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Most gastropods exhibit sinistral (right hand) winding of their shells. But very few species are anti sinistral. Have there been any theories as to why such a great difference?

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    $\begingroup$ "Sinistral" means left-handed. I believe most gastropod species have a dextral coil (which works out to be your intended meaning). $\endgroup$ – Ben Kovitz Jul 5 '15 at 22:29
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    $\begingroup$ @BenKovitz If I accept an answer, it doesn't lock out other answers , does it? I don't believe so. If you have a good answer, please go for it. Thanks. $\endgroup$ – docscience Jul 6 '15 at 1:52
  • $\begingroup$ It doesn't lock out other answers, but it does discourage them. It says "The OP is satisfied with this answer," suggesting that the conversation—and competition—are basically finished, unless someone persuades you to reconsider. On most SE sites, answerers tend to focus on questions that don't have an accepted answer. Here's a nice message about this on a different SE. I don't have another answer for this question, but I'm very skeptical that "non-independent trials means that no explanation is possible". $\endgroup$ – Ben Kovitz Jul 6 '15 at 2:15
  • $\begingroup$ @BenKovitz Thanks for the advice. I'm about a year working with SE posts, but still learning the ropes. $\endgroup$ – docscience Jul 6 '15 at 16:16
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Why so many molluscs exhibit sinistral winding?

The estimates of the number of molluscs vary quite greatly between 50,000 and 200,000 species. Of those molluscs species, about 70'000 are Gasteropoda. Gasteropa is most diverse Mollusca phylum.

The winding you describe is present in all Gasteropoda and is often called the torsion. So the answer to why there are so many molluscs that make a torsion is simply, because of phylogenetic independence. The torsion evolved only once in the gastropods. The answer to the more specific question, why is the torsion right handed rather than left handed in all gastropods is 1) phylogenetic independence again. The torsion evolved only once and was therefore either sinistral or anti-sinistral. There is no need for an explanation of why they are all sinistral because the observations are not independent.

Why has the torsion evolved at first place?

One may ask but why did the torsion evolved at first place?. I think the reasons are still to be discovered. The following is a summary of what I read on wikipedia (torsion#evolution)

"Why torsion is bad"

As a result of this torsion, the anus is found next to the mouth which is an obvious hygiene issue and therefore seems rather deleterious. Moreover, there are a whole bunch of issues about organs spinning around and entwining. Also, ventilation seems to be reduced by the torsion which is pretty deleterious.

"Why torsion is good"

However, because there's no "hole" left in the posterior position, the torsion may help preventing sediments. Some have suggested that the torsion may allow to move sensory organs closer to the head. The most likely explanation is that the torsion might have evolved as a defense mechanisms against predation as torsion allow an organism to hide its head behind its shell. Finally, citing from wikipedia:

The evolution of an asymmetrical conispiral shell allowed gastropods to grow larger but resulted in an unbalanced shell. Torsion allows repositioning of the shell, bringing the centre of gravity back to the middle of the gastropod’s body, and thus helps prevent the animal or the shell from falling over.

Note also that

Whatever original advantage resulted in the initial evolutionary success of torsion, subsequent adaptations linked to torsion have provided modern gastropods with further advantages.

Why sinistral rather than anti-sinistral

To repeat myself, we only have one single observation of torsion (as it evolved only once) and this observation is either sinistral or anti-sinistral. It sounds therefore quite likely that stochastic processes have driven the evolution of sinistral (rather than anti-sinistral) torsion. In other words, the first mutation allowing for some degree of torsion was probably causing a sinistral torsion and this is it.

But there might eventually be a more functional reason for why torsion evolved to be sinistral. The reasons would then be related to the already existing asymmetry of organs. For example, anti-sinistral torsion may yield to more entwining between the gut and the respiratory system, or to not squeeze too much the one lung, who'd be smaller than the other one due to the presence of circulatory organs. I don't have enough knowledge in the anatomy of the molluscs ancestors to have a good intuition of whether I'd expect sinistral or anti-sinistral torsion to be more beneficial.

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  • $\begingroup$ all of what you posted is very interesting and all makes sense, my main interest was in why so many gastropods have right hand coiling, and so few left hand. What evolutionary factor might have selective favor of right hand over left? $\endgroup$ – docscience Jul 5 '15 at 17:55
  • $\begingroup$ I developed the second paragraph and the added a section to try to better address this point. $\endgroup$ – Remi.b Jul 5 '15 at 18:47
  • $\begingroup$ Good hypotheses. Perhaps worth a rudimentary analysis, study. $\endgroup$ – docscience Jul 5 '15 at 18:53
  • $\begingroup$ Stimulating answer! Here are some reasons for doubt, which could be addressed in an edit or by other answers. 1) How do we know that only one torsion mutation ever happened? 2) Most gastropods actually have dextral coils. 3) A few gastropod species and genera are sinistral. 4) Amphidromus perversus has both. 5) A few sinistral snails appear in populations of mostly dextral snails. (Info mostly from Wikipedia. I am not an expert on molluscs.) $\endgroup$ – Ben Kovitz Jul 6 '15 at 4:55

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