Right does not equal left. Snails may not be reflected, as their
shell is asymmetric. [RN]
|
Right does not equal left. Snails may not be reflected, as their shell is asymmetric. [RN] |
In nature there are numerous examples for the rule that right is rarely the same as left. One example is the tetrahedral structure of the carbon atom. Another is the coiling of a snail's shell.
Basically a shell, as snails have one, is a common character of all higher molluscs. As such, the shell is nothing special for a mollusc. What makes a snail's shell special, is the fact, that it is asymmetrically coiled to one direction, either left or right. Compared to a Nautilus's symmetrical shell, looking the same from left or right, the whorls of a snail's shell mount to a clearly visible tip - the apex - on one side of the animal.
If finally a snail's shell is coiled to the right or to the left, is based on several factors.
which is why this species also is called cheese snail. Upper and lower side, though, are very different. Picture: Helmut Nisters. |
The shell of Helicodonta obvoluta seems to look the same from above and below, as flat and round like a loaf of cheese, which is why the species is also called cheese snail. However, upper and lower side of the shell are clearly different, the upper side showing the spire of whorls, though without a clearly visible apex. On the lower side there is the shell's conspicuous navel, also called an umbilicus.
That means there is no geometrical mirror plane to divide the shell in two equal halves, which is why a snail's shell may not be reflected horizontally in a picture. The picture will then show something completely different.
How to
determine
the coiling direction of a snail shell.
The coiling direction of the shell is basically as specific for a certain snail species, that it may be used for identification purposes. Often whole snail families are definite in this matter, such as the Helicidae, to which belong the largest land snail in Central and Western Europe, Helix pomatia. Helicids, like most snails, are dextral or right handed.
(dextral), Cochlodina laminata und Macrogastra ventricosa, both sinistral. See text. Pictures: Helmut Nisters. |
In other snail groups, there may be single sinistral among the larger part of dextral species. For the former the sinistral is again specific in contrary to their relative species. For example, among the Enidae with their characteristic towering shell there is the large zebra snail (Zebrina detrita) from Southern Germany (the Kaiserstuhl area near Freiburg). Zebrina detrita is always dextral. In contrary, the four-toothed bulin, Jaminia quadridens, a smaller relative with four apertural teeth (hence the name), is always sinistral and is such opposed to the family rule.
The only land snail family with generally sinistral species are the door snails (Clausiliidae). Those small snails living on rocks and trees have got a spindle-shaped shell which makes it easy for them to hide from dryness in cracks and behind bark. They occur in an especially rich number of species on the Balkan peninsula and in Greece, but are also spread of the rest of the Palaearctic (see zoogeographical faunal provinces). There are also door snails in Central and Western Europe, such as the plaited door snail (Cochlodina laminata) or the bulbous door snail (Macrogastra ventricosa), both of them sinistral. In contrary to that rule, there is the genus Alopia mainly occurring in Romania, that is, with some exceptions, dextral, as are some species of the Greek genus group Albinaria on the Peloponnesian peninsula.
 This picture has not been mirrored: The snail is really sinistral - so it is a true snail king! Picture: [RN] |
There is another very well documented phenomenon among the Roman snail (Helix pomatia). Among thousands of specifically dextral shells there may be one or the other rare sinistral specimen. Those then are colloquially referred to as snail kings.
Snail Kings - Sinistral Helix Shells.
Of course sinistral specimens of dextral snail species not only may be found among Helix. From the Renaissance era shells like that have found their way into naturalists' collections as "conchylia sinistralia" and so can be found in natural history museums, that often enough evolved from those collections. Also among modern conchologists, sinistral shells are a very special addition to a shell collection, sometimes even a basis for specialisation.
Additional information:
Sinistral shells in collections:
