Do you mean that the soft parts are covered with a ferruginous layer? Or
just the shell? Iron is capable of replacing calcium in ionic compounds.
Therefore molluscs which live where there is an unusually high level of
dissolved iron often develop a reddish-brown ("rust-colored") shell.  This
is often seen in molluscs which live in association with shipwrecks. On Cape
Cod, Massachusetts, two large suspension bridges cross the widest manmade
canal in the world, the Cape Cod Canal.  Specimens living on the concrete
supports of these bridges often show such a red-brown color, due to the iron
washing down from the bridges.  Specimens on the rocks a hundred feet away
in either direction are white.

Paul M.

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