Snail shells an evolutionary saga

Forest and Bird, Issue 279, 1 February 1996, Page 42

Snail shells an evolutionary saga

Ann Graeme

E ARE ALL familiar with the unwelcome garden snail. It is an introduced member of a group of animals called molluscs that also include the slugs, shellfish, chitons, octopus and squid. The mollusc way of life is very old and they appear far back in the fossil record — more than 500 million years ago. Over 100,000 different kinds of molluscs are alive today, and over 40,000 extinct species have been found. Snails and slugs — together known as gastropods — make up the largest group of the living molluscs. The most primitive of known fossil snails are called "amphigastropods". Their shells did not have the trademark spiral coil of today’s snail shells, but the paired muscle scars inside the shell suggest that the living animal was bilateral (meaning it had a head and tail end and could be divided into two equal halves, as humans can). In a bilateral animal, the head carries the eyes, feelers, and the mouth, while the tail end, being at the end of the digestive system, is the route for defecation. This was the basic body plan of these ancient snails, as they glided along carrying their conical or caplike shells on their backs. Then, around 500 million years ago, a

gene mutation appeared that was to permanently change the evolution of the gastropods. Gastropods, like all molluscs, hatch from eggs and develop. through distinct larval stages before appearing as

the recognisable adults of the species. One of the larval stages is called a veliger. In marine molluscs, the minute veliger swims amongst the plankton, a tiny, shelled animal propelled by a structure like a paddle wheel. It was in a veliger, back all those years ago, that this revolutionary mutation appeared. As the veliger grew, its head and foot remained stationary but the other body organs twisted, bringing the anus to lie above the head. This caused the organs on one side of the body to atrophy. The lop-sided body now grew in a spiral coil, and secreted a matching, spiral shell. The mutated veliger carried this trait to adulthood, resulting in a snail

that carried a compact portable spiral shell, rather than the long cumbersome cone or token cap that gastropods had carried up to that time. The new spiral shell also offered better protection because it was sufficiently

roomy for the entire animal to withdraw into, and the entrance could be plugged with its foot. Such were the adaptive advantages offered by this ancient twisted veliger and its descendants that they cornered the market in the evolution of all future gastropod species. Today there are more than 50,000 species of land, sea and fresh-water snails carrying a wonderful array of spiral shells. But however well designed, a secure home has its costs. For a snail, the cost is a loss of speed and agility. In some species the shell is shrunken, and the slugs have gone even further and reduced their shell to a tiny internal organ. Although slugs have outwardly straightened their bodies

to a bilaterally symmetrical form, they still retain a contorted digestive system. Without a protective shell, land slugs have become nocturnal to avoid the burning sun and beady-eyed birds. Despite the diverse forms of today’s gastropods, every species still carries the secret of its ancestry in the twisted gut of

its veliger larva.

ANN GRAEME is the national coordinator of Forest and Bird’s Kiwi Conservation Club.

ORE THAN A thousand species of native land snails have been identified in New Zealand. They range from carnivorous snails — so large the extended animal would cover a human hand — to species the size of pin-heads. The best known of the large carnivores are the beautiful Powelliphanta and Paryphanta which live on the forest floor and hunt earthworms, slugs and smaller snails, devouring them with the hundreds of tiny dagger-like teeth that cover their tongues. Why do we have such an abundance of species, when the whole of the similarly sized United Kingdom has only 112? The geological record suggests that New Zealand has always had a moist, snail-friendly climate. The past fragmentation of our land masses together with the variety of forest habitats has provided opportunities for species to become physically separated from their fellows, adapt to different niches and evolve into different species. Today, both ancient and more recent species make up our enormous diversity of native snails. The snails evolved in this country along with natural predators such as weka which meant that snail numbers were maintained at a stable level. This balance was shattered by forest clearing and the introduction of mammals such as rats, pigs and possums. The result has been a decline in native snail populations throughout mainland New Zealand and today more than half of the threatened invertebrates listed by the Department of Conservation are native snails. One of best ways in which you can help native land snails is to resist souveniring their shells. Firstly the shell may not be empty at all with the snail only aestivating inside — to emerge later on the mantlepiece — and, secondly, shells are eaten by other snails and are an important source of calcium for the next generation. One researcher who removed many shells from an area of snail habitat in Northland, was dismayed to discover on his return some years later, a large proportion of snails with deformed shells.