In humans, about 10% of us are left-handed. It used to be thought that this was a uniquely human phenomenon – for example, there is no conclusive evidence for genuine handedness in chimpanzees – but surprisingly, it seems that most kangaroos are left-handed! Or to put it more precisely, Eastern grey kangaroos and red kangaroos appear to show a preference for using their left forelimbs when doing everyday tasks such as grooming and grazing!
Our left and right hands are mirror images of each other. Which means they are chiral objects. Chirality is a form of asymmetry, where two objects are identical in every way, except that they are mirror images of each other. You can tell whether something is chiral if it cannot be superimposed onto its mirror image (below). Left and right hands each have a thumb and fingers in the same order, but they are not the same (below).
Chiral molecules in nature also have the same things attached in the same order, but are mirror images, and so are not the same as each other. They are called enantiomers. At first sight, you might not think that it made much difference? The molecules have the same molecular formula and the same number of atoms of each chemical element. The critical factor is however, that they are arranged differently in space (just like your hands). That is makes a vital difference. The classic example is the molecule known as thalidomide. One enantiomer caused the desirable sedative effects, while the other caused birth defects. A tragic error.
Another interesting example of an enantiomer in nature, is the pheromone known as frontalin, which is found male Asian elephants (and insects, but that is another story!). It turns out that that it causes sexual activity and aggression in elephants (a state called musth). The molecule exists in two chiral forms ( mirror images), the ratios of which change as the elephants mature. So here we have a subtle balance of two enantiomers causing different responses in both male and female elephants!
Which brings us to snails! Snails are said to be the only animal group to ordinarily produce mirror-image forms! (Davison, 2020). Hold a snail with the tip of its shell pointing upwards and the opening towards you, and the shell will normally coil away to the right (dextral). Less commonly, it will coil to the left (sinistral). The words sinistral and dextral, are derived from the latin words for “left” (sinister) and “right” (dexter).
So snails can come in two enantiomorphs, but generally speaking, sinistral (coiling counter-clockwise) snails are rare. Sometimes extremely rare. However, there are some families which regularly produce left-handed shells, as an article in the British Shell Collectors’ Club explains. The easiest ‘lefties’ or ‘sinistral freaks’ to find, reportedly belong to the genus Physa (family Physidae) and are commonly known as tadpole or bladder snails (below).
There is a bay in South Africa, Jeffreys Bay, near Port Eizabeth, where sinistral cowries get washed up on a regular basis! There’s even a shell museum there! Sinistral cowries are very rare. A certain professor Franz Alfred Schilder, having examined “more than 150,000 cowries” confidently predicted that “nobody will find a sinistral cowry” (Schilder, 1964 in Lee, 2010). But in 1967, somebody found one in Australia, and soon they were popping up all over the place, especially on the beaches of Jeffreys and Algoa Bays, where more than two dozen sinistral cypraeids have now been found. Here’s a photo of a beautiful Toothless Cape Cowrie from South Africa.
Left-handed shells are particularly rare in marine ecosystems for some reason, but left-handedness is more common in freshwater and land pulmonates, as these beautiful land snail shells for Vietnam (below) illustrate. Amphidromus species are one of the few species with definite sinistral and dextral specimens occurring in mixed populations (Gittenberger et al., 2012). The photographers who took these photographs, Claude & Amandine EVANNO, have the most amazing collection of photographs on their Flick website, here.
In the UK, the most easily obtained sinistral land snails apparently belong to the family Clausiliidae (the door snails). These door snails really are unusual, because most of them are left-handed.
There can be distinct advantages for a snail in coiling in a counter-clockwise direction, the so-called sinistral advantage! For example, that rare left-handed coiling promotes survival from attacks by right-handed crabs, as described here. The crabs (Clappa species) are used to cracking open dextral shells; they do it ion a certain way. But when confronted with a left-handed snail, they fiddle and fumble around and find it difficult to insert their right pincer into the aperture to break the shell! A right-handed crab often abandons a left-handed snail before it breaks the shell. The authors point out that left-handers (southpaws for example) occasionally enjoy an advantage over their right-handed opponents in human sports like boxing and fencing. Another example, is a Japanese snail-eating snake – Iwasaki’s snail-eater, Pareas iwasakii – which has a problem grasping sinistral snails with its mouth! (Hoso et al., 2010).
