Slow, slimy and surprising!
New Zealand has around 1000 species of native land snails, yet most people have probably never seen a single one. Warren Judd reveals something of the glory of these creatures and their shell-less relatives the slugs.
The slaughter WAS dreadful, and went on for years. Each morning crushed corpses lay strewn over the battle zone, and yet within days—often by the very next night—plentiful reinforcements would pour out from their concealed lairs only to be trampled beneath father’s solid slippers.
My mother’s periodic complaints about the unsightly and slippery desecration of paths and lawns did little to stem the carnage. But, like many of history’s more prolonged battle campaigns, there was no clearcut winner. Indefatigable persistence won my father a measure of control, but never victory. As every gardener knows, it’s hard to eliminate slugs and snails. Those nocturnal armies seem to spring from the ground, leaving a wake of perforated plants, silver trails and green faeces.
Although Helix aspersa, the common garden snail, and its many pestiferous slug cousins are European immigrants, New Zealand is well endowed with its own native snail fauna, plus a sprinkling of slugs. These natives spurn agricultural delicacies, far preferring the dank underworld of the forest floor, and it is likely that most New Zealanders have never seen a single one, although we may have as many as 1000 species.
Most land slugs and snails constitute a distinct group of air-breathing gastropod (one-shelled) molluscs named pulmonates. The best way to understand snails is to think of them as an amalgamation of four components: a shell (for protection), a head (for sensing the world outside), a foot (for movement) and a body mass (to keep the rest functioning).
One of the features that separate molluscs from all other animals is the mantle, a membranous extension of the body which is responsible for making the shell. In gastropods there is a space between this mantle and the main body of the animal. The space is large when the animal is extended and crawling, and small when the animal has retracted into the shell.
Pulmonates have enclosed this space — termed the mantle cavity to form an air-breathing lung. A small hole, the pneumostome, allows air in and out.
Each individual snail usually has both male and female reproductive organs, so they are true hermaphrodites. The reproductive system is elaborate, and its details are used to separate one species from another.Snails also possess sophisticated digestive, circulatory, nervous, excretory and locomotory systems. Surprisingly for such complex animals, they seem able to regenerate all body parts except the central nervous system.
While most people may not find snails a particularly riveting subject,there is a smattering of enthusiasts in whom snails inspire intense devotion. Guru of this select company is Dr Frank Climo,Curator of Molluscs at Wellington’s National Museum. Frank readily concedes that for him snails are more than a mere passion.”Snails are the key to my world view. In their spiral shapes I see a reflection of the world’s symmetry, the movement of tectonic plates, and much more.”
Frank’s search for keys to understanding the relationships and distribution of our snails has led him to embrace the panbiogeographical ideas of Frenchman Leon Croizat.
Panbiogeography emphasises the prominence of earth movements such as continental drift and plate tectonics in accounting for present-day distributions of organisms. When land masses collide, the organisms get mixed together. As one land mass slides under another, life forms pile up at the boundary, much as goods accumulate at the end of a conveyor belt. By looking at present distributions of organisms one can reconstruct prehistoric geology.
Although this sounds fairly reasonable, in some of Frank’s recent writing panbiogeography seems to burst out from the prosaic manacles of snail taxonomy to recast the world. In the New Zealand Journal of Zoology Frank writes: “Panbiogeography will remove much fear …. from the international community. This reduction in fear levels (a measure of the reducing power of the collective unconscious) will improve international relations, make rabid ‘nationalism’ irrelevant and thus make humanity more healthy.”
Despite Frank’s somewhat unorthodox ideas on panbiogeography and snails as the key to the cosmos, he does know a lot about these creatures. His special interest is the myriad species of small snails found throughout New Zealand. The largest of them would be the size of the extinct one cent piece, but most are much smaller, and plenty are no larger than a pinhead. Shy and retiring, they are the sort of thing that you rarely find unless you are looking for them, and not too many people grub in the dark heaps of decaying leaves and wood favoured by the snails. Even then, it takes a trained eye to spot them.
Often they are more abundant on the fringes of bush than in the impenetrable depths. Occasionally, they crop up in unexpected places. Grafton Gully in central Auckland sported an attractive stand of bush last century, but now there remains only a smattering of scrubby natives leavening a mass of indifferent exotics. The small area is squeezed between encroaching buildings on one side and concrete capillaries of the motorway system on the other. Wandering Jew climbs rampant over everything. Yet, wonder of wonders, there are a surprising number of small native snails under this lush carpet. Perhaps 10 or 15 native species are in residence, and at least one is flourishing.
