally have more-flattened, cap-like shells.

In Queensland the vast majority of marine snails are predatory, feeding on worms, barnacles, other molluscs or even sponges, and several species (e.g., dog whelks) are scavengers. Sand creepers and mud whelks feed on the detritus lining the surface of sandy or muddy sediments while groups such as limpets and many top snails feed predominantly on algae. 

Land Snails, Slugs and Freshwater Snails

The terrestrial environment is home to a vast array of snails and slugs. Many native land snails and slugs live in the moist layers of litter on the forest floor and other moist habitats such as rotting logs, under rocks or beneath debris. In eastern Australia, they are particularly diverse in rainforest areas. By contrast, fewer species occur in the dry eucalypt forests, but these dry-adapted species can have greater ranges than the rainforest species.

Whereas marine snails typically use gills for breathing, land snails and slugs breathe air through a modified mantle chamber which acts like a lung. Some land snails have an operculum that seals the aperture which helps prevent water loss. In others, the shell aperture may be sealed by an epiphragm (hardened mucus) during extreme dry spells. The close association of land snails with rainforest means that they are sensitive indicators of biological change. Many species are threatened by habitat loss due to human activity and forest degradation and/or fragmentation. Others can constitute severe environmental pests such as the Giant African Snail which has established itself throughout the south Pacific region. The freshwater environment also hosts a variety of snails. Many are native species but some such as the Spike-top Apple Snail are invasive, ecological threats to our rivers and streams.

Nudibranchs

Nudibranchs (naked-gilled sea-slugs) include some of the most colourful and flamboyant of sea creatures. There are around 3,000 valid species in this Molluscan group. Many have bright and elaborate colour patterns as spectacular as those seen in some butterflies. Like butterflies these colour patterns can serve to warn-off predators indicating they are toxic and taste foul - and if eaten it could be their last meal! These creatures may produce their own toxins or obtain them from their diet. Some nudibranchs that feed on fire corals keep their stinging cells to use for their own defence, whereas other nudibranchs that are not toxic use the colour patterns of toxic species to pretend to be dangerous and warn-off predators (mimicry).

All nudibranchs are carnivorous exhibiting quite a diverse range of diets -some species feeding on sponges, others consuming hydroids, and still others eating lace corals (bryozoans). There are species that feed on other sea-slugs or their eggs, and some are occasionally cannibalistic – preying on individuals of their own species.

Common questions

Images