Free Spirals in Time: The Secret Life and Curious Afterlife of Seashells by Helen Scales Page A

coiling direction, artists would commonly etch what they saw into metal plates; their shells would then become reversed as mirror images in the printing process. In Rembrandt’s case, though, it’s thought he reversed his shell on purpose, for aesthetic reasons: he just felt it looked better that way. Pleasingly, other artists who copied Rembrandt’s cone did so directly and faithfully, without thinking to reverse the etching, so these printed shells were restored to their rightful place as right-coilers.
    The abundance of right-coiling shells in the natural world, and lack of left-coilers, comes down to one simple but inescapable truth: if right- and left-coiling snails try to mate, their genitals don’t match. Not only are shells coiled one way or another but the rest of the snail’s body is also asymmetrical. Female snails have a genital pore offset to one side into which a male will inject sperm through his penis. Most gastropods in the oceans have separate sexes – they are shes and hes; land snails are commonly hermaphrodites, each one with both bits of equipment, but they will pair up and take turns being male and female. Face-to-face is a popular position for snail sex, and for this to work it’s crucialfor the female pore and male penis to overlap: this only happens if both snails coil in the same direction (a little like when you go to shake someone’s hand – it only works if you both offer the same hand). The shells and bodies of left- and right-coiling species are mirror images of each other. Even the corkscrew-shaped penis of the Asian Trampsnail twists the other way in lefties, and the choreography of their circular mating dances is reversed. In a tryst between right- and left-coiling snails, everyone is confused, and everything is in the wrong place.
    To gauge just how much of a problem coiling direction is in mating molluscs, researchers place pairs of mismatched snails together in cosy containers. Roman Snails, known and eaten in France as escargots (and highly protected in England), are often used in these sorts of sex studies because most of them are right-coiling, but once in a while a lefty shows up. No matter how much the left-right partners are feeling in the mood, the slurp of a baby snail’s feet never issues from the mating cubicles.
    An alternative mating tactic adopted by some snails is for one to clamber up from behind on the shell of the other. Similar snail-in-a-box studies show that shell climbers have more success in crossing the left-right divide than face-to-facers, but things are still rather awkward. Far fewer offspring will result from a right-left union than from snails paired up with same-shelled partners.
    All of this means that for sinistral snails in a mostly dextral world, life can be lonely. It’s not that right-coiling shells are inherently any better than their left-coiling brethren, it’s really just a matter of chance. Whichever form is less abundant within a species will be less likely to find a matching mate and therefore not as successful at passing on its genes; this pushes a population towards one dominant coiling direction. It just happens that at the moment right-handed shells are most abundant and get the best chances to mate. But that hasn’t always been the case, and the fossil record shows that fashionscan change, although exactly why this happens remains a mystery. In The Natural History of Shells , Vermeij describes the eight or nine ancient groups of cephalopods that, through time, evolved right- and left-coiling shells, with no particular inclination towards twisting one way or the other.
    It is tempting to link the coiling of gastropod shells to the fact that when they are very young, their soft bodies also undergo a major twist. This process, torsion, is unique to the gastropods and involves all the major organs spinning around 180 degrees (clockwise in sinistral and anticlockwise in dextral shells). Among many things that move, the anus shifts to