One Snail Species Prefers to Do It Anti-Chiral

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 et al. 2007 report in the latest Journal of Evolutionary Biology (open access!) that one disgusting species of snail actually selects for anti-chiral mates (Fig. 3b, d to the right reproduced under the Creative Commons Deed, Attribution 2.5). 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."-Schilthuizen et al. 2007 (references removed)

So typically is a pretty rare thing and it usually dies 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 (evilleft-opening) individuals. They keep their damned "whole-body chiral dimorphism", but why would a snail do such a thing? Schilthuizen et al. 2007 proposed a series of hypotheses that 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 2 separate populations of Amphidromus inversus, with the same predation frequency. Though the dextral morph was slightly less than 50% (~35%) of the population, predation was the same hence hypothesis 1 can be ruled out (i.e. no increase in predation frequency with the more frequent evilsinistral morph). Additionally, a PCA analysis using 10 biometric features shows there is 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, habitat preference, and molecular data shows they are 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 (Fig 3f to the left, reproduced under the Creative Commons Deed, Attribution 2.5). 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 fro this study onA. inversus is consistent with. Still waiting for the cool part?? See below.

Fig. 5 reproduced in its beautiful entirety under the... you get the drift. These pictures are of the spermatophore. Important to note the spiral tip.

Oh yeah, thats one mighty spermatophore, 60mm 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)."-Schilthuizen et al. 2007, italics my own.

Its all about the fit not the size... So what's the 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