Product Code Database
The gastropod shell is part of the body of many gastropods, including snails, a kind of mollusc. The shell is an exoskeleton, which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium storage. Some gastropods appear shell-less () but may have a remnant within the mantle, or in some cases the shell is reduced such that the body cannot be retracted within it (semi-slug).
Some snails also possess an operculum that seals the opening of the shell, known as the aperture, which provides further protection. The study of is known as conchology. The biological study of gastropods, and other molluscs in general, is malacology. Shell morphology terms vary by species group.
| Morphology of typical spirally coiled shell. The shell of Zonitoides nitidus, a land snail, has dextral coiling. Upper image: Dorsal view, showing whorls and apex Central image: Lateral view showing the profile of the shell Lower image: Basal view showing umbilicus in the centre. | Photo of the shell of Zonitoides nitidus with an apical view, an apertural view and a basal view |
Gastropod shell morphology is usually quite constant among individuals of a species. Controlling variables are:
Some of these factors can be modelled mathematically and programs exist to generate extremely realistic images. Early work by David Raup on the analog computer also revealed many possible combinations that were never adopted by any actual gastropod.
Some shell shapes are found more often in certain environments, though there are many exceptions. Wave-washed high-energy environments, such as the rocky intertidal zone, are usually inhabited by snails whose shells have a wide aperture, a relatively low surface area, and a high growth rate per revolution. High-spired and highly sculptured forms become more common in quiet water environments. The shell of burrowing forms, such as the Olive snail and Terebra, are smooth, elongated, and lack elaborate sculpture, in order to decrease resistance when moving through sand. On land, high-spired forms are often associated with vertical surfaces, whereas flat-shelled snails tend to live on the ground.
A few gastropods, for instance the Vermetidae, cement the shell to, and grow along, solid surfaces such as rocks, or other shells.
If a coiled gastropod shell is held with the spire pointing upwards and the aperture more or less facing the observer, a dextral shell will have the aperture on the right-hand side, and a sinistral shell will have the aperture on the left-hand side. This chirality of gastropods is sometimes overlooked when photographs of coiled gastropods are "flipped" by a non-expert prior to being used in a publication. This image "flipping" results in a normal dextral gastropod appearing to be a rare or abnormal sinistral one.
Sinistrality arose independently 19 times among marine gastropods since the start of the Cenozoic.Geerat Vermeij (2002). "The Geography of Evolutionary Opportunity: Hypothesis and Two Cases in Gastropods". Integrative and Comparative Biology 42(5): 9359–40. . This left-handedness seems to be more common in freshwater and land pulmonates. But still the dextral living species in gastropods seem to account for 99% of the total number.Asami, T. (1993) "Genetic variation of coiling chirality in snails". Forma, 8, 263–276
The chirality in gastropods appears in early cleavage (spiral cleavage) and the gene NODAL is involved.Myers P. Z. (13 April 2009) "Snails have nodal!". The Panda's Thumb, accessed 3 May 2009. A more recent study (2013) correlates the asymmetric coiling of the shell by the left-right asymmetric expression of the decapentaplegic gene in the mantle.Keisuke Shimizu, Minoru Iijima, Davin HE SetiamargaIsao Sarashina, Tetsuhiro Kudoh, Takahiro Asami, Edi Gittenberger, Kazuyoshi Endo; "Left-right asymmetric expression of dpp in the mantle of gastropods correlates with asymmetric shell coiling." EvoDevo 2013, 4:15. .
A possible exception may concern some of the European Clausiliidae of the subfamily Alopiinae. They are obligatory living in isolated colonies on limestone outcrops. Several sets of species differ only in the direction of coiling, but the evidence is inconclusive as to whether left- and right-handed shells live together. Soos (1928, pp. 372–385) summarized previous discussions of the problem and concluded that the right- and left-handed populations were distinct species. Others have stated that these populations were not distinct, and the question is far from settled. The Peruvian clausiliid, Nenia callistoglypta Pilsbry (1949, pp. 216–217), also has been described as being an amphidromine species.
The genetics of reverse coiling in a rare dextral mutant of another clausiliid, Alinda biplicata (Montagu), has been studied by Degner (1952). The mechanism is the same as in Radix peregra (Müller), with the direction of coiling determined by a simple Mendelian recessive.
The most simple form of sculpture of the gastropod shell consists of longitudinal ridges, and/or transverse ridges. Primary spirals may appear in regular succession on either side of the first primary, which generally becomes the shoulder angle if angulation occurs. Secondary spirals may appear by intercalation between the primary ones, and generally are absent in the young shell, except in some highly accelerated types. Tertiary spirals are intercalated between the preceding groups in more specialized species. Ribs are regular transverse foldings of the shell, which generally extend from the suture to suture. They are usually spaced uniformly and crossed by the spirals. In specialized types, when a shoulder angle is formed, they become concentrated as nodes upon this angle, disappearing from the shoulder above and the body below. Spines may replace the nodes in later stages. They form as notches in the margin of the shell and are subsequently abandoned, often remaining open in front. Irregular spines may also arise on various parts of the surface of the shell (see Platyceras).
