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.
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.
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:
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.
Reticulate phylogeny of gastropod-shell-breeding cichlids from Lake Tanganyika--the result of repeated introgressive hybridization.(Clinical report)
Jan 01, 2007; Abstract Background: The tribe Lamprologini is the major substrate breeding lineage of Lake Tanganyika's cichlid species flock....