rust-like patches on shells

[dave r]

It certailly looks like rust. I think the shells may have been kept in a metal container or on something metal at some time which has rusted and stained the shells. This often happens in the tropics where the humidity is high or near the sea where the salt spray gets into things.It should not transfer itself to other shells unless they are wet and touching. As to how to get rid of it, I have no idea. I have seen this type of thing before and the rust seems to eat into the shell itself, all I can suggest is soaking in a strong bleach solution for a few days.
Sorry I can be of no further help.
Dave