A far greater range of species can be found in Auckland’s Waitakere Ranges. Jim Goulstone, a knowledgeable snail enthusiast and a professional tomato grower, has compiled a checklist of 120 species found in the Waitakeres. An extra 30 species occur in other bush areas in the greater Auckland area. Many have a surprisingly narrow distribution. “You would expect there to be north-south differences in snail distribution in New Zealand,” explains Jim, “and there are some, but there are also marked east-west differences, even when the country is as narrow as it is around Auckland. Quite a few extra species can be uncovered in the Hunuas that aren’t present in the Waitakeres.”
I asked him how he went about collecting small snails.
“The most important thing I take is a good heavy duty pair of leggings.
In the bush I seem to spend a lot of time grovelling around on hands and knees, and it’s always wet. Usually I head for a spot that isn’t too damp, because dead shells don’t last long in the wet. Somewhere a bit drier, ideally beside a vertical wall, a place the snails might fall off, can be good.
“One of the very best spots is a really big old rimu tree. Snails live under the flaky bark on the lower trunk, and this is constantly falling off and rotting at the foot of the tree. Once you’ve got under the dry surface leaves into the fine tilth, you’re where the snails are. Fine, black compost is what you want most of all. Another good spot is under the bark of a fallen tree. After the tree has rotted to the right stage the bark lifts off in large plates. Snails like the black humus between bark and trunk.
“Some sorts of fungus do a great job on fallen wood. They destroy everything except the cell walls, and you can push your foot right through the log. In there you always find Charopa. Rotting nikau fronds are not a bad place to find snails either. The bases are tough and smooth, so they provide a good surface to see the regularity of the snail shell on. There are quite a lot of things that resemble shells at first glance—seeds, rolled up arthropods and especially coiled millipedes. Some trees such as kauri and tanekaha never have many snails associated with them. Roots overgrow their tough leaves before they ever compost.”
So, having found a likely spot and done a bit of grovelling in the humus, what next? “Finding some snails indicates more in hiding, so I collect a few litres of leaf mould to sort through at home. After drying it out in the boiler house used for heating the glasshouses, I put it through two sieves. The coarse one was put together from plastic mesh used to stop leaves blocking downpipes, and the second one is a fine garden sieve. Other people use different methods. David Roscoe [a snail enthusiast from Nelson] somehow manages to float the small snails off in a dish of water. It’s amazing watching him pluck those tiny little shells off the surface of the water with his fingers.”
Jim prefers to use an artist’s brush that has had most of its bristles removed. He waves a particularly emaciated brush at me and explains that the snails get held between the hairs, making it easy to transfer them from the sieved leaf fractions to their final resting place: customised matchboxes stored in shallow tray filing cabinets. Inspection of the matchboxes reveals that each (of the thousands in Jim’s collection) is fitted with a polystyrene insert in which a hole has been cut to contain the snails. A small sheet of thin glass covers each box.
The shells look perpetually secure, but apparently not so. Most small land snail shells are precariously fragile—mainly protein with little calcium carbonate. Leaf litter tends to be acidic, and thin calcium shells would be in danger of being dissolved by acid. A protein skin on the shell—termed a periostracumprotects the shell from this hazard.
There are other problems with storage, as Jim explains: “Frequently—especially if they are put away damp—a white crystalline substance forms on the shell and starts to eat into it. They call it museum disease, and once it gets a hold the shells don’t last long. A lot of old shells in the core museum collections have completely disintegrated.”
Fortunately, these collections are constantly being replenished by the efforts of amateur collectors who send the minute fruit of their searches down to Frank Climo. He adds interesting specimens to the museum’s collections and uses the material as grist for his taxonomic and biogeographical studies.
These collectors are a vital part of the scientific process and are experts in their own right. Most have discovered new species of snails, and some have been rewarded by having species named after them.
David Roscoe, inspired by the expanding database being built up on snails, is pushing for the government to provide funding for a national database on all animal and plant species. “It would contain detailed distributional data as well as information on the characteristics of species, and be widely available. A teacher waiting to lead a biology field trip could access the database to ascertain detailed information on species in the area of the proposed visit. It would be of great use to amateur and professional alike, whether interested in taxonomy, environmental monitoring or whatever.”
