To expand on some of the issues Rich brought up, has anyone entertained the
possibility that the shell color and pattern of those shell that are
encrusted with sponges, bryozoans, etc., may help attract the encrusting
organisms? I am assuming that encrustations help camouflage the shell (and
that's where selection comes in). The color arises from the differences in
the concentration of pigment molecules within the shell. In an appropriate
light we see the concentration gradients as colors, but the shells have not
evolved for our eyes. Other organisms, e.g., sponges, may not see the colors,
but may be able to detect the concentration gradients (pigment molecules
probably diffuse out of the shell in very small amounts). In other words, the
color patterns may be a way of a mollusc announcing to the potential
encrustors "Here is a suitable surface to attach yourself."
 
A.