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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