David, along with Pauline Mayhill, an avid Tauranga collector, has already started the scheme for snails.
Norman Gardner, a retired cabinet maker in Birkenhead, Auckland, was perhaps the first of the scientific amateurs to become really interested in snails, and he has been pursuing them for close to 50 years. Although still very active in snail work, he has followed a somewhat different trajectory. In recent years he has left New Zealand snails to others and concentrated on the many neglected Pacific Island species.
Colourful tree snails are common in the islands, whereas New Zealand has no true tree snails. There are also fewer small species than in New Zealand, but more larger ones, though there is nothing to compare with the magnificence of our Powelliphanta.
The minute New Zealand species are also more notable for architectural exuberance. Sculptural detail of such delicacy that it requires a scanning electron microscope to unlock its intricacies is common. Some species are tall spired, others flat, and every conceivable intermediate form exists. Some resemble beehives while others compensate for more pedestrian form with elaborate surface plates, giving them the appearance of mini-turbines. Hairs and bristles cover the surface of many species, and yet others sport complex arrays of plates in their apertures.
The functional significance of much of this sculpture is obscure, but one guess is that the sculpture stops dirt clinging to the shell. To a two-millimetre snail, even a speck of dirt could represent a sizeable bur den. Longer shells (10mm-plus) are much less elaborate. Apertural plates perhaps serve to reduce water loss from the resting animal To a slimy, soft-bodied mollusc, living in air is hazardous. Drying out is a constant danger, hence the damp habitats and nocturnal behaviour.
Yet droughts do occur, and snails have to cope. Many species are able to go into a state of quiescence—termed aestivation. Some secrete a tough mucus membrane (dn epiphragm) to close out the world, and pulmonates can lose an astonishing amount of their body weight (5080 per cent) by dehydration without suffering irreversible damage. In cold areas snails may also hibernate in winter.
If you are a slug or snail, just getting around uses up a lot of water—it’s almost a fuel. Mucus, that silver trail snails leave, is essential in locomotion, and water is its main constituent. Prevent the snail secreting mucus and it can’t move. Although all snail mucus seems glutinous and clinging, it apparently has different properties depending on whether it’s on the back of the animal, in the gut or lubricating the sole of the foot.
Mucus also has a role in mating, with some introduced slugs copulating while suspended from long mucus ropes. Thus separated from the confines of terra firma, they unfurl their copulatory organs—in one Swiss species to a remarkable 80cmand exchange packets of sperm. After disentangling their entwined parts, one slug falls to the ground, leaving the other to climb back up its slimy tresses. Partners often store the sperm “for a rainy day”—literally. They fertilise their eggs only on wetter days more suited to egg laying.
The mating habits of our 20 or 30 native slugs are unknown. These species occur mostly in the South Island, and all belong to one distinctive group. They bear a close resemblance to a slender four-centimetrelong dead leaf of yellowish hue with darker veins. The slime they exude has a distinct yellow tinge. A few species have the eyes on short stalks, but there are no other tentacles. Some favour damp leaves or rotting wood while others prefer the closeness of flax and nikau palm leaf bases.
Slugs are really just land snails without shells, but many still possess internal shell rudiments. A few snail species seem to be halfway between slugs and snails, with shells still clearly visible but reduced to fig-leaf proportions. Members of the genus Schizoglossa are one such group. The shell looks like that of a 15mm paua, but provides meagre protection for the 50mm animal.
Schizoglossa is allied to our most spectacular native snails—indeed, some of the most striking land snails to be found anywhere. These are members of the family Paraphantidae, tidae, particularly the genus Powelliphanta. It is named after Dr A.W.B. Powell, a former conchologist at Auckland Museum who studied the snails during the 1930s and ’40s. Unlike the smaller natives talked about earlier, these snails are large (diameter 40-110mm) and strikingly beautiful. Some of the largest are plain old-gold coloured, but most resemble polished wood with patterns of axial or spiral stripes in blacks, browns and gold.
Headquarters of the group are the forests of north-west Nelson, but others extend down to Fiordland, Southland and across the Marlborough Sounds, with a few in the North Island—around Levin and a smattering through the southern axial ranges as far north as Egmont and Waikaremoana. Some species live at low altitudes, but a number prefer the subalpine environment at 700-1200m.
