Powelliphanta snail

Hidden away in New Zealand’s forests and grasslands is a bewildering array of native land snails, which range from the microscopic to the massive.

Among these are at least 20 species and 59 sub-species of Powelliphanta – which represent some of the most distinctive invertebrates in New Zealand.

Powelliphanta snails are not your common garden snail. In fact, they are totally unlike garden snails, which are a European import and an unwanted garden pest. Powelliphanta are giants of the snail world. They are also beautiful. Their oversize shells come in an array of colours and patterns, ranging from hues of red and brown to yellow and black.

They are as representative of New Zealand’s unique evolutionary history as the kakapo, moa or kiwi. Unfortunately, they are also one of the most threatened of New Zealand’s invertebrates. A total of 61species or subspecies are ranked as being of national conservation concern.

Quick facts

• Powelliphantasnails are carnivores. They particularly like earthworms and suck them up through their mouth just like we eat spaghetti. They are also known to eat slugs.
• The largest species is Powelliphanta superba prouseorum, found in Kahurangi National Park and measuring about 9 cm across. These are the sumo wrestlers of the snail world, weighing in at 90 g, or the equivalent of a tui.
• The genus was named after Dr A.W.B. Powell, a former scientist at Auckland Museum who studied the snails during the 1930s and 1940s.
• Powelliphantasnails used to be known as Paryphanta snails, until the 1970s. Now, Paryphanta refers only to kauri snails, which live north of Auckland.
• Powelliphantasnails are hermaphrodites, meaning they possess both male and female reproductive organs and therefore can mate with any other adult 
• Powelliphantasnails lay about 5-10 large eggs a year. Each egg is up to 12 mm long, pearly pink and hard-shelled - just like a small bird’s egg.
• It is estimated that Powelliphantasnails can live up to 25 years. In snail terms, that is an incredibly long-life span.

Rare white snail discovery

The fascinating find of a rare white-bodied giant Powelliphanta snail was made in the Flora Stream area of Kahurangi National Park in November 2011. The albino snail was a Powelliphanta hochstetteri hochstetteri that had its characteristic gold and brown-spiralled shell but had a body that was a glowing white rather than the usual deep black colour.

Albinism is known to occur in many animal species around the world. The absence in pigment, which could be partial or complete, was due to a genetically inherited defect in the enzyme which produced melanin.

DOC Powelliphanta expert Kath Walker said it was exceptional to come across an albino snail. From the photos it looked to be an adult snail at least 10 years old. It is amazing it survived so long as its white body would make it clearly stand out to be picked off by weka or other predators.

Where to find Powelliphanta

Powelliphanta are mostly "spot endemics". This means each species and subspecies are confined to its own small area, with lots of country in between without any Powelliphanta at all.

This is probably because of the patchiness of suitable habitat. and they can't move very fast or far. It also reflects past barriers to snail movement such as glaciers, rivers, lakes, mountains and volcanic ash. Whatever the reason, it makes for a rich and interesting pattern of occupation, which is helping shed light on the past biogeography of New Zealand.

The stronghold for Powelliphanta snails is in northwest Nelson and north Westland, with more species here than anywhere else.

They are also found in the Marlborough Sounds and Richmond Ranges, and on coastal west coast mountain ranges as far south as Fiordland and Southland. In the North Island they are found in tiny parts of the Horowhenua plains, in some of the mountains of central North Island and on Mt Taranaki

Because Powelliphanta snails are prone to dehydration, they cannot survive in dry conditions. For this reason, they are more common in the wetter western parts of New Zealand and in moist high-altitude forest than in drier forests at lower altitudes in the east.

Different Powelliphanta species can be found from sea level, where they live in rich temperate rainforest, to above the bush line. Most of the alpine species must contend with prolonged snowfalls and bitterly cold winters.

Empty snail shells on the forest floor are the most usual sign of their presence.

You are unlikely to spot a live snail, except at night or occasionally on rainy days with high humidity. Powelliphanta are nocturnal and come out every few weeks to forage for food and to mate. They live buried in leaf mould or under logs. The snails are most likely to be active on warm, moist nights after a long dry spell.

Threats

Despite being legally protected, New Zealand’s Powelliphanta land snails are under serious threat. 34 taxa are in danger of extinction, and another 13 are endangered, particularly from predation, habitat loss and degradation, and from increasingly dry conditions brought by climate change.

Habitat loss

As a result of major habitat loss in the past, many Powelliphanta populations are now restricted to tiny pockets of native bush, where they have a precarious toehold on existence. These isolated populations can be threatened further if the bush is removed entirely or degraded by ungulate browsing and trampling, or drainage of neighbouring land, as the snails need moist conditions to survive.

In recent years, open-cast coal mining has been reducing the range of two species – Powelliphanta augusta and Powelliphanta patrickensis. Both are found only on the Buller Plateau. This is an unusual habitat for large land snails as it is so acidic and poorly drained that it requires special adaptations in both snails and their earthworm prey—both normally lovers of alkaline and well drained soils— to thrive there.

