[Ronen]

Ronen asked the Naked Scientists:
   
I noticed that all the sea shells with spiral growth patterns (I have not yet seen otherwise) all grow in the same direction.

What would the reason for this be?

What forms or creates sea shells to "grow"?
What do you think?

[RD]

The coil of a snail shell can be either right-handed (dextral) or left-handed (sinistral), based on whether the shell spirals out clockwise or counterclockwise when viewed from above. Most species are composed entirely of individuals that are one or the other type; in exceptional cases, populations may differ in their handedness, or chirality, but within a single population, all individuals tend to be alike. This makes sense, since the mechanics of reproduction are harder between two individuals of opposite chirality (their genitalia are also reversed), reducing the likelihood that they will successfully mate and produce offspring. Over time, therefore, the rarer type will become rarer and rarer until it goes extinct.

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1182695

[peppercorn]

Good question!  ... And good answer also, RD!! []

Most illuminating!

esp. the reversed genitalia bit [:0]   !!!
 []

[Oceans Helen]

Just like humans, molluscs are mostly right handed. Left handed shells are far rarer at the moment - but looking back through time, the fossil record reveals periods when shells have flipped from right handed to left handed and back, through the eras. I don't think we have a good idea yet, why that might be!

[thedoc]

We discussed this question on our  show
 Diana -  It sounds like you've got a right-handed shell there which is called a dextral shell. You do get other shells which are left-handed and they're more sinister called the sinistral shells, but there’s no real obvious reason why you get more right-handed shells right now than you do get left-handed shells. Many people think that if you have all shells within a certain population, which are the same “handedness”, it’s much easier for them to mate with each other. So therefore, if you get one turning up that's left-handed, it’s going to be difficult for the right-handed one to mate with it. But looking back over paleontological records, you do get periods when more left-handed shells appear and then you get periods when more right-handed shells appear, and it just seems to be something that fluctuates and changes with time. At the moment, we’re getting mostly right-handed shells.
Click to visit the show page for the podcast in which this question is answered. Alternatively, [chapter podcast=2913 track=10.11.28/Naked_Scientists_Show_10.11.28_7581.mp3] listen to the answer now[/chapter] or [download as MP3]