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Sunday, 22 April 2012

Handedness in shells

Most 'snails' have a spiral shell, but not all of them. Slugs have no visible shell, and the shell of a mature limpet is a flattened cone, like an upside-down wok. (A wok, if you do not know what it is, is like a large upside-down limpet.)

Actually, most "spiral" shells are helical, not spiral, but let's stay with the common word.

Left-handed shell on the left, right-handed shell on the right.
(Confession: the left side of the picture is a fake,  produced 
by my digitally reversing the image of a right-handed shell.)
This image has been created from Dover copyright free  art.
Spiral shells come in two kinds, right-handed and left-handed, just like creeping plants (see my next entry when I get around to it). Some of the spiral snails come in left-handed spirals only, some come in right-handed spirals only, and some come in both forms.

The easy way to identify the 'handedness' of a shell is to hold the shell, with the top pointing up, and turned so that you are looking into the entrance hole. If the entrance hole is on the right, the shell is right-handed, if it is on the left, the shell is left-handed.

In one odd example, Sir Theodore Cook (Theodore Cook, The Curves of Life. New York: Dover, 1979), describes a beach at Felixstowe in Britain, where a snail called Fusus antiquus may be found living on the beach, and fossilised in the cliffs behind the beach. Cook said that 99% of the living specimens were right-handed, while all the fossil specimens were left-handed.

Some twenty years ago, I examined several thousand Pacific shells, and several hundred Indonesian shells. They were all right-handed, and so were all the shells on display at the Australian Museum, and the shells of Australia and New Zealand that I examined in several reference books. Even the Australian Museum's fossil shells were all right-handed! I decided to give up my hunt.

Then one day I was cleaning out an aquarium tank. The snails that I kept in the tank to clean the glass were all left-handed! I suspect the snail is an import from overseas, but the lesson here is to never give up looking!

So collect a range of snail and shellfish shells. Sort your shells into species, and into right-handed and left-handed forms (if any). What conclusions can you draw, and why?

While you are at it, look at your snails, and see if you can find any differences within a species. If they are striped, do all of them have the same size stripes? If they have knobs on their shells, are all of them just as knobby, or are some of them smoother? Are they all the same colour?

How could you explain the difference between the fossil Fusus antiquus snails which were left-handed, and the modern ones which are nearly all right-handed? How could such a change be driven by evolution? Right-handed snails go counter-clockwise when mating, and left-handed snails go clockwise, so the two types cannot mate.

If snail species vary in numbers over time, this might explain why a species becomes entirely of one form or the other, just by chance. How could it work across species? Could some predators find left-handed shells easier to attack? Why?

Or could there be another explanation? Maybe the right-handed form has a real advantage, but they used to be eaten by an animal which is now extinct, or the shells dissolved more easily, or the shells of dead right-handed or left-handed animals rolled away into deeper water. See how creative you can be in generating interesting hypotheses.

You can't leave it there, of course. Design a good scientific test which could be used to test your hypotheses.

Helical playwiths
The first man of science was he who looked into a thing, not to learn whether it furnished him with food, or shelter, or weapons, or tools, or playwiths, but who sought to know it for the gratification of knowing.
— Samuel Taylor Coleridge, (1772-1834)
  • Crazy speculation: Martin Gardner ( Ambidextrous Universe, 2nd edition. Ringwood: Penguin Books Australia, 1982) claimed that left-handed shells are quite common. As a North American, he was writing in the northern hemisphere. Could there be a difference between the handedness of snails in the two hemispheres? Could it relate to Coriolis forces, maybe, or the way the sun's shadow goes around a vertical stick? Develop your own hypothesis, and draw up a plan to test it. (By the way, I don't really believe the Coriolis idea. There may be something in the sun theory, but even that strikes me as profoundly unlikely. There isn't a lot to be gained in science from clinging to wacky ideas, but equally, there isn't a lot to be gained from rejecting wacky ideas without savouring them a bit first.)
  • Oddity: There is a television sequence showing an 'alien' being dissected, supposedly back in the 1950s. You can tell that the dissection is a fake, because there is a phone in one shot which has a coiled lead—phones did not have coiled leads in the 1950s!
  • Curiosity, a bit daft at the end: When I first got interested in this question, computer keyboards and telephones all had helical cords, and I found that you could usually deduce the country of origin by looking to see if the twist went to the left or the right. Helical cords are now less common, but look out for them, and see which way they go. Make a study of these leads, and where they were made. See which handedness is favoured in which countries. Does it correlate with the side of the road cars drive on? (Indonesians, Japanese and Malaysians all drive on the left.)
  • Useful line of enquiry: What conclusion should we make from the lack of left-handed shells in the southern hemisphere? There is the basis of a good project here for somebody. Collect information from a wide range of sources, comparing east coasts and west coasts, northern and southern hemispheres. Use real shells, photographs and drawings, and be wary of accidentally reversed images in books and on the web!
By the way, I have a whole website full of science playwiths. It's a bit tired and in need of a good dust and polish—and I'll get there one day, but if you ignore the primitive layout, you may find some amusement there.

It began about 15 years ago when I was teaching computing in a high school, and whipped the first page up for a workshop for teachers from the primary 'feeder schools'. It kind of growed, but the design is putrid.  After all, my web-authoring software was the incredibly sophisticated Notepad!

Final note: the two books cited in this entry were also mentioned in and I think it is worth having a look at that as well. The next entry looks at twining plants, which are also "handed".

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This blog covers quite a few different things, so I tag each post. I also blog about history, and I am currently writing a series of books called Not your usual... and the first two have been accepted by Five Mile Press, The offcuts appear here with the tag Not Your Usual... . For a taste of Australian tall tales, try the tags Speewah or Crooked Mick.   For a miscellany of oddities, try the tag temporary obsessions. And language us covered under the tags Descants and Curiosities, while stuff about small life is under Wee beasties.

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