IN TOUCH WITH NATURE

Otago Daily Times, Issue 21018, 6 May 1930, Page 2

IN TOUCH WITH NATURE

THE WATER SNAILS. By J. Dbdmmond, F.L.S.. E.Z.S. Fresh-water snails are plentiful in New Zealand in only a few places. One species has been found in the Avon, another in the Heathcoto, other species in Lake Lyndon, in Auckland and Otago lakes and streams, and in out-of-the-way places, such as Governor’s Bush Creek, near the Mount Cook Hermitage, Birch Hill Lagoon, which is several miles from the Hermitage, a small lake at Arthur’s Pass, and at Ashburton. In distribution, the group Limnsea. is world-wide. ’lts members have almost boundless capacity to adapt themselves to physical conditions. They arc found in tarns on the Himalayas at an altitude of 17,000 feet, at a depth of 800 feet in Lake Geneva, in the geysers of Iceland, in sulphur springs, in brackish water, and in all kinds of polluted water. They are true snails. For some reason they have taken to life in the water, preferring it to life on the land, but they still retain the lung, functioning properly, which their ancestors had as dwellers on the land.

The methods by which these creatures breathe have excited a good deal of interest and speculation. Covered in by a roof of ice, water snails have suffered no inconvenience. Some individuals kept under water for 19 days retained their health. Others remain beneath the surface during cold weather; when warm weather returns they rise to the surface to take in a supply of ain The facto seem td be that water snails usually breathe through their lungs, but sometimes through the skin. One species never comes periodically to the surface. It breathes, probably, by receiving into its lung minute quantities of oxygen given off by. the vegetation on which it feeds.

snails can endure for a fairly long time complete deprivation of atmospheric air. Common garden snails, placed in a jar from which the air was exhausted, showed no signs of being inconvenienced for about 20 hours. They survived for three days. Kept under water, they were active for six hours. They then become motionless, their bodies swelled, and they died in 36 hours. If immersed for only 24 hours, they usually recover. The pauses of death of land snails in these experiments were not so much deprivation of air as compulsory absorption of water through the skin. In this way, they absorb surprisingly large quantities of water. Even slugs enclosed in moist paper soon gain in weight.

A European water snail, nalis, introduced into the Avon Tnany years ago as food for trout, is influenced in the size of its 'Shells, and in other ways, by the volume of water in which it lives. The largest individuals occur in only pieces of water of fair size. All water snails seem to deposit their eggs in masses of jelly on the leaves of water plants or on all sorts of debris, occasionally on stones in the water. In the Old Country, where pond life is a favourite study of many people, water snails make sudden and surprising appearances. Wherever a new pond is formed water snails may be seen in it, unless it contains something repugnant to them. Members of a species of Limntea suddenly appeared in small gravel pits in Cambridgeshire. They were present in such large numbers that they could have been scooped out in handfuls. Their numbers gradually decreased until, after four seasons, they disappeared. '

When frosts and cold winds come along at this season, land snails retire beneath stones, creep into holes in trees, cracks in rocks, and other cosy places, or bury themselves in earth, moss, or heaps of leaves. There they hibernate throughout the winter. Several may be found attached to one another, not for warmth, as their temperature is low, but to share the comforts of a cosy home. Slugs usually hibernate alone. They excavate a nest in the ground, contract their bodies until they are almost round, and cover themselves with slime. Both land snails and slugs see that, when theit winter sleep begins, they are in good condition. The action of their hearts almost stops during hibernation, a result of the cold.

Water hnails do not seem to hibernate, but some bury themselves deeper in the mud. Most water snails bury themselves in the mud during long droughts, when the water in their ponds dries up. A sudden fall of water in a ditch caused water snails that lived there to bury themselves three/inches deep. On the other hand, there are water snails that favour dry ditches. A water snail recorded from the Heathcoto River, yellowish white in colour, and semi-transparent —that is the snail itself, not its shell—sometimes leaves the water to roam the land. Other water snails abandon the water during thO whole winter. Large and beautifully painted land snails iu Brazil, Ecuador, and Peru bury themselves deeply in the ground during the dry season. In the rains they climb to the topmost branches of lofty forest trees. The introduced watqr snail, Limnoea stagnails, is generally believed to be a strict vegetarian, but it sometimes takes to flesh food. In an aquarium in which it had ample vegetable food, an Individual of this species suddenly attacked living, healthy water-newts, overcame them, and devoured them. Other members o£ the same species have been seen eating their own eggs. _ Deprived of other food, these water snails have caught minnows and partly devoured them.

Water snails, it has been pointed out, cannot migrate across country from one piece of water to another. Barriers bo tween pond and pond, lake and lake, stream and stream are almost insuperable to them. It might be expected that many species of water snails would bo confined to very restricted areas. The reverse is the case. The range of groups of water snails is astonishingly. with. Liraiia}a and other groups arc spread almost over the world. Limnaea stage alls is a native of the whole of Europe, and of Northern Asia, Turkestan, Afghanistan, Northern Persia, Greenland, North America from the Pacific to the Atlantic, Northern Canada, British Columbia, and Texas. In contrast with this extraordindistribution, a small black water snail is abundant in almost every New Zealand pond and stream where the water i it not too stagnant and muddy, but it lias been found in no other country. Yet its capacity to adapt itself to its conditions is so great that it can live in the tidal reaches of rivers, even in estuaries. ; Another water snail, like a limpet in appearance, is known from the Avon only, | Efforts to colonise land snails usually fail. Snails from the Riviera taken to England and placed on southern coasts under climatic conditions similar to those of their native country lived for n while, but seldom took root permanently. Yet lira common garden snail of the Old Country, Helix aspersa, and several species of slugs from the same place, now are the most plentiful molluscs in New Zealand. They are in almost every garden. They have spread along river-beds, up valleys and gullies, on roads and railway lines, even into remote districts, and , have changed the face of the Dominion’s j fauna, as far as creatures that' are fet- | tored to the soil are concerned. ! ■ The common garden snail, hardy, robust, and adaptable by art or by acci- , dent, has been induced to invade with l notable success, not only New Zealand, | but also Nova Scotia, Maine, South Carolina, New Orleans, California, Mexico,