Unlike the small snails, which feed on the microscopic fungi and algal films on detritus, Powelliphanta are carnivores. For the most part they live deeply buried in leaf mould and feed on large (20cm-plus) native worms, sucking them in like spaghetti. Because the worms are large, the snails need eat only occasionally.
Powelliphanta are not exactly outgoing. At times photographer Michael Schneider and I crouched for four or five hours vainly waiting for snails to emerge from their shells. And if they were coy about public appearances, eating in front of humans must have been the ultimate breach of snail etiquette. Pushing enormous worms into their faces elicited not the slightest interest (though later Michael was successful with garden worms.)
Anne Strachan, who kept Powelliphanta for several years, noted peculiarities in the snails’ eating behaviour. “It seemed to me that the snails put out a substance that affected the worms when they wanted to feed,” she told me. “Usually the worms crawl over the snail’s mucus trails no trouble at all, but when the snail is hungry, the worm blanches and goes limp as soon as it touches snail mucus.”
Unfortunately, the snails themselves seem to be a desirable item on quite a few other menus.
Kath Walker, who has had a long interest in these snails and now works for the Department of Conservation in Nelson, showed us a selection of mutilated shells. “Rats are a real menace, especially in lowland situations. They start chewing the lip of the shell and slowly work around the main whorls, devouring the animal as they go. Wekas tend to destroy the spire of the shells with their beaks in their efforts to extract the animal. Pigs simply crush the whole shell. Other birds, probably keas and kakas, peck into the body whorl beside the aperture, although sometimes the shell is sufficiently tough that they don’t succeed in getting in.”
Predation by vertebrates is the main cause of mortality in Powelliphanta. Considering all species, probably two thirds of dead shells bear damage attributable to one of these predators.
Kath has set up snail houses in the grounds of several homes she has occupied and after a lot of tedious work in providing the moisture, humidity, calcium and food they need, has succeeded in growing and breeding Powelliphanta, making many unique and fascinating observations along the way. For instance, she recalls seeing two snails trying to eat the same worm. “One evening a medium-sized snail had almost completely ingested a big worm when a larger snail latched onto the other end. Next morning, both snails were still mouth to mouth, tugging at the worm, but the bigger snail finally got it all.”
Her measurements and weighings of captive snails and her field work in monitoring two South Island Powelliphanta populations have put Kath in a unique position to know how rapidly the snails grow. “Generally the shell’s lip extends by 5- 8mm per year, so large snails could be 10-20 years or older.”
Such ages place Powelliphanta amongst the Methuselahs of the snail universe, where lifespans of 1-5 years are common.
Like many longer-lived animals, Powelliphanta lays a small number of large eggs. Although hermaphrodites, Powelliphanta usually mate and fertilise each other. Mating involves one snail sitting on top of another and the two exchanging sperm packets. It is accompanied by a lot of bubbles.
It seems that fertilisation does not always require two individuals. Kath Walker kept a solitary Powelliphanta in monastic confinement for 18 months, and this individual eventually produced fertile eggs, suggesting that snails may self-fertilise if necessary. (Some doubt remains, because snails can store viable sperm from another snail for long periods.)
According to Kath, Powelliphanta lay one or two eggs per week during the spring, and they are huge compared to the size of the snail. Shaped like birds’ eggs, the largest are 12- 14mm long—not much smaller than a sparrow’s egg! Eggs are laid in the ground (from an aperture near the mouth, not the rear end) and require 2-9 months to hatch out a young snail.
I can’t help wondering if some of these youngsters ever see daylight. They are nocturnal, and with one species the only live specimens we unearthed were buried beneath 300mm of decayed beech litter and sinewy roots—deep and inaccessible enough to escape all predators… except man.
In the past, some shell collectors took both live and dead snails, but there were never more than a score of people interested, and the snails were protected by their remoteness. Dead shells are also easier to find. Since 1980, however, Powelliphanta has joined the elite list of invertebrates protected by the Wildlife Act, which makes it an offence to collect either live or empty shells. Empty shells? It is claimed that it is difficult to tell whether the snails were alive or dead when collected. In reality, the only uncertainty arises with freshly dead snails, but the law consigns all those beautiful shells to decompose in obscurity.
