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The way a shell coils is described as its chirality.

The way a shell coils is described as its chirality. Photo: iStockphoto

Question

Do all shells coil the same way?

Answer

Seashells are from gastropods or snails, a kind of mollusc. Much like garden snails, marine snails use their shells as protection from predators, damage and the sun.

The way a shell coils is described as its chirality. Chirality is the way an object like the hand is not identical to its mirror image. Just like there are left-handed and right-handed people, there can be left-handed and right-handed shells.

The shell is largely made from calcium carbonate, like eggshells or some blackboard chalk. A snail shell is built in layers, usually in a coiling spiral.

You can tell how quickly a shell has been made by looking at how big the opening, or aperture, of the shell is. A big aperture, like that of an abalone, has grown very quickly. A tightly coiled shell, with a small aperture, has grown slowly.

The aperture of the shell is essential to identifying the chirality, or handedness. If you pick up a coiled shell and hold it with the pointy end upright, the direction in which the aperture faces indicates its chirality.

If the aperture is on the right-hand side then the shell is right-handed or 'dextral'. If the aperture is on the left then the shell is left-handed, or 'sinistral'.

In most snails, shells are dextral. A smaller number are sinistral and in rare cases you can have some species with a mix of dextral and sinistral shells.

For collectors of seashells, chirality can be one factor that influences the value of the shell. Finding a sinistral specimen of a largely dextral species is a collector's dream!

However being a chiral rarity isn't so good for a snail's love life. If a snail coils the opposite way to all the others, its genitalia is also reversed making it very hard to reproduce.

■ Response by Dr Merryn McKinnon, Australian National Centre for the Public Awareness of Science at the ANU, and lapsed marine biologist.

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