Paul Monfils wrote about reddened shells,
"What is not clear in such situations is whether the snails actively
extract iron from the water and deposit it in their shells along with the
calcium, through their mantles - or - whether iron replaces calcium in the
shell from without, by a simple inorganic reaction, without metabolic
involvement of the snail."
 
We have all seen dead, reddened shells on the beach, of course. What seems
to be happening in that case is that shells are buried in sediment that
lacks oxygen and has a low redox potential. Iron sulfide (pyrite) is
precipitated in holes and interstices within the shell, particularly near
its surface. Pyrite is brass-colored in large crystals and black in small
crystals; if you rub a pyrite crystal on a piece of porcelain, the
resulting streak is colored a very dark greenish-black. Occasionally, this
happens in living Mercenaria; the late Bob Frey showed me live-collected
Mercenaria with brassy exteriors.
 
If the shell is later eroded and returned to a well-oxygenated environment,
the iron sulfide oxidizes to iron oxide, hence the rusty color. Rust is
also iron oxide (hematite), or I should say ONE of the iron oxides, just as
pyrite is only one of several iron sulfides. And another common iron
compound, goethite (iron hydroxide), is yellow to brown; some species of
the brachiopod Lingula are brown owing to this compound, even after
hundreds of millions of years. Well, never mind.
 
So. To find out whether live-collected reddened shells are red because the
animals extract iron from the water, or whether the shells are red because
of inorganic reactions, break or slice the shell and examine it under
magnification. If the red color is uniform throughout the shell, then the
animal probably put it there. If the red color is concentrated in only the
outermost layer, then perhaps an inorganic reaction is the cause. But snail
shells typically are built up of several layers with crystals arranged in
different directions, like the grain of plywood, and I suppose that it's
possible for the animal to introduce iron into only one of the layers.
 
Anxiously awaiting the results of scientific inquiry from shell slicers,
cutters, and breakers...
 
Andrew K. Rindsberg
Geological Survey of Alabama