The main human threat to these large snails has never been shell collectors, but habitat destruction. Home for some of these snail varieties has frequently been a very restricted patch of bush. One type found at the tip of the West Coast survives in only a hectare of very disturbed bush on a farm. Among the worst affected snails have been those on the Horowhenua plain, inhabiting diminishing bush remnants in a busy agricultural area where human activities have obliterated a number of colonies.
Yet the most wholesale destruction of snails and habitat has occurred on the northern West Coast, under government auspices and subsequent to the snails’ protection by the Wildlife Act. While the Wildlife Service had responsibility for snails, it had no authority over the land they were on. Consequently, it could not prevent the Forest Service razing large tracts of West Coast forest and replanting in exotics. Drainage demands of pine and eucalypt were met by mounding the recalcitrant pakihi soils into 1.5m-high ridges.
Powelliphanta lignaria rotella was just one subspecies that was largely exterminated by these endeavours. The Department of Conservation has ironically now been presented with some of this mutilated snail habitat (unlike the Wildlife Service, DoC has responsibilities for habitat) and has considered felling the young exotics, reflattening the land and replanting native trees! Frank Climo is right. The snail’s spirality does capture that clearsighted, unflinching directness we associate with public administration.
The stately snail has another message to teach us too, this about our past. It is hard to imagine the large buried eggs of these creatures being dispersed across seas by breezes or on the feet of wekas. Despite Kath Walker finding a live Powelliphanta washed up on a Golden Bay beach, it strains credibility to envisage these snails as powerful swimmers or even blithe windsurfers. They have been dispersed by measured crawling or tectonic movements (which is faster?).
The presence of similar snails on both sides of Cook Strait is good evidence for the existence of land bridges between what is today the northern South Island and the southwest North Island. On a larger scale, members of the Paryphantidae (the family to which Powelliphanta belongs) occur in southern and eastern Australia, South Africa, Indonesia, Melanesia, Samoa and Tonga. Land connections between these disparate places perhaps extend back to Gondwanaland, which is thought to have broken up 150 million years ago.
Paryphantidae are not the only members of this ancient club. A number of elements of the New Zealand fauna—frogs, lizards and other land snails—all suggest that New Zealand has been isolated from other land masses for the last 60 million years and possibly longer.
New Zealand has other interesting genera of snails in the Paryphantid family besides Powelliphanta. In Northland, Paryphanta busbyi, the kauri snail (termed pupurangi or whistling snail by the Maori) is still common in places, and human intervention has re-established it in other areas, such as Auckland’s Waitakere Ranges. This snail has a black animal inhabiting a dark greenish shell 45mm in diameter. The colour resides in an exterior proteinaceous periostracum made of conchin, and this overlies a fairly solid calcareous shell. Shell structure in Powelliphanta is similar, but the calcareous layer is much thinner.
Scavenging sufficient calcium to build an adequate home seems to be a problem for many land snails, especially large ones. Their distribution tends to correlate with limy rocks.
Shells of both genera have been known to explode in continental climates where there are rapid changes in temperature and, especially, humidity—the sort of changes which occur when air conditioning or heating is switched on in the day and off at night. Dr Alan Solem of the Field Museum of Natural History, Chicago, reports that temperature-induced swelling and shrinking in shell layers causes them to explode “like a BB gun shot”.
Rhytida is a genus containing smaller dull-brown calcareous-shelled paryphantids which are cannibalistic. The shell of Wainuia resembles a small, completely decalcified Paryphanta. It is very fragile and translucent when empty. Murray Efford of Ecology Division, DSIR, has recently observed those snails feeding on amphipods (sandhopper types)—the first snail found to prey on live arthropods anywhere.
Paryphantids (like most snails) apparently evolve at a speed that makes continental drift look like large scale drag-racing, so each of these genera may be 20-50 million years old.
Another ancient group of large snails is represented locally by the flax snails Placostylus.
The affinities of the Placostylus lie only to the north—Lord Howe to the Solomons, but not Australia or further west. It, too, lays the large eggs that mandate land dispersal.
Far northern New Zealand in the Pliocene, 10 million years ago, was a series of islands, and it is considered that the three New Zealand species of Placostylus developed on these.
One island would have included the present Three Kings Islands, a second (possibly several islands) was the “hammer head” of the far north isthmus and the third, the rest of the north Auckland peninsula. Tombolos of sand now link the second and third islands, while the Three Kings remain separate.
