I have a theory about most of the mass demises of mollusks that appear to be
spontainous (one thing I have learned in the world of biology Nothing is
spontanious)
Most mollusks lay eggs in massive quantities right? That would mean that the
fertilization would occur around the same relative time. Since development
would be in a similar time frame then the new population would reach adulthood
around the same time. Animals have a set pattern (including us) Eat, Mate,
Die. Most mollusk (notice I said Most not all) have a relativly short life
span. If the development was in sync then the mating would be around the same
time inorder to exchange gametes (or in some cases self fertilization) Then
after the mating process I would expect for the mollusks to die. While the
next generation is in its larval stage, the parent generation would die off in
order to maybe help promote the survival of the species by leaving space or
food resources (some have specific diets) Most insects have this pattern. For
example Odenata (dragonflies) stay in a larval stage for most of their life
and when they reach adulthood they quickly find a mate and die. Their adult
lifespam is no more that about 36 hours.  Remember this is just is theory. I
am using the examples of other invertabrate phyla. I am also thinking about
certain cephlopods (ex.. The Octopus) The female lays massive amounts of eggs,
gives her life to raise them to hatch, then she dies. I don't think this would
have anything to do with die-offs due to pollution, storms etc. know if this
sounds reasonable. I am intersted in finding out if this is a documented
theory because it would be cool to learn more about it. It may even be
possible to calculate when some die offs could occur (Grinning for shellers).
I doubt it but its still worth a shot. Amy, maybe you could add your 2 cents.
 
Sarah Watson
Silver Spring MD
http://www.geocities.com/SouthBeach/Tidepool/8845