Sunday, June 21, 2009

Mack the Snake

"Oh, the shark, babe, has such teeth, dear
And it shows them pearly white"
Bobby Darin, 1958

As reported here:
Asaki Hoso reported that the snail-eating snake, Pareas iwasakii, has lopsided jaws to better enable it to tug snails out of their shells.
Apparently these snakes feed mostly on snails...

...and the lopsided upper jaw with about 17 teeth on the left and 24 on the right has evolved to extract snails from their shells which, in nature, seem to usually turn clockwise making the shell opening asymmetric.

All was well until one clever snail, Satsuma c. caliginosa, got in touch with her inner genes and got them to turn her shell around to swirl counter clockwise. Clever Miss. So, for the moment S. caliginosa is not on the menu for p. iwasakii. See the two web sites linked to above (here and here) for exciting videos and the full story. Quite a sinister situation, it seems.

So, two big questions come to mind reading this:
1. Why do most snails have clockwise rotating shells?
2. What mechanism did Miss S. caliginosa use to switch the rotation of the shells from right to left?

Snails are pretty interesting guys. According to Wikipedia "Snails lacking a shell or having only a very small one are usually called slugs." (One of the more interesting slugs is the Nudibranch, but I digress.) I also did not know that snails were next to insects in number and that sea snails are more common than land snails. Some land snails are predatory omnivores. Oh, my:

So, onward to the snail shell. Did you know that snails need extra calcium to develop their shells? (daily Tums, please). And that some snails have a "door" called an Operculum ? Gosh, Mr. Wizard (or should I say Mr. Wiki).

The snail shell is a logarithmic spiral. Counter to popular belief, the mollusk shell is not a Golden Spiral. Even more disheartening is that it is not even a Fibonocci Spiral, which (hold on) is not even a logarithmic spiral!! (the tenets of my firmament are being shaken!)

Sensibly, the snail uses a logarithmic spiral :
"... it allows the organism to grow at a constant rate without having to change shape. Spirals are common features in nature; golden spirals are but one special case of these."
One has to intrigued how the human brain sees beauty in the spiral and the golden rectangle. Is it possible that such complex mathematics is somehow built into our brain? Of course we are idiot savants when it comes to this but why such things are beautiful is intriguing. Then there is Phyllotaxis "
The beautiful arrangement of leaves in some plants, called phyllotaxis, obeys a number of subtle mathematical relationships. For instance, the florets in the head of a sunflower form two oppositely directed spirals: 55 of them clockwise and 34 counterclockwise. Surprisingly, these numbers are consecutive Fibonacci numbers."
That's better. Snail shells are also interesting in that:
"Due to the way snails solve the growth problem (enlarging their shell), they always have to carry the whole construction with them, even if they do not inhabit the oldest whorls. Crustaceans and insects, for example, use another option to overcome the growth problem: they regularly shed their whole skin when they grow and replace it with a larger one. The advantage of that method above the snail-grow method is that they do not have to carry around obsolete body parts. On the other hand, a crustacean or an insect is a very vulnerable when it has shed its old skin and the new skin still has to harden. "
And we all know who sheds her/his shell like a strip teaser:

But this says nothing about how the snail determines the direction of the whorl. We have to believe that it is related to the embryonic organizer. That little group of cells in the embryo which seem to run the show known as the primitive knot. This group of cells appears to be the most active secretors of growth factors which, in addition to jump starting the anatomical differentiation of the embryo also, due to "differential secretion of factors by the node ... causes development of the right-left axis in the embryo.: So it is here that is the nitty gritty of the whorl direction.

I am more at a loss to postulate a molecular mechanism for this . Clearly a left right axis means one group of cells is behaving differently than a similar group, but is this due to differential secretion of growth factors within a single cell (at first), or do all the cells of a small group suddenly go one way and the others go another. I don't know enough about embryology to even begin to answer this question.


Pat Dennis said...

"I don't know enough about embryology to even begin to answer this question." ... maybe not, but I'd bet that, if forced to hazard a guess, you'd suggest that we look at chirality.

Julie Heyward said...

Dr. C ... dude, you are so far behind. See this that nicely ties in neurology, memory, and snail shells.

Also an ecdysiast requires music.

Dr. C said...

Pat, Thanks for stoppoing by. Chirality crossed my mind but, as you know, it exists at the level of a single carbon atom in its tetrahedral configuration. Since at that level the majority of nature is the same, i.e. left handed, it is hard to envision how that atomic configuration could translate up to the macro configuration of cells. And, of course, it has to be a gene mutation, probably in one of the regulatory genes. The ability to form a left hand whorl is probably inherent in all snails if one ascribes to the current theories of evo-devo, though it takes a single gene mutation to make it dominent. This really is the problem here, i.e. what is the underlying genetic mechanism. It is hard to imagine how the alteration of one amino acid in one protein causes such an important change in development.

Dr. C said...

Julie, you got me there. I liked Ray's linking to cellular automata. I guess "dumb as a slug" is still operative but not "dumb as a snail."

Felix Grant said...

My brain hurts...