Most snails walk the line and stick with doing the dirty deed missionary-style. No one even thinks about any of that kinky, low-down, unholy ways of propagating the world. But Schilthuizen and colleagues report in the Journal of Evolutionary Biology (open access!) that one disgusting species of snail actually selects for anti-chiral mates (Figure to the right). This scandalous sixty-nine of satan is even sexually selected for by spermatophore morphology! Just when I thought I heard it all.

Chirality in snails refers to what directions the coils run, clockwise or anti-clockwise (known as counter-clockwise in some localities).

“The cause of directional asymmetry is clear in only a few groups (Vermeij, 1975), such as in gastropods (snails). Their coiling direction (not only of the shell but also of the entire body organization), determined by a single locus of maternal effect, is usually fixed within a species because mating among D and S individuals (so-called ‘inter-chiral mating’) is either impossible or very difficult. Consequently, a rare reverse-coiled morph will normally not persist because of frequency-dependent selection.”

Typically, anti-chiral mating is a pretty rare thing and snails who do not keep in line usually die out of the population fairly quickly. But not satisfied with being a typical gastropod, members of the southeast asian tree snail genus Amphidromus do NOT, yes I repeat do NOT, deviate from a 1:1 ratio of dextral (right-opening) to sinistral (evil left-opening) individuals. They keep their damned “whole-body chiral dimorphism”, but why would a snail do such a thing? Schilthuizen and colleagues proposed a series of hypotheses which they tested:

1) Frequency-dependent selection countered by extrinsic balancing selection. Basically, one morph is present in higher number and is predated upon unto rare than other morph which is then, in turn predated on, etc.
2) Occupy separate niches
3) Dimorphism is maintained by intrinsic factors such as coiling genetics or reproduction.

They observed two separate populations of Amphidromus inversus with the same predation frequency. Though the dextral morph was less than half (~35%) of the population, predation was the same hence hypothesis 1 can be ruled out since there was no increase in predation frequency with the more frequent evil sinistral morph. Additionally, another analysis using 10 biometric features showed there was no difference between D and S morphs. They are in fact mirror images of each other, which suggests to the authors that they are ecologically equivalent. They provide other evidence from previous work led by the first author that both morphs have identical dispersal patterns and habitat preferences. Additionally, molecular data showed they were panmictic to argue against hypothesis 2.

Which brings us to hypothesis 3. To study inheritance of chirality, they grabbed all the egg bunches they could and waited till the juveniles hatched out (Figure to the left). Except for one clutch, all other clutches were invariant for coiling direction. In other snails, coiling direction has been found to be maternally transmitted from a single locus, which the data from this study on A. inversus is consistent with. Still waiting for the cool part? See below.

Fig. 5 showing the spermatophore. Important to note the spiral tip.

Oh yeah, that’s one mighty spermatophore, a 60 mm schlong that curves either to the right or the left. This curvature reflects the chirality of the shell.

“The oviduct is connected with the SRO [spermatophore receiving organ] at an angle to the left or to the right in a sinistral or a dextral recipient, respectively, whereas the spermatophore tail tip points to the left or right in a dextral or a sinistral donor respectively (viewed from the front of the recipient animal). Consequently, it can enter the oviduct more easily in inter- than in intra-chiral copulation (Fig. 5e–h).”

Its all about the fit not the size… So what is their conclusion? Why do I think this is a cool study?

“We have shown that mutual mate choice for a partner with mirror-image asymmetry stabilizes antisymmetry in these snails. As far as we are aware, this is the first confirmed case of heritable antisymmetry in the Metazoa.”-Schilthuizen et al. 2007

SCHILTHUIZEN, M., CRAZE, P., CABANBAN, A., DAVISON, A., STONE, J., GITTENBERGER, E., & SCOTT, B. (2007). Sexual selection maintains whole-body chiral dimorphism in snails Journal of Evolutionary Biology, 20 (5), 1941-1949 DOI: 10.1111/j.1420-9101.2007.01370.x