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Snails and slugs

by  Paddy Ryan

New Zealand has an estimated 1,400 native species of slugs and snails – many of them endangered. Native slugs are patterned to look like a leaf. The largest snails, Powelliphanta species, can grow up to 10 centimetres across, and have beautiful coloured shells.


New Zealand land molluscs

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.

Land molluscs

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.

Walking stomachs

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.

Native species

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.

Introduced species

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.

Reproduction

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.


Flax snails, giant snails and veined slugs

Flax snails (pūpūharakeke)

Flax snails belong to the genus Placostylus, and were known to Māori as pūpūharakeke. Three native species are recognised. They live in coastal broadleaf forests and surrounding scrubland in northern New Zealand.

The genus is distributed in the South Pacific, with different species in New Caledonia, the Solomon Islands, Fiji and Lord Howe Island. It is not certain whether these species trace their ancestry to Gondwana, or arrived in New Zealand more recently.

The tall, spired shell of the largest flax snails can reach up to 8 centimetres in length. The young live in trees and feed on micro-organisms that grow on the leaves. When they mature, they move down to the ground. Despite their name they do not feed on flax, but eat the fallen leaves of trees such as karaka, kohekohe and rangiora.

A different Placostylus species lives in the forests of New Caledonia, where locals regard it as a delicacy. In New Zealand, there is some archaeological evidence that Placostylus were once eaten, but today neither Māori nor Pākehā have any enthusiasm for eating snails .

Giant snails

The largest and most visually striking New Zealand snails are members of the family Rhytidae.

Kauri snails

The large kauri snail (Paryphanta busbyi, known to Māori as pūpūrangi), is found in northern New Zealand. Although it occurs in kauri forest regions, it does not live close to kauri trees, as the nearby ground is probably too dry for the worms on which the snail feeds. It also eats insects and snails, and may live more than 20 years. The shell is a dark, greenish-brown flattened spiral, up to 10 centimetres in diameter.

Super size snail

Powelliphanta snails are giants of the New Zealand snail world. The largest species, Powelliphanta superba, can grow to about 10 centimetres across (about the size of a hamburger), and weigh up to 90 grams – as heavy as a mobile phone.

Powelliphanta snails

The closely related genus Powelliphanta is widespread in the wetter parts of central New Zealand, especially north-west Nelson and the West Coast of the South Island. At least 21 species have been recognised, in habitats ranging from temperate rainforest near sea level to mountains near the bushline. Some have beautifully coloured and patterned shells.

Powelliphanta are carnivores, eating worms (which they slurp like spaghetti) and slugs. They are mainly nocturnal, living in leaf mould or under logs, and only appearing at night to forage.

Powelliphanta snails lay up to 10 eggs a year. Each egg is 12–14 millimetres long, with a hard pink shell like a tiny bird’s egg.

Snail namesakes

Mollusc scientist Baden Powell made detailed studies of New Zealand’s largest snails, then included in the genus Paryphanta, in the 1930s. When some of the snails were moved into a separate genus, it was named Powelliphanta in his honour.

Veined slugs

There are about 30 species of native New Zealand slugs. All have a characteristic leaf-vein pattern on their upper side, for camouflage. Māori knew them as putoko ropiropi. Their biology is poorly known, but they are thought to live mainly on algae and fungi on the surface of plants. Slugs may huddle together, apparently to create a humid microclimate. They may also cluster around small pools of water or wet humus on larger leaves.

Native leaf-veined slugs have only one pair of tentacles – unlike introduced species, which have two. Also, the native species do not damage garden plants.


Conservation

Endangered snails

All the large native land snails and many of the smaller ones are endangered, and numbers fell dramatically in the latter part of the 20th century. They have been threatened by changed habitats, introduced predators (both mammals and molluscs), and possibly by introduced diseases. Human collectors are also a threat, and in 1982 it was made illegal to take Powelliphanta snails and their shells.

There is strong evidence that predation by rats and pigs has taken a high toll on kauri snails, Powelliphanta and flax snails. Hedgehogs and mice may also be implicated. The Department of Conservation has developed a recovery plan for Powelliphanta snails, primarily controlling predators.

High-tech backpacks

In 2006, around 1,500 rare Powelliphanta snails were moved to safety from the Stockton mine area on the West Coast. Sixty had radio transponders and antennae glued to their shells. This allows Department of Conservation staff to track the snails – but it may interfere with mating. ‘The snails climb on top of each other when they mate, and it might disturb them that there’s this bulge where the diode is,’ said scientist Ingrid Gruner. 1

Powelliphanta near the Stockton mine

The Powelliphanta ‘Augustus’ snail (named for its habitat, on Mt Augustus) is only known in a 5-hectare area near the Stockton mine on the South Island’s West Coast. Coal producer Solid Energy plans to strip-mine the area. There was controversy in 2006 when the Minister of Conservation gave the company permission to move the entire snail population to a new location 800 metres away. The expense was considerable, and there has been public debate over whether the snails should have been left undisturbed.

Threat from the garden snail

Cantareus aspersus, the common garden snail, poses a threat to native land molluscs as well as to some rare plants. Unlike native snails and slugs, it eats herbaceous vegetation as well as seedlings. It is highly gregarious, and groups can monopolise plant litter and food resources, as well as leaving mucus and faeces. They can also spread disease and parasites.

Although the garden snail is seen mainly as a pest to the gardener, there is concern that it may spread into sensitive habitats and damage them.

Footnotes
  1. Quoted in Sandra Cox, ‘Snails tuned in to DOC’s radio hits,’ The Press, 5 December 2006. › Back

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How to cite this page: Paddy Ryan, 'Snails and slugs', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/snails-and-slugs/print (accessed 24 September 2019)

Story by Paddy Ryan, published 24 Sep 2007