The more famous species is Powelliphanta augusta. It was only discovered in a few hectares on the northern edge of the Stockton Plateau in 2005 when most of its habitat had already been mined. Before the rest was mined about 4,000 snails were collected and moved to nearby sites. Another ~2,000 snails were used to start a captive colony to increase the now limited genetic diversity of the species, and as an insurance against failure of the transferred populations. As P. augusta had not previously lived in the sites they were transferred to, we won't know for many years whether these sites can support snails long-term.

The less well-known species is Powelliphanta patrickensis which lives only on the southern part of the Stockton Plateau and on adjoining Denniston Plateau. While it has a bigger range than P. augusta, it is affected by the same problem: specialisation to a habitat niche on coal measures which are also sought after for coal mining. In the past this was not a great problem as coal extraction was primarily by underground mining. But since the 1980s all mining has been open cast which removes all the soil and vegetation supporting the snails to get at the coal under it.

Predation

The main predators of Powelliphanta are introduced rat, possum and pig and the native weka also eat them. While all these predators can kill even the largest Powelliphanta, the introduced song thrush and hedgehog which also prey on snails, can only kill small and medium sized Powelliphanta.

Powelliphanta superba prouseorum is a gorgeous giant of a snail, with an old-gold coloured shell and huge size (nearly 10 cm across) but it’s now very rare to find such a large old individual as possums, and to a lesser extent at lower altitudes pigs and rats, have nearly wiped them out.

These massive snails are confined to the forests inland of Kahurangi Point in Northwest Nelson, and the introduced possum didn't reach this remote site till the late 1950's.  As the invading possum population peaked in the following 2 decades and ate out all the most palatable vegetation, possums began eating the snails instead, and snail numbers plummeted. While possum numbers have now been greatly reduced, pig, rat and weka numbers have increased there, and drier conditions brought by climate change are putting recovery of the snail population in doubt. This, the largest of all Powelliphanta, is a treasure the world should not lose.

1080 poison helps native snails

Possums and rats cause massive problems for New Zealand's native species, and the periodic use of the biodegradable poison 1080 to kill these pests has reversed the decline in some snail populations.

It is a particularly valuable tool for protecting remote back country snail species which have been difficult to help using ground pest control methods.

Climate change

Since DOC began close monitoring of Powelliphanta snails in the 1990s, summer soil moisture deficits have increased significantly in the areas being monitored, whilst snail numbers have declined. Drier soils make it hard for such big land snails and their eggs to survive; they are reliant of moist conditions year-round.

The warming climate has also allowed rats to expand into mountainous snail habitat which previously was free of rats due to the cold.

Our work

DOC undertakes a number of programmes to protect Powelliphanta populations.

Fencing out predators and habitat modifiers

On D'Urville Island in the Marlborough Sounds, numbers of live Powelliphanta hochstetteri obscura snails almost immediately doubled when pigs were fenced out of two small areas of forest in the late1990s.

Somewhat surprisingly, this was not just because snails inside the fence were no longer being eaten by pigs, but because:

• the snail eggs had a much better chance of hatching
• the young hatchling snails had a much better chance of surviving, and
• they had more earthworms to eat once the forest floor wasn't being regularly dug up by rooting pigs in search of their main food which is earthworms (pigs compete with snails for worms).

Since that trial on D’Urville Island was so successful, DOC has begun to fence out pigs and weka from small areas of snail habitat elsewhere, and to also exclude deer, goats and hares which degrade the snail’s habitat when possible.

Powelliphanta gilliesi brunnea in Golden Bay

One species, Powelliphanta gilliesi brunnea, is now doing well in Golden Bay, thanks to a combination of habitat protection and predator control measures. Restricted to half a hectare of farmland, DOC erected an outer fence to exclude farm stock, an inner fence to exclude rodents and hedgehogs, and planted native trees to increase the habitat available.

By 2003, there were about 1,000 individual snails compared to about 350-500 in 2001. Numbers have remained high ever since.

However, until the trees in the replanted areas have grown enough to allow the formation of a proper densely littered forest floor and so become suitable habitat for Powelliphanta gilliesi brunnea, the size of the snail population and the extent of habitat is just too small to be sure this subspecies is safe.

You can help

The Powelliphanta snails found today are the culmination of million years of evolution on New Zealand’s isolated landmass, developing a set of peculiar characteristics that is totally unique in the world. They represent a small but significant part of our natural heritage which deserves to be saved.

Suport predator control

You can help by supporting efforts to reduce the numbers of possums, rats, pigs, deer and goats in native forests your area.

Supporting efforts to reduce New Zealand’s carbon emissions will help not just Powelliphanta but all those other land snail species also dependant on  cool moist conditions and refuges too cold for rats.

Don't take or move shells

It's against the law to take or hold Powelliphanta shells without a permit.

Leave Powelliphanta shells where they are.

It's difficult to determine if a shell is really empty. Many live snails have been accidentally taken from their home and later left in a place where they cannot survive.

If Powelliphanta are released outside their natural range, all the small resident animals at the new site suddenly have to contend with a top predator they are not equipped to handle.

Human-aided translocations destroy the intriguing natural patterns of distribution which typifies New Zealand Powelliphanta.