|MadSci Network: Zoology|
Question: Why do shells seem to be mostly clockwise in their rotation and not evenly split between clockwise and anticlockwise?
I'm going to break this question into two parts to answer it: (a) why do shells mostly coil in one direction? (b) Why is that direction mostly clockwise?
The answer to the first question is that the evolution of spiral shells--and some other evolutionary changes--required and resulted in big changes to snails' anatomy and these can't be easily switched from one side to the other.
The answer to the second question is: We don't really know!
Now some additional information...
It helps to know a little about the evolution of the gastropods (the group of animals which includes the seashells). The gastropods probably evolved from an animal which looked at bit like a modern limpet. It had a broad flat foot, with a head and mouth at one end, and an anus and gills at the other inside a special space (the mantle cavity). Its body was covered and protected by a broad flat shell.
During the evolution of the gastropods two important events occurred: their shell became coiled to one side (asymmetrically coiled), and their internal organs became twisted (called torsion). Asymmetrically coiled shells have obvious advantages--they are compact and stable--but, because of the way they sit, they have the disadvantage of squashing the internal organs on one side of the animal's body. Biologists are still arguing about the advantages of torsion but one disadvantage of it is that the animal's wastes end up being released over its head. To fix this problem, snails evolved to have water flowing into the mantle cavity from one side, over the gills, then out the other side. The gill on the "out" side, which would be fouled by waste, was gradually reduced and lost. Other organs on that side of the body also became reduced in size.
Thus the coiling of the shell to one side has had, and does have, many complicated effects on the animal. For this reason, it is probably hard for snails to change from coiling in one direction to coiling in the other. The various changes to their body would simply be too complicated.
As to why shells should nearly all be coiled clockwise (also called right-handed or dextral), rather than anticlockwise (also called left-handed or sinistral), no-one really knows. This is, however, only one of several examples of such "handedness" in the universe (see the article by Hegstrom & Kondepudi for others).
For further information about gastropods see any zoology textbook, such as "Invertebrate Zoology" by R.D. Barnes (1987, Saunders College Publishing).
For further information about "handedness" see "The handedness of the universe" by R.A. Hegstrom & D.K. Kondepudi in Scientific American, Volume 262, pages 108 to 115, and "Snail handedness" by R. Robertson in National Geographic Research & Exploration, Volume 9, pages 104 to 119.
For further information on the www about snails see Conchnet and Internet Resources for Conchologists.
Try the links in the MadSci Library for more information on Zoology.