When a row of spines is formed at the edge or outer lip of the shell during a resting period, this feature sometimes remains behind as a varix as in ( Murex) and many of the Ranellidae. Varices may also be formed by simple expansion of the outer lip, and a subsequent resumption of growth from the base of the expansion.
The aperture or peristome of the shell may be simple or variously modified. An outer and an inner (columellar) lip are generally recognized. These may be continuous with each other, or may be divided by an anterior notch. This, in some types ( Fusinus, etc.) it is drawn out into an anterior siphonal canal, of greater or lesser length.
An upper or posterior notch is present in certain taxa, and this may result in the formation of a ridge or shelf next to the suture ( Clavilithes). An outer (lateral) emargination or notch, sometimes prolonged into a slit occurs in certain types (Pleurotomidae, Pleurotomaridae, Bellerophontidae, etc.), and the progressive closing of this slit may give rise to a definitely marked slit band. In some cases the slit is abandoned and left as a hole (Fissurellidae), or by periodic renewal as a succession of holes ( Haliotis). The outer emargination is often only indicated by the reflected course of the lines of growth on the shell.
On the inside of the outer lip, various ridges or plications called lirae are sometimes found, and these occasionally may be strong and tooth-like ( Nerinea). Similar ridges or columellar plicae or folds are more often found on the inner lip, next to the columella or central spiral twist. These may be oblique or normal to the axis of coiling (horizontal), few or numerous, readily seen, or far within the shell so as to be invisible except in broken shells. When the axis of coiling is hollow (perforate spire) the opening at the base constitutes the umbilicus. The umbilicus varies greatly in size, and may be wholly or in part covered by an expansion or callus of the inner lip ( Natica).
Many Holocene shells, when the animal is alive or the shell is freshly empty, have an uppermost shell layer of horny, smooth, or hairy epidermis or periostracum, a proteinaceous layer which sometimes is thick enough to hide the color markings of the surface of the shell. The periostracum, as well as the coloration, is only rarely preserved in fossil shells.
The apertural end of the gastropod shell is the anterior end, nearest to the head of the animal; the apex of the spire is often the posterior end or at least is the dorsal side. Most authors figure the shells with the apex of the spire uppermost. In life, when the soft parts of these snail are retracted, in some groups the aperture of the shell is closed by using a horny or calcareous operculum, a door-like structure which is secreted by, and attached to, the upper surface of the posterior part of the foot. The operculum is of very variable form in the different groups of snails that possess one.
The terminology used to describe the shells of gastropods includes:
The following are the principal modifications of form in the gastropod shell.George Washington Tryon, Structural and systematic conchology, 1882, pp. 43–44
Detailed distinction of the shape can be:Hershler R. & Ponder W. F.(1998). "A Review of Morphological Characters of Hydrobioid Snails". Smithsonian Contributions to Zoology 600: 1–55. .Dance P. S. Shells.
In this context, the height (or the length) of a shell is its maximum measurement along the central axis. The width (or breadth, or diameter) is the maximum measurement of the shell at right angles to the central axis. Both terms are only related to the description of the shell and not to the orientation of the shell on the living animal.
The largest height of any shell is found in the marine snail species Syrinx aruanus, which can be up to 91 cm.Wells F. E., Walker D. I. & Jones D. S. (eds.) (2003) " Food of giants – field observations on the diet of Syrinx aruanus (Linnaeus, 1758) (Turbinellidae) the largest living gastropod ". The Marine Flora and Fauna of Dampier, Western Australia. Western Australian Museum, Perth.
The central axis is an imaginary axis along the length of a shell, around which, in a coiled shell, the whorls spiral. The central axis passes through the columella, the central pillar of the shell.
The shells of some gastropods have been reduced or partly reduced during their evolution. This reduction can be seen in all , in semi-slugs and in various other marine and non-marine gastropods. Sometimes the reduction of the shell is associated with a predatory way of feeding.
Some taxa lost the coiling of their shell during evolution. According to Dollo's law, it is not possible to regain the coiling of the shell after it is lost. Despite that, there are few genera in the family Calyptraeidae that changed their developmental timing (heterochrony) and gained back (re-evolution) a coiled shell from the previous condition of an uncoiled limpet-like shell.Collin R. & Cipriani R. (22 December 2003) "Dollo's law and the re-evolution of shell coiling". Proceedings of the Royal Society B 270(1533): 2551–2555. .
== Variety of forms ==
This article incorporates public domain text from references,John. B. Burch (1962). "How to Know the eastern land snails; pictured-key for determining the land snails of the United States occurring east of the Rocky Mountain Divide" WM. C. Brown Company Publishers, Dubuque, Iowa. 214 pages. Laidlaw F. F. & Alan Solem (1961). . Fieldiana Zoology 41(4): 505–720.Grabau A. W. & Shimer H. W. (1909) North American Index Fossils . A. G. Seiler & Company, New York. pages page 582–584. and CC-BY-2.0 text from reference.Pfenninger M., Hrabáková M., Steinke D. & Dèpraz A. (4 November 2005) "Why do snails have hairs? A Bayesian inference of character evolution". BMC Evolutionary Biology 5: 59.
|
|
