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The gastropod shell is a shell which is part of the body of a gastropod or snail. It is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage. In land snails the shell is an essential protection against the sun, and against drying out.
The gastropod shell has several layers, and is typically made of calcium carbonate precipitated out into an organic matrix known as conchiolin. The shell is secreted by a part of the molluscan body known as the mantle.
Not all gastropods have a shell, but the majority do. The shell is in one piece, and is typically spirally coiled, although some groups, such as the various different families and genera of limpets, have simple cone-shaped shells as adults.
The study of mollusc shells including gastropod shells is called conchology.
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Chirality in gastropods
Because coiled shells are asymmetrical, they possess a quality called chirality, the "handedness" of an asymmetrical structure.
By far the majority of gastropod shells are dextral (right-handed) in their coiling, but a small minority of species and genera are virtually always sinistral (left-handed), and a very few species (for example Amphidromus perversus[1]) show an even mixture of dextral and sinistral individuals.
In species that are almost always dextral, very rarely a sinistral specimen will be produced, and these oddities are avidly sought after by some shell collectors.
If you hold a coiled gastropod shell with the aperture down and the spire pointing upwards, 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 often 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 and abnormal sinistral one.
The chirality in gastropods appears in early cleavage (spiral cleavage) and the gene NODAL is involved.[2]
Forming the gastropod shell
Morphology
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. |
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Gastropod shell morphology is usually quite constant among individuals of a species, and with exceptions, fairly constant among species within each family of gastropoda. Controlling variables are:
- The rate of growth per revolution around the coiling axis. High rates give wide-mouthed forms such as the abalone, low rates give highly coiled forms such as Turritella or some of the Planorbidae.
- The shape of the generating curve, roughly equivalent to the shape of the aperture. It may be round, for instance in the turban shell, elongate as in the cone shell or have an irregular shape with a siphonal canal extension, as in the Murex.
- The rate of translation of the generating curve along the axis of coiling, controlling how high-spired the resulting shell becomes. This may range from zero, a flat planispiral shell, to nearly the diameter of the aperture.
- Irregularities or "sculpturing" such as ribs, spines, knobs, and varices made by the snail regularly changing the shape of the generating curve during the course of growth, for instance in the many species of Murex.
- Ontologic growth changes as the animal reaches adulthood. Good examples are the flaring lip of the adult conch and the inward-coiled lip of the cowry.
Some of these factors can be modeled 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 adapted 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 and Terebra, are smooth, elongated, and lack elaborate sculpture, in order to decrease resistance when moving through sand.
A few gastropods, for instance the Vermetidae, cement the shell to, and grow along, solid surfaces such as rocks, or other shells.
 Apertural view of shell of Valvata sincera |
 Abapertural view of shell of Valvata sincera |
 Basal or umbilical view of shell of Valvata sincera |
 This dorsal view of the living animal Calliostoma bairdii also shows the dorsal view of its shell |
Description
The shell begins with the minute embryonic whorls of the protoconch, which is often quite distinct from the rest of the shell. From the protoconch, which forms the apex of the spire, the coils or whorls of the shell gradually increase in size. Normally the whorls are circular or elliptical in section, but from compression and other causes a variety of forms can result. The spire can be high or low, broad or slender according to the way the coils of the shell are arranged, and the apical angle of the shell varyies accordingly. The whorls sometimes rest loosely upon one another (as in Epitonium scalare). They also can overlap the earlier whorls such that they may be largely or wholly covered by the later ones. When an angulation occurs, the space between it and the suture above it constitutes the area known as the "shoulder" of the shell. The shoulder angle may be simple or keeled, an may sometimes have nodes or spines.
The most primitive sculpture of the gastropod shell consists of revolving ridges or spirals, and of transverse folds or ribs. Primary spirals appear in regular succession on either side of the first primary, which generally becomes the shoulder angle if angulation occurs. Secondary spirals 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 this 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. These simple varices may project from the shell (Epitonium) or be reflected backwards (Harpa). Periodic enlargements of ribs (Murex, Cerithium) are not to be classed as varices.