Placostylus are as distinct from the paryphantids as east is from west. Whereas paryphantids are carnivores, Placostylus fatten on shed karaka and a few other broadleafs. The shells of Placostylus are solidly calcified with only a thin brown skin of protein, while those of Powelliphanta contain much less calcium. Powelliphanta are most at home in montane beech forests, while Placostylus stays within sight of the sea. Placostylus gets its name from a fancied resemblance between its tall spire and a writing stylus, while paryphantids have no spire. Where Powelliphanta is generally glossy and patterned, Placostylus is matt and plain. Nonetheless, the shell of Placostylus is rather handsome—a chocolate brown exterior set off by a rich, shiny orange aperture.
Placostylus is more endangered than most paryphantids, and is also fully protected under the Wildlife Act. The more southern of the two mainland species, P. hongii, once extended sporadically down the east coast as far south as Bream Bay. Now very few remain on the mainland, but they thrive at the Poor Knights Islands. From Cape Maria Van Diemen, round North Cape and down to Parengarenga, the second species of Placostylus (P. ambagiosus) clings to life. Even within this limited area, snails survive only in minute pockets as isolated subpopulations.
Dr A.W.B. Powell also studied these snails, naming most subpopulations as separate subspecies, but some uncertainty exists as to whether they deserve this status or whether they are just ecological variants.
Does anyone care about the distinction? Surprisingly, yes. A subspecies is held to be genetically distinct, and therefore worth quite an effort to preserve it. A variant looks different because of some environmental factor, and tends not to be as highly regarded. To ascertain whether the nine living P. ambagiosus subspecies are valid or not would normally involve comparing the details of the animals inhabiting the shells, but since there are so few live Placostylus in some of the subspecies this cannot be countenanced.
Norm Gardner was interested in Placostylus as long ago as the mid-1940s. “Even then they were not abundant,” he says. “Major predators were (and still are) rats and pigs. Wild pigs are especially devastating in that they root right into the bases of flax bushes where the snails often hide. As well as eating snails, the pigs destroy their habitat. Wild cattle also roamed over the entire area, trampling snails and damaging habitat. Since roads were improved in the Far North in the early 1970s, thousands of visitors have flocked into the area over summer, and these haven’t done anything for the snails.”
One of the major reasons that the North Cape scientific reserve was established during the 1960s was to protect two Placostylus varieties found there.
The late Norman Douglas, a dairy farmer of Waiuku and a remarkably dedicated and talented naturalist with a particular interest in molluscs, kept Placostylus for some years. His widow Lorna relates how he and some friends found a single live P. ambagiosus in the Cape Maria area, at a place where rats had devoured all other individuals.
“They buried this sole live specimen in the flax,” she recalls, “but eventually decided it was doomed to become rat fodder, so Noman reluctantly brought it home to Waiuku. There it laid fertile eggs for three and a half years, and he was eventually able to return a number of individuals to the wild.”
Sand is another threat to Placostylus. In addition to the nine named recent varieties of P. ambagiosus, there are six subfossil species. Their bleached, sand-blasted shells festoon dunes at a number of locations on the far northern coast. Carbon dating says they lived 6006000 years ago, presumably in bush that has been inundated by encroaching sand. Lighthouse keepers at Cape Reinga used to collect shells from the big dunes behind Cape Maria and sell them as souvenirs.
Government attempts to save the snails have not always met with success. In the early 1980s it was decided that relic live Placostylus varieties in the Far North were doomed. Attempts were made to collect all live individuals from the most threatened sites, and these were transferred to holding cages at Kerikeri. Many snails died in captivity, but the survivors were transferred to various islands in the Cavalli group, off Matauri Bay. Unfortunately, not all the selected isles proved suitable, and most transferred Placostylus perished, although they still persist on one island.
The only mitigating aspect of this sorry saga was that eventually a few snails surfaced back in the Far North at the sites where all were supposed to have been collected. More recent conservation efforts seem to be enjoying greater success. DoC has fenced some areas to exclude pigs and cattle, and systematic rat poisoning has been carried out.
Gratifyingly, some subspecies seem to be thriving, although others nearby remain very sparse. For instance, in one part of the North Cape reserve Pa. watti is now very difficult to find, whereas 2km away Pa. micheii is flourishing. An idea of its abundance can be got from the hundreds of broken shells beside thrush and blackbird anvil stones in the area. This exotic bird predation has only arisen in the last decade. Near Cape Maria two other varieties are on a similar seesaw, with one variety quite abundant and another teetering on the brink of oblivion.
