> I am curious about deep sea mollusks found in association with hydrothermal
> vents.  Some of these are quite large, and look like they are thoroughly
> calcified shells.  Are the pictures decieving, are they just not so deep
> that CO2 in the water disolves the shell material, or are there other
> chemical reactions to consider in proximity to these vents?

The animals are actively producing the shells, and may have
periostracum or encrustations that provide additional protection
against dissolution (also typically well-developed in freshwater
taxa).  After death, the shells may be very prone to dissolution,
especially if they are aragonite (like most mollusk shells)-the
aragonite saturation depth is much shallower than the calcite
saturation depth.  Complicating factors would include the rate of
supply of carbonate (e.g., high or low abundance of planktonic forams,
coccolithophores, and pteropods in the overlying water) and of organic
material (high levels would produce organic acids and deplete oxygen,
promoting more acidic conditions).

Hydrothermal vents are usually shallower than the average abyssal
seafloor-they're most common along mid-ocean ridges.  However,
hydrocarbon seeps, dead whales, logs, and other organic sources of
food popular in the deep ocean can be found in standard abyssal or
even trench settings.


--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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