So there may be advantages in being sinistral, but there is one big disadvantage: dextral and sinistral snails have difficulty in mating. Copulation between dextral (clockwise coiled) and sinistral (counter clockwise coiled) snails is impeded by the fact that that both their genitalia, and their behaviour is mismatched. They are just not made for each other! They are reversed in terms of the genital position on their bodies, as well as in their coiling direction.
Jeremy the left-handed snail made headlines all around the world in 2017. A rare (‘one in a million’) left-coiling snail garden snail (below), the story is told here and here, and in told in this video. Jeremy needed a left-handed mate who could cope with his sinistral ways. After a Europe-wide search, two other sinistral individuals were found – Lefty and Tomeu – but alas, they found each other more attractive than Jeremy! Eventually however, Jeremy did get a chance to mate with the Tomeau, the Spanish left-coiler, and they produced a batch of 56 baby snails. The offspring were all dextral though: right coilers. This is the default position in snails.
One of the best things that came out of this story though, was a delightful song written by Lydia Hiller (below). She had heard the story, and in what must have been a burst of inspired creativity, came up with this lovely ballad. This song is terrific! I’m surprised it did not become a No. 1 hit. The lyrics are just so clever!
“For those not in the know of the lives of escargot. This is a rare and unfortunate trait, ‘Cause the way that our shells twist determines where our bits exist, Which sadly means I cannot procreate.”
Lydia went on to write a eulogy for Jeremy the snails (below). I hope she goes on to write more songs, as she is real talent.
In July 2018, the research team at the University of Nottingham announced the arrival of St Stephen, a ‘lefty’ snail of the species Cepaea nemoralis – “the only lefty grove snail in the whole of Europe, perhaps the world” – and stated in a tweet that they were looking for potential mate. I took this photograph of a dextral one in Spain (below).
So what have we learnt? Left-handed snails can be incredibly rare, although there are some species where they are more common, for reasons of selection or survival. People value their rarity, and more importantly identify with their oddity.
Links
References
M. Abe, R. Kuroda, “The development of CRISPR for a mollusc establishes the formin Lsdia1 as the long-sought gene for snail dextral/sinistral coiling,” Development, doi:10.1242/dev.175976, 2019.
Davison, A. (2020). Flipping Shells! Unwinding LR Asymmetry in Mirror-Image Molluscs. Trends in Genetics, 36(3), 189-202.
Dietl, G. P., & Hendricks, J. R. (2006). Crab scars reveal survival advantage of left-handed snails. Biology Letters, 2(3), 439-442.
Giljov, A., Karenina, K., Ingram, J., & Malashichev, Y. (2015). Parallel emergence of true handedness in the evolution of marsupials and placentals. Current Biology, 25(14), 1878-1884.
Gittenberger E, Hamann TD, Asami T (2012) Chiral Speciation in Terrestrial Pulmonate Snails. PLoS ONE 7(4): e34005. https://doi.org/10.1371/journal.pone.0034005
Hoso, M., Kameda, Y., Wu, S. et al. A speciation gene for left–right reversal in snails results in anti-predator adaptation. Nat Commun 1, 133 (2010). https://doi.org/10.1038/ncomms1133
Lee, H.G. (2010b). Absence of evidence is not evidence of absence, or never say “never.” American Conchologist 38(3): 6-9. September.. Available from: https://www.researchgate.net/publication/303873800_Lee_HG_2010b_Absence_of_evidence_is_not_evidence_of_absence_or_never_say_never_American_Conchologist_383_6-9_September [accessed Oct 25 2020].
Marchant, L. F., & McGrew, W. C. (2013). Handedness is more than laterality: lessons from chimpanzees. Annals of the New York Academy of Sciences, 1288(1), 1-8.
Ray Cannon's nature notes 