>The smaller Donax species are hard to distinguish, to the point where I suspect that only a lady Donax can tell the laddies of different species apart. Or David Campbell in his molecular genetics lab!<

In fact, Dr. Laura Adamkewicz did some molecular work on Donax.  For the U.S. east coast, variabilis seems to be one big happy species from New Jersey to Texas, but the slightly deeper water species (i.e., just subtidal rather than intertidal) are different on the Gulf coast versus the Atlantic.

    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