Looking at albinism in mammals and trying to apply those observations to
molluscs is probably not a valid approach.  In mammals, albinism is
relatively simple, at least in concept.  There is one major pigment involved
- melanin - which is responsible for most of the color seen in mammalian
skin, hair, eyes, and some of the color in certain internal organs.
Mammalian albinism is the inability to synthesize melanin, probably due to
the absence of a particular enzyme involved in melanin formation.  No enzyme
- no melanin - no color.  The concept at least is simple, even if some of
the genetic pathways causing the condition are fairly involved.  However,
where multiple pigments are involved, possibly controlled by multiple genes,
the situation becomes more complex, as Tom's reptilian examples illustrate.
To use another herpetological example, our local green frog has two skin
pigments, a yellow one and a blue one, which blend to create the appearance
of green.  This is a different situation from plant leaves, which look green
because of a single, actually green pigment.  Sometimes an individual frog
lacks the yellow pigment.  Result - a blue frog.  Now, ordinarily we
wouldn't refer to a blue frog as an albino.  Yet, the frog has exactly the
same genetic condition as an albino mammal - the genetic inability to
produce a single pigment.  So, how do we define albinism?  Is it an actual
genetic aberration, or only its visible outward manifestation?  If we use
the general definition "a genetically mediated inability to produce normal
pigmentation" then the blue frog is an albino.  If we define albinism as the
inability to produce any pigmentation at all, then it must be a far more
complex phenomenon in multipigmented animals than it is in mammals.  This is
particularly true in molluscs, where the shell and the soft parts may have
different pigmentation, each having multiple pigments controlled by
different genes, and created by diverse biochemical processes.  For this
reason, I would have to disagree with a couple of statements in Tom's last
paragraph.  Since the shell pigments may be different from the soft tissue
pigments both in composition and in synthesis, I don't think there is
necessarily any reason to expect that the "animal" in an albinistic shell
would also be albinistic.  If both shell and soft parts were albinistic,
that would indicate that either (1) the shell pigments and tissue pigments
were similar, or (2) that a more complex cause was operating, quite possibly
multiple causes, acting independently.  Also, I don't subscribe to the
theory of environmental causes.  It is true that albinism tends to be more
prevalent in certain localities.  However, the fact remains that most of the