The Department of Conservation is not the only body interested in the well-being of Placostylus. S.O.S.(Save Our Snails) was started in Auckland nine years ago and is dedicated to “protecting and promoting endangered invertebrates,” according to Andrew Jeffs, one of the founders. The organisation has made several attempts to safeguard Placostylus populations, including laying rat poison in the worst affected colonies at either end of Spirits Bay and around Cape Maria. Seeds and cuttings of snail food plants in the Far North were raised to plantable size in Auckland and then replanted to improve habitat and food supply in snail colonies. SOS members even fenced a steep hillside above Spirits Bay to protect a couple of hectares of snails from larger animals.
Much of our knowledge of the biology of Placostylus derives from an MSc thesis by Andrew Penniket, now with Television New Zealand’s Natural History Unit. He found that between 20 and 30 large eggs are laid in a shallow nest in loose earth, but perhaps only every two or three years. They hatch in 6-15 weeks to yield a 5- 7mm snail. Young snails may spend a period living in trees, but teenagers and adults dwell in fallen leaves and flax. Older snails (5-6 years) develop a thick lip which provides protection against the fangs of rats, but young snails have a thin shell which can be easily fractured. Mortality among very young snails is high, perhaps due to dessiccation via their gaping aperture.
Even these coastal snails seem to have a calcium problem. Andrew Penniket speculates that removing dead shells from an area depletes accessible calcium, so that remaining snails produce deformed shells. He has sprinkled calcium in the area as a dietary supplement.
The main food plants are karaka, kohekohe, Coprosma, rangiora, Planchonella and lace bark—all coastal broadleaf species. Pohutukawa, flax, five finger and taraire provide shelter only, and are not eaten.
The brown shells bear more than a passing resemblance to the dead leaves in which they live and feed. Snails are active only on wet nights, which means they spend a lot of time resting. By marking individuals, Andrew discovered that they do not range more than five metres in a year, and many will still be under the same flax bush they were placed in 12 months earlier!
Perhaps this relaxed pace of living is responsible for the snail’s longevity: it is suspected that individuals live for at least 10 years. provided they escape predators. Rhvtida and Paryphanta probably eat young Placostylus, and on the Poor Knights an unexpected predator is the large shore crab Leptograpsus, which can crush the spire of the shell with its pincers.
In New Caledonia a major predator on Plactostylus is man. New Zealanders, being a gastronomically cautious lot, largely eschew snails. Of course, the mercurial French are renowned for their penchant for escargots. Afficionados assure me that, laced with garlic sauce and olive oil, snails are great. And the opportunity exists to try snails locally (though not Placostylus!)
A few years back retired agricultural consultant Hans Pos encouraged school children to collect the common introduced Helix aspersa. with promises of a new export industry catering to discerning European palates. Unfortunately, the venture never really prospered, but there are a few people farming snails locally and supplementing their incomes by selling them to restaurants.
“I’ve been interested in culturing snails for 14 or 15 years now—it’s a hobby of mine,” Hans told me. “I produce them for my own consumption, and am also trying to select faster-growing, bigger, superior eating snails. It’s actually a pretty ancient preoccupation. Scattered throughout Europe and North Africa you can find pockets of larger, more colourful Helix aspersa, often called Helix aspersa maxima. I think that these represent the results of snail breeding carried out in the Roman Empire. The evidence suggests that the Romans bred them for colour, fecundity and growth rate.”
Local snail farmers use brassicas for food and can carry up to 150 snails per square metre, or 1.5 million per hectare. (Eat your heart out, sheep farmers, where 10 per hectare is about average!) Above 150 per square metre everything becomes covered with slime.
And how’s this for a new agricultural twist? Snail eggs. Hans tells me that in Europe people regard them as a health food delicacy, and they fetch the same price as caviar—around $50 per 50 grams. “You can buy them in Harrods,” says Hans.
“Laying” snails excavate a small hole in the ground and in it deposit 80-120 eggs, each 3-4mm in diameter. They manufacture such a brood 2-3 times a year (which goes some way to explaining my father’s ongoing battle).
Apart from the eggs, snail meat is being increasingly sought as a food. The traditional French fare is Helix pomatia (a bit bigger than H. aspersa), although H. asperse, the “little grey”, is also popular. Demand is outstripping orthodox supply, resulting in exploitation of alternative species.
