>  Could they be Crassostrea virginica (Eastern  Oyster) from North America? The
>  young mollusks often look much like what  you describe. Being on buoyant
>  debris indicates float from another  source is a possible explanation as you
>  indicate. The Gulf Stream is famous for  transporting sea life to Northern European
>  shores from the Western  Atlantic.

Furthermore, humans are infamous for transporting sea life all over the world.  Oysters have been imported from one place to another for aquaculture numerous times, despite both the existence of good oysters already in the area and the past establishment of pests and diseases from transplanted oysters.  Also, oysters can attach to ships and be transported all over the place.  Thus, a mystery oyster population could be almost anything.

Details of shell structure and form can be important in identifying oysters.  Hyotissa, Neopycnodonte, and related genera have a spongy texture to the shell, if you look closely at a broken surface, whereas the standard oysters such as Crassostrea, Ostrea, Lopha, and Saccostrea, do not.  Some habitually favor certain habitats (under rocks, in sponges, on soft corals. etc.).  There's also a lot of molecular data on the group; you could try sending a specimen to a lab working on oysters.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

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