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 Common door snail (Alinda biplicata). Picture: Jiří Novák, biolib.cz. |
Door snails are a family of terrestrial pulmonate snails (Stylommatophora) living on tree trunks, walls and rocks, where they can be seen crawling around in humid weather. Their outward appearance differs widely from that customary for snails, as door snails' shells are form a round spiral, but are coiled into a tower-like spire, albeit a very small one – door snails usually are not larger than 20 mm.
 Common door snail (Alinda biplicata). [RN, biolib.cz] |
The largest Central European species, the ventricose door snail (Macrogastra ventricosa), is only little smaller than that, with an average shell height of 19 mm. In contrary to that the smallest species, adequately named Clausilia rugosa parvula, has got a shell of averagely 9,5 mm height.
In addition to their characteristic form, door snails' shells are also set apart from those of most other snails in usually being sinistral or left handed.
Door snails feed on algae, which they rasp from the ground using their radula. Like other terrestrial pulmonates they are hermaphrodites. There are, though, some door snails, such as the Central European common door snail (Alinda biplicata), which are ovoviviparous. That means, oviposition is delayed long enough for the young snails to hatch inside the parent's body and to be born alive. Alternatively sometimes also eggs with far developed embryos are laid.
Their characteristic shell means a decisive evolutionary advantage to door snails: During periods of dryness they are able to hide in the smallest of crevices in bark or rock. This is especially important where the largest number of different door snail species is distributed: On the Balkan peninsula, mainly in Greece, additionally also in Asia Minor (Turkey) and in the Caucasus mountains. Apart from that door snails have a palaearctic pattern of distribution.
Externally many door snail species are hard to tell apart. But enhanced by a magnifying glass or a binocular microscope differences in the surface sculpture of elsewhere similar species reveal themselves. There are, on one hand, species with an almost smooth shell surface, such as Cochlodina laminata. On the other hand there are many of the remaining door snail species, different by a variety of ribs running across the shell whorls. The density and form of those ribs makes it possible to determine different species. As the first picture on the top of this page shows, in nature this surface of ribs can however be hidden under a camouflage of dirt or faeces.
 Common door snail (Alinda biplicata). Picture: Jiří Novák, biolib.cz. |
Besides form and sculpture of the shell, also the shell aperture can be used for determination purposes. There are round, oval and even pear shaped apertures among door snails.
 Closing apparatus (Clausiliar) of a common door snail (Alinda biplicata). Picture: Jiří Novák, biolib.cz. |
Looking more closely and with sufficient magnification at a door snail's shell aperture, many species reveal an apertural armature with folds appearing as toothlets to the outside. According to their position, the following major folds are distinguished: The palatal fold is located at the aperture whorl's external wall. On the opposite, columellar wall there is the lower or columellar fold. Above, on the upper wall of the apertural whorl, there is the upper or parietal fold. Finally below the columellar fold there may be an additional subcolumellar fold.
The mentioned folds, which on the parieto-columellar side are also referred to as lamellae, are part of the so-called clausiliar, a closing apparatus unique among all snails, the reason to this particular snail family's name. The overall composition of a door snail's clausiliar, however, can only be seen, when the outer wall of the apertural whorl is opened with a set of tweezers and observed through a binocular microscope.
Among most door snail species then a plate from calcium carbonate can be discovered, shaped like a spoon the elastic handle of which is connected to the shell spindle (columella). This clausilium is pressed to the shell's outer wall, when the snail is stretched out of the shell and crawling around. As soon as the snail withdraws into its shell, the clausilium swings forward and closes the aperture.
It is not fully clear, what the determining evolutionary advantage of the clausiliar has been to door snails. It seems highly probable, though, that it means an additional protection against dryness, given door snails' distribution centre around the Balkan peninsula and Asia Minor.
 Door snails (Alinda biplicata?), a juvenile specimen in the back- ground. [RN] |
Scientifically door snails are of very high interest especially because quite numerous species occur in a relatively small area. This is because single populations occur even on single rocks separated though dry areas exposed to the sun. Due to this isolation there can be allopatric speciation leading to the development of new species. There is also the possibility of sympatric speciation due to bastardization between closely related species.
Especially then characters of the shell and the composition of the clausiliar apparatus may not suffice to determine a species. Then an anatomical examination of the genital apparatus, as described on clausilia.de, becomes necessary. On that homepage there is also further information on collecting and treating material for such examinations.
Facing the distribution of door snail species, which may cover only very small areas, human influence may have a very high importance, as, similar to the situation of endemic island snail populations, changes due to agriculture, road construction etc. can cause sudden unpredictable and irreversible harm.