>as i understood it, tanganyika was such a lake that was a closed-off part of the ocean that slowly became freshwater.  please correct me if this is wrong.<

I believe that this has been suggested, but more recent studies do not support the idea of past marine connections.  The lake basically occupies part of a large crack where Africa is splitting in two.  Some of the gastropods have been studied phylogenetically and found to be related to other freshwater groups.  There is extremely high diversity and endemism, so the mollusks represent an excellent opportunity for studying evolution.  The fish are more famous in that regard, but the fossil record of the mollusks makes them a better subject for study.  This diversity and endemism also results in high concern for the conservation of the mollusks and other lake species.

    Dr. David Campbell
    Old Seashells
    46860 Hilton Dr #1113
    Lexington Park MD 20653 USA
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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 Droigate Spa




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