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 below 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 (chiefly old age) types, 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 lira are sometimes found, and these occasionally may be strong and tooth-like (Nerinea). Similar ridges or columellar plicce 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).
Most modern shells are covered by a horny smooth or hairy epidermis or periostracum, which sometimes hides the (often brilliant) color markings of the surface. The periostracum, as well as the coloration, is rarely preserved in fossil shells.
The apertural end of the gastropod shell is the anterior end, the apex of the spire the posterior. Most authors figure the shells with the apex of the spire uppermost. French authors generally figure them with the anterior end uppermost. The aperture is often closed by a homy or calcareous operculum, of very variable form in the different groups. It is secreted by and attached to the foot of the animal.[3]
Parts of the gastropod shell
- Periostracum: a thin layer of organic "skin" which forms the outer layer of the shell of many species
- Protoconch: the larval shell, often remains in position even on an adult shell
- Apex: the smallest few whorls of the shell
- Spire: the part of the shell that protrudes above the body whorl
- Whorl: each one of the complete rotations of the shell spiral
- Body whorl: the largest whorl in which the main part of the viseral mass of the mollusk is found
- Aperture: the opening of the shell
- Peristome: the part of the shell that is right around the aperture
- Columella: the "little column" at the axis of revolution of the shell
- Umbilicus: in shells where the whorls move apart as they grow, on the underside of the shell there is a deep depression reaching up towards the spire; this is the umbilicus
- Sculpture: ornamentation on the outer surface of a shell
- Varix: on some mollusk shells, spaced raised and thickened vertical ribs mark the end of a period of rapid growth; these are varices
- Operculum: the "trapdoor" of the shell
- Siphonal canal: an extension of the aperture in certain gastropods
- Parietal callus: a ridge on the inner lip of the aperture in certain gastropods
- Suture: The junction between whorls of most gastropods
Dimensions
The most frequently used measurements of a gastropod shell are: the height of the shell, the width of the shell, the height of the aperture and the width of the aperture. The number of whorls is also often used.
The largest height (also known as length) of any shell is found in the marine snail species Syrinx aruanus, which can be up to 91 cm.[4]
Evolutionary changes
The shell of some gastropods have been reduced or partly reduced during the evolution. This reduction can be seen in all slugs, in semi-slugs and in various other marine and non-marine gastropods. Sometimes the reduction of the shell is associated with predatory way of feeding.
Some taxa even lost the coiling of their shell during evolution.[5] 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.[5]
Gastropod shells showing variety of forms
 Turritella communis, many-whorled shell of tower snail |
 X-ray image of Turritella |
 Shell of marine cowry snail - Cypraea nebrites |
 X-ray image of Cypraea |
 X-ray image of the shell of Tonna galea |
 Planispiral shell of freshwater operculate snail - Marisa cornuarietis |
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References
This article incorporates public domain text from reference [3] .
- ^ Amphidromus perversus (Linnaeus, 1758)
- ^ PZ Myers. (April 13) 2009. Snails have nodal!. The Panda's Thumb, accessed 3 May 2009.
- ^ a b Grabau A. W. & Shimer H. W. 1909. North American Index Fossils Invertebrates. Volume I.. A. G. Seiler & Company, New York. pages page 582-584.
- ^ 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.
- ^ a b 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. doi:10.1098/rspb.2003.2517.
External links
- Gastropods by J. H. Leal - Information on some gastropods of the tropical Western Atlantic, specifically the Caribbean Sea, with relevance to the fisheries in that region
- Radiocarbon Dating of Gastropod Shells
- Nair K. K. & Muthe P. T. 1961. Effect of Ribonuclease on Shell Regeneration in Ariophanta sp. Nature 192, 674 - 675 (18 November 1961). doi:10.1038/192674b0.
- (Spanish) Antonio Ruiz Ruiz, Ángel Cárcaba Pozo, Ana I. Porras Crevillen & José R. Arrébola Burgos Caracoles Terrestres de Andalúcia. Guía y manual de identificación. 303 pp., ISBN 84-935194-2-1. (from website)
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