New Zealand has a rich and distinctive land mollusc fauna. There are an estimated 1,400 native species of slugs and snails, few of them found elsewhere. By comparison, the United Kingdom, with a similar land area, has only 112 native species. This amazing diversity of species is due largely to New Zealand’s long isolation, its varied landscape, and possibly a lack of predators.
Although there are about 80 families of land molluscs worldwide, only 11 occur naturally in New Zealand. These are not the families that are dominant overseas. They include families that originated in the Gondwana supercontinent and have been isolated in New Zealand for the last 85 million years.
Slugs and snails are gastropod molluscs, which belong to the subclass Pulmonata. Gastropods are the only molluscs that have successfully adapted to life on land, by developing a primitive lung. Māori called the whole group ngata, and also named a few distinct species.
Slugs and snails are land molluscs that belong to the class Gastropoda – from the Greek ‘gaster’ (stomach) and ‘podus’ (foot), referring to the way they move around.
Differences and similarities
Snails have an external shell. Slugs do not, although some, called semi-slugs, have a small partial shell. Both slugs and snails have a flat muscular foot on which they move around. They typically have one or two pairs of tentacles on their head – the larger pair usually has eyes at the tip.
The internal organs of slugs and snails are within the soft hump of the body. For snails this is inside a coiled shell, so that only the head and foot can be seen. Slugs and snails lose moisture freely through their skin, and need a humid environment. They try to stay out of the sun, and most are nocturnal.
They feed using a tongue-like organ called the radula, which is covered with rows of sharp teeth used to rasp their food. Different species can be distinguished by these teeth and the way they are arranged.
Most native slugs and snails are small and easily overlooked. It is estimated that less than half of the species have been formally described.
Most species live in moist litter on the forest floor, and are detritivores – they feed on the micro-organisms that break down plants. Tree-living species primarily graze on organisms that grow on leaf surfaces, rather than eating the leaves.
There are also a few giant native snails, especially Powelliphanta species. The largest are up to 10 centimetres in diameter.
New Zealand also has about 30 introduced species of snails and slugs, most associated with human habitats and crops. Unlike native snails, many of them eat plants. In urban areas, you are more likely to find two introduced species – the common garden snail (Cantareus aspersus) and the grey field slug (Deroceras reticulatum) – than the smaller and more reticent native species. High numbers of garden snails can destroy crops in vegetable gardens.
Most land molluscs are hermaphrodites – they contain both male and female sex organs. As far as we know, when they meet, individuals exchange sperm and then use it to fertilise their eggs. Some snails lay eggs with a hard shell. Most slugs and many snails lay soft jelly-like eggs in a moist environment to avoid them drying out. There is no parental care once an egg hatches.