One of the most bizarre stories concerns Achatina fulica, the giant East African snail. This beauty reaches 200mm in length, grows at a prodigious rate and can survive in most tropical and subtropical climates. It has become undisputed winner of the title “world’s most dreaded snail”. Man is the agent by which it has spread—from fascinated aristocrats taking it home, to eggs carried on tank tracks during World War II. Many tourists have taken home shells (sometimes even painted ones) certain that they were empty, when in reality the snail was just biding its time. A few years later, abundant progeny of the neglected souvenir wreak devastation on ornamental and agricultural plants.
In some parts of the world Achatina has actually made agriculture impossible. In Florida, a state of emergency was declared when the snail was discovered in 1969. Using all the powers of the state, it still took four years to eliminate 18,000 snails and eggs from just a few infested kilometres.
Recently, in a “if you can’t beat them, join them” (and eat them) move, some people have abandoned the futile attempt to produce food from their plants and are harvesting the snails instead. Perhaps because Achatina is not universally admired, its flesh is being discreetly marketed in Helix pomatia shells, creating some shortages of empty H. pomatia shells!
(In case anyone is salivating over Powelliphanta as a possible toheroa substitute, it’s been tried and found really revolting. Perhaps something to do with its carnivorous diet.)
In bygone days man’s regard for snails was not entirely tied to their gustatory possibilities. Snails were also appreciated as medicine, with particular efficacy in overcoming severe colds, consumption and other ailments. “Beating snales which be in shell with bay salt and mallowes, and laid to the bottomes of your feet and to the wristes of your hands” was an example of sixteenth century preventative medicine for warding off fits associated with high fevers. Crushed snails wrapped in linen, moistened with brandy and applied to the forehead were a favoured Spanish cure for headache.
These and similar remedies seem to have been omitted from the vademecum of modern socialised medicine.
Snails do, however, make other contributions to human society. En snails was not entirely tied to their gustatory possibilities. Snails were also appreciated as medicine, with particular efficacy in overcoming severe colds, consumption and other ailments. “Beating snales which be in shell with bay salt and mallowes, and laid to the bottomes of your feet and to the wristes of your hands” was an example of sixteenth century preventative medicine for warding off fits associated with high fevers. Crushed snails wrapped in linen, moistened with brandy and applied to the forehead were a favoured Spanish cure for headache.
These and similar remedies seem to have been omitted from the vademecum of modern socialised medicine.
Snails do, however, make other contributions to human society. Engineers continue to be inspired and challenged by the shells not just of land snails but of all molluscs. The essence of concrete is cement (calcium carbonate) which is the main component of a shell. How is it that the strength to weight ratio of a shell is so superior to man-made concrete? The answer proves to be partly in the long protein fibres mixed in with the calcium, and this is inspiring new forms of concrete containing glass or carbon fibres to produce stronger and lighter products. An Australian experimenter has even made concrete microlight wings!
One extreme environment in which snails have recently been found to play a significant role is in Israel’s semi-desert Negev region. All gastropods, including snails, have large numbers of fine teeth attached to a flexible ribbon termed a radula. This device is dragged back and forth like a rasp to grind off particles of food. As teeth at the front are worn away, the whole ribbon just moves forward.
Three small related snail species in the Negev subsist on a lichen that bores into limestone rock. In getting at the lichen, the snails grind away the rock with their radulas, turning 100 grams of rock into dust each year in every square metre of desert. Their faeces supply a significant amount of nitrogen to desert shrubs, so snails there are helping the desert to blossom.
Related snails occupy alpine environments, and may perform similar functions. After all, snails are second only to insects in success amongst land animals, with some 30,000 species worldwide. Just as surprising are the numbers of individuals. Dr Alan Solem estimated 3,000,000 to 12,000,000 small living snails per acre under favourable conditions.
Alas, my backyard must come close to falling into this category. Every Scotch thistle harbours dozens of snails, but, unfortunately, the thistles seem unaffected. On a recent night a neighbour and I picked a plastic bag full of snails off two young citrus trees in about 10 minutes. Plenty were right at the tops of the trees, two metres off the ground. Now, father is not as young as he used to be, but I wonder whether the prospect of some aerial combat might lure those slippers out of retirement?