I don't know of any research that has been done on the ability of mollusks
to see in the UV range.  Seems like it would be a difficult thing to
measure.  Many other invertebrates, especially insects, can do so.  However,
many of the mollusks which have fluorescent compounds in their shells live
at depths far beyond the penetration of UV radiation.  Therefore, they never
fluoresce in their natural habitat, even though they have the potential to
do so.  Others, like the fluorescent cowries, may live in shallow water, but
tend to be nocturnal, hiding in dark recesses during daylight hours.  So it
would seem that their potential to fluoresce would likewise be of little
value to them.  Therefore, I think that fluorescence is simply an incidental
characteristic of compounds which are present for some other reason.

Paul M.