Over the years I've seen a number of what appear to be iron shells growing on odd bits of rusting metal. These shells look exactly like bi-valve molluscs, can be any size up to about 2" long, are filled with liquid (water?) and will crush and break if squeezed.
Noting the striations (or concentric 'growing rings') it appears that some bacterial process is going on here. I guess that nonobacteria may be at work or some form of bio-chemical reaction such as chemoautrophy.
The shells are identical to mussel shells: two halves joined together in a watertight seal AND showing a thicker variation at the main join-point (as in the 'hinge' of a live mussel shell).
Does anyone know exactly what's going on?
Thanks, Noodles