The manner in which a snail shell coils holds a lot of intrigue for scientists. A gastropod shell generally exhibits spiral growth (chirality), coils tightly around a single axis clockwise (dextral) but some mutant members of the same species, which usually makes of only a small percentage of the population, do grow its shell anti-clockwise (sinistral).
To me, one of the enduring images of how scientists can be so “mean” to their lab animals, in their pursuit of scientific knowledge, is when they try to make these mirror-image snails copulate with each other: their shells’ asymmetry causes mechanical obstacles giving rise to difficult or even impossible mating, leaving the poor snails entwined, and huffing and puffing for days with unconsumated passion.
Now, standard evolutionary theory will tell you that only animals (and plants) that can mate and reproduce will populate the next generations of like-animals (and plants); mutants will make for only a small proportion of the population. And so it is for most snails found so far…until scientists found a species of air-breathing land snail, Amphidromus inversus, in the rainforests of Sabah. (To be fair, these snails are found in many parts of South-east Asia, but the study was carried out in Sabah.) What excites, and initially stumped the researchers was that both dextral and sinistral individuals of this species occur in roughly equal proportions (so-called ‘antisymmetry’) living in the same area, and thereby, on face value, running counter to known evolutionary theory.
As always, there is a under-lying reason of course. Apparently, the snails prefer to mate with their mirror-image partner. (These hermaphrodite snails do sex always simultaneously and reciprocally, with both partners donating and receiving a spermatophore at the same time. Wow!) So, while they do have to go through difficult acrobatics and contortions to mate, the scientists found that the comparatively better anatomical advantages (more spermatophores are deposited) of such inter-chiral matings produces greater fecundity, and thereby more offspring.
In short, in scientific lingo, “In Amphidromus, there is sexual selection for dimorphism, rather than selection for monomorphism.” The authors of the paper said this was a first confirmed case of heritable antisymmetry in Metazoans (higher animals).