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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. It 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.
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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 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.
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
Ideal 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.
Certain shell shapes are found in certain environments, though there are many exceptions. Wave-washed high-energy environments are inhabited by snails with a wide aperture, a relatively low surface area, and a high growth rate per revolution. High-spired and highly sculpturd forms become more common in quiet water environments. Burrowing forms such as the olive and terebra shells are smooth and lack sculpture to decrease resistance to moving in sand. A few gastropods, for instance the vermetidae do not grow a coiled shell to cary as they move about, but instead cement themselves to and grow along the surface of a rock.
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
- lira: one kind of shell sculpture
- plait: another kind of shell sculpture
- 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
A selection of different kinds of gastropod shells
 Planispiral shell of freshwater operculate snail - Marisa cornuarietis |
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 Shell of marine cowry snail - Cypraea nebrites |
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See also
External links
- Gastropods by J. H. Leal - guide to shell morphology, terminology and sea species of Central America
