Keywords: Succineidae, gastropod, pulmonata, mating, sex, evolution, shell coiling, dextral
Snail shells come in 2 basic shapes: tall or wide. Shell shape is usually expressed as the ratio of shell height to shell diameter; therefore, tall shells are taller than wide, while wide shells are wider than tall. This fundamental dichotomy and the scarcity of shells that are about as tall as they are wide was first pointed out by Cain (1977).
It so happens that the mating positions of terrestrial pulmonate snails also fall in 2 groups: face-to-face and shell-mounting. Interestingly, which position a given species mates in depends, with exceptions, on its shell shape: most species with tall shells mate by shell mounting, while most species with wide shells mate in the face-to-face position. Evolution by itself is an exciting enough research area and when you add sex to it, you can imagine the simulation, arousal of curiosity and the gradual building up of intellectual tension only released by occasional climaxes provided by the publication of a landmark paper. Research on the evolutionary origins and implications of mating positions of snails is no exception; for more information, I will refer the readers to Asami et al. (1998), Davison et al. (2005) and Davison & Mordan (2007).
The almost-amphibious land snail Oxyloma retusa (family Succineidae), which previously was the subject of this post and this post, has a tall shell and expectedly, mates by shell-mounting. Here are 2 individuals mating.
The snail on the right is on the shell of the slightly lower snail on the left and has its head slightly twisted. The twisting of the head is apparently necessary, because these are both dextral individuals and so, their genital openings are on the right sides of their heads. Only by twisting its head can the snail on top bring its genital opening next to that of the lower snail.
A sinistral snail, on the other hand, would have its genital opening on the left side of its head. Could it still mate with a dextral individual of its own species? Yes, if they had tall shells. According to Asami et al. (1998), 2 individuals with tall shells and with opposite shell coiling could mate "with small behavioral adjustments." In the case of O. retusa, for example, the necessary behavioral adjustment would be that if the snail on top were sinistral, it wouldn’t need to twist its head. Therefore, an aberrant individual with a tall sinistral shell among tall-shelled dextral conspecifics is more likely to be able to mate and leave offspring than one with a wide sinistral shell among wide-shelled dextral conspecifics. The evolutionary result of this is that sinistral species or closely related species with oppositely coiled shells are more common among tall-shelled lineages than among wide-shelled ones.
Pascal over at Research at a snail's pace has some information on the reproduction of another succined, Succinea ovalis.
Asami, T., Cowie, R.H. & Ohbayashi, K. (1998): Evolution of mirror images by sexually asymmetric mating behavior in hermaphroditic snails. American Naturalist 152:225-236.
Davison, A., Wade, C.M., Mordan, P.B. & Chiba, S. (2005): Sex and darts in slugs and snails (Mollusca: Gastropoda: Stylommatophora). Journal of Zoology 267:329-338.
Davison, A. & Mordan, P. (2007): A literature database on the mating behavior of stylommatophoran land snails and slugs. American Malacological Bulletin 23:173-181.
Cain, A.J. 1977. Variation in the spire index of some coiled gastropod shells, and its evolutionary significance. Philosophical Transactions of the Royal Society, London 277:377-428.