Jan,
  If my memory serves me well the tridacnids are on the CITES list of
endangered species and cannot be sold in countries abiding by the
agreement.  I do not have the URL for CITES but a web search or another
conch-ler might be able to provide it.
 
Charlie
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Charlie Sturm, Jr
Research Associate - Section of Invertebrate Zoology
                     Carnegie Museum of Natural History
Assistant Professor - Family Medicine
 
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