Homies Series

Gastropod shell

Chirality in gastropods

Shells of two species of sea snail: on the left is the sinistral shell of Neptunea angulata, on the right is the dextral shell of Neptunea despecta

Because coiled shells are asymmetrical, they possess a quality called chirality, the "handedness" of an asymmetrical structure.

By far the majority (over 90 %) of gastropod species have dextral (right-handed) shells 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) 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 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 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.

Formation of the shell

Main articles: Mantle (mollusc) and Mollusc shell

Morphology

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.

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.

Parts of the shell

A drawing of an abnormal scalarid form of the shell of the garden snail, Helix aspersa

The terminology used to describe the shells of gastropods includes:

Aperture: the opening of the shell

Apex: the smallest few whorls of the shell

Body whorl: the largest whorl in which the main part of the viseral mass of the mollusk is found

Columella: the "little column" at the axis of revolution of the shell

Operculum: the "trapdoor" of the shell

Parietal callus: a ridge on the inner lip of the aperture in certain gastropods

Periostracum: a thin layer of organic "skin" which forms the outer layer of the shell of many species

Peristome: the part of the shell that is right around the aperture

Protoconch: the larval shell, often remains in position even on an adult shell

Sculpture: ornamentation on the outer surface of a shell

Lira: one kind of shell sculpture

Plait: another kind of shell sculpture

Siphonal canal: an extension of the aperture in certain gastropods

Spire: the part of the shell that protrudes above the body whorl

Suture: The junction between whorls of most gastropods

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

Varix: on some mollusk shells, spaced raised and thickened vertical ribs mark the end of a period of rapid growth; these are varices

Whorl: each one of the complete rotations of the shell spiral

Apertural view of the shell of the sea snail Syrinx aruanus, the species which has the largest shell of any living gastropod

Schematic representation of the apical, apertural and basal view of a shell, showing 14 different commonly used measurements. Dotted lines represent the orientation axes.

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.

Evolutionary changes

Among proposed roles invoked the variability of shells during evolution include mechanical stability, defence against predators, sexual selection and climatic selection.

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. 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.

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

Charonia

Murex pecten

Planispiral shell of freshwater operculate snail - Marisa cornuarietis

Shell and live animal of edible land pulmonate snail - Helix pomatia

Shell of marine turban snail showing operculum - Turbo chinensis

Shell of marine limpet, probably a Patella species

References

^ Schilthuizen M. & Davison A. (2005). "The convoluted evolution of snail chirality". Naturwissenschaften 92(11): 504515. doi:10.1007/s00114-005-0045-2.

^ Amphidromus perversus (Linnaeus, 1758)

^ Myers P. Z. (13 April 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.

^ Britton J. C (1995) "The relationship between position on shore and shell ornamentation in 2 size-dependent morphotypes of Littorina striata, with an estimate of evaporative water-loss in these morphotypes and in Melarhaphe neritoides". Hydrobiologia 309: 129-142. abstract.

^ Wilson A. B., Glaubrecht M. & Meyer A. (March 2004) "Ancient lakes as evolutionary reservoirs: evidence from the thalassoid gastropods of Lake Tanganyika". Proceeings of Royal Society London Series B - Biological Sciences 271: 529-536. doi:10.1098/rspb.2003.2624.

^ Schilthuizen M. (5 June 2003) "Sexual selection on land snail shell ornamentation: a hypothesis that may explain shell diversity". BMC Evolutionary Biology 3: 13. doi:10.1186/1471-2148-3-13.

^ Goodfriend G. A. (1986) "Variation in land-snail shell form and size and its causes a Review". Systematic Zoology 35: 204-223.

^ a b Pfenninger M., Hrabkov M., Steinke D. & Dpraz A. (4 November 2005) "Why do snails have hairs? A Bayesian inference of character evolution". BMC Evolutionary Biology 5: 59. doi:10.1186/1471-2148-5-59

^ a b Collin R. & Cipriani R. (22 December 2003) "Dollo law and the re-evolution of shell coiling". Proceedings of the Royal Society B 270(1533): 2551-2555. doi:10.1098/rspb.2003.2517.

Further reading

Wandelt J. & Nagy L. M. (24 August 2004) "Left-Right Asymmetry: More Than One Way to Coil a Shell". Current Biology 14(16): R654-R656. doi:10.1016/j.cub.2004.08.010

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. (18 November 1961) "Effect of Ribonuclease on Shell Regeneration in Ariophanta sp.". Nature 192: 674-675. doi:10.1038/192674b0.

(Spanish) Antonio Ruiz Ruiz, ngel Crcaba Pozo, Ana I. Porras Crevillen & Jos R. Arrbola Burgos Caracoles Terrestres de Andalcia. Gua y manual de identificacin. 303 pp., ISBN 84-935194-2-1. (from website)

Categories: Gastropods | Mollusc anatomy
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