For whatever this is worth....
 
Some years ago, I noticed that 23 of 25 cockle shells my kids gathered from Casey Key beach were, I believe the left valves.  I mentioned this to one of my co-workers nad he said that he had done a short paper on that phenomennon while an undergraduate.  The jist was that the valves spin in opposite directions when caught up in an alongshore current.  He painted several of them, dropped them in the "first trench" and spent a few days watching them.  Very soon, half of them would disappear into deeper water while the others remained.  When the tide changed, the direction of the current reversed and the effect was the same - but the valves that remained in the first scenario spun into deeper water while the others remained.
 
I've often wondered whether the ratio changes from summer to winter - assuming that the alongshore di8rection changes.

From: Conchologists List [mailto:[log in to unmask]] On Behalf Of Andy Rindsberg
Sent: Tuesday, January 17, 2006 3:14 PM
To: [log in to unmask]
Subject: Re: FW: Re: Effects of collecting shells from beaches, and changes in the numbers of shells on beaches

Hi Bas,
 
Glad you're looking into this. It sounds like fun!
 >  Your suggestion about right and left bivalves is elegant, but could be complicated at least in this case by natural sorting - one of my target taxa was a robust cardiid (Fragum hemicardium or similar), and the conts showed significant inequality between the numbers of right and left valves.   
 
Thanks. That's why you have to perform the experiment with a "control beach" where you collect both the right and the left valves. As a grad student at the University of Georgia, I shared an office with Stephen W. Henderson, who wrote a paper along with Bob Frey summarizing what was then known about left-right sorting. In 1986 I think. Truly an odd phenomenon, isn't it? Steve teaches at the Oxford College of Emory University these days.
 
Mike, the topic of beach renourishment is a broad one, but if you want to delve into the literature, a lot of government agencies have posted their studies on the Web as pdf files in recent years. It's a measure intended to save paper while making information more readily available to the public. It may take a high-speed connection to access them. -- Okay, I just typed "beach renourishment pdf" on Google Scholar and got 440 hits. This is a boon for conservationist groups, though the emphasis is typically on vertebrates rather than mollusks. These reports often have big bibliographies on the geology and biology of local areas.
 
Cheers,
Andy
 
Andrew K. Rindsberg
Geological Survey of Alabama
 
 

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