THE HANDEDNESS OF SEASHELLS

All snails (shelled gastropods) possess shells which spiral with a certain sense or handedness. This renders them ‘chiral’ objects, which means that they lack a plane of symmetry and that their mirror images are not superimposable. The most obvious example of chiral objects are our hands. Indeed, the term chiral is derived from the Greek word for hands (kheir). We tend to think of them as identical, however, each of our hands is the mirror image of the other, and they are not identical. One only has to try putting on a left glove on your right hand to demonstrate this.

The vast majority of marine gastropods possesses shells which spiral in a single direction, referred to as ‘dextral’ [ i.e. the spiral grows in a clockwise direction from the spire (tip) to the aperture (opening) of the shell ]. For nearly all marine species, all of the individuals spiral in this dextral manner. The opposite direction of spiraling is termed ‘sinistral’. There do exist a very small number of sinistral species including Neptunea contraria and Sinistrofulgur contrarium.

In principle, all normally dextral species have the potential of generating mutant, sinistral offspring. Such individuals are often not viable, or the asymmetry of their sexual organs precludes successful mating with a dextral partner, so the sinistral trait ends at this point. It is presumed that the handful of sinistral species which currently exist stemmed from the fortuitous encounter of two sinistral individuals which established a viable community that eventually led to the new species. Some species are more prone to having sinistral adult mutants than others (eg. Cryptospira ventricosa), however, such freak specimens of most gastropod species are usually extremely rare and command very high prices.

Finally, a wonderful example of how a mirror image is not necessarily identical to its original is provided by an error committed by the Dutch master, Rembrandt van Rijn. An avid shell collector himself, his engraving of a Conus marmoreus is a lovely depiction, however, the print shows a sinistral shell. All living Conus are dextral (see the Conidae section). The artist understood that his signature would have to be engraved backwards to appear correct in the print, however, he failed to realize that the same applies to the shell!

Chirality in Nature

Asymmetry is a common occurrence in nature, so many instances of chirality exist, from rams’ horns to climbing vines. My interested in the handedness of shells stemmed, surprisingly, from my work as an organic chemist. Countless molecules, notably those produced by biological systems (sugars, amino acids/proteins, DNA) also exhibit chirality. This molecular asymmetry arises from the tetrahedral geometry of a saturated carbon atom, and the discovery of this phenomenon is arguably Louis Pasteur’s greatest contribution to knowledge. Indeed, much of the field of organic synthesis has been devoted to finding methods of preparing molecules of a single handedness (or ‘stereochemistry’) as opposed to a mixture of both.

Landsnails

A group of gastropods which stands out in terms of handedness are a family of terrestrial mollusks (or landsnails). These are members of the genus Amphidromus which roughly translates to ‘running on both sides’. As opposed to the vast majority of gastropod species that normally exist as populations exclusively of a particular handedness, many species of Amphidromus exist as both dextral and sinistral individuals within mixed populations. Apart from the novelty of having individuals of either chirality, they are often brightly coloured and adorned with attractive patterns, making them quite popular among collectors. They are native to tropical jungles/forests throughout southeast Asia.

Unlike the case of marine species, those that are sinistral are far from rare among landsnails. Although Amphidromus are clearly the most prone to having mixed populations, this phenomenon occurs in other terrestrial snails including some members of the genus Liguus, also popular among collectors.

BELOW: GALLERY OF DEXTRAL/SINISTRAL PAIRS

BELOW: GALLERY OF OTHER TERRESTRIAL GASTROPODS