Thanks for confirming that I've suspected for some years. I have Chione gnidias that were collected from several different places around the Guaymas, Sonora area. Some from inside Estero Soldado have a rusty tinge in concentric rings of where they may have paused their growth for periods. The soil in the estuary is red with iron.  About 20 miles down the coast is Playa El Cochorit, where there is blue mud in a nearby estuary, and just over the low dunes. There the C. gnidias have dark grayish rings. Moon snails from the area may may be a pale gray color instead of the usual buff color, or they may have dark black coloring in the sutures or around the apex. The color doesn't seem to be inside on any of these creatures. But not all genera or species seem to be affected the same way.

---- David Campbell <[log in to unmask]> wrote:

=============
I think that manganese can contribute to the black as well.  Decay of
the animal can contribute to the lack of oxygen, but sometimes live
shells have a fair amount of black.  It depends on the habitat.
Lucines pump anoxic water from below for their bacterial symbionts and
oxic water from above for themselves, so where they live the
oxic-anoxic boundary is fairly shallow in the sediment.

On Sun, Jun 17, 2012 at 6:57 AM, Marlo Krisberg <[log in to unmask]> wrote:
> Seeking comments from Conch-Lers regarding the process that blackens dead
> shells.  Susan has pretty well described it, but I’m wondering if some of
> you can provide more details.
>
>
>
> “…shells sometimes get buried in sediments in a layer that has no oxygen,
> and then bacteria living in the sediment produce iron sulfides which can
> turn a shell partly or completely black.”
>
>
>
> http://z14.invisionfree.com/Conchologist_Forum/index.php?showtopic=2424
>
>
>
> Marlo



--
Dr. David Campbell
Collections Assistant
The Paleontological Research Institution
1259 Trumansburg Road
Ithaca NY 14850

----------------------------------------------------------------------
[log in to unmask] - a forum for informal discussions on molluscs
To leave this list, click on the following web link:
http://listserv.uga.edu/cgi-bin/wa?SUBED1=conch-l&A=1
Type your email address and name in the appropriate box and
click leave the list.
----------------------------------------------------------------------

----------------------------------------------------------------------
[log in to unmask] - a forum for informal discussions on molluscs
To leave this list, click on the following web link:
http://listserv.uga.edu/cgi-bin/wa?SUBED1=conch-l&A=1
Type your email address and name in the appropriate box and
click leave the list.
----------------------------------------------------------------------