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