>Does anyone know of other molluscs that are species quite sensitive to ecosystem disturbance, and whose dissapearance or dramatic reduction in numbers might be taken as an Early Warning sign of developing problems?   I know many Unionids fall into this category, but what about marine species or fresh water gastropods?<

Some freshwater gastropods seem quite tolerant of disturbance (e.g., Physa), but others are or were rather sensitive.  For North America, many of the pleurocerids have declined greatly or disappeared due to disturbance.  All species of Neoplanorbis (not a pleurocerid) and Gyrotoma are among the victims.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 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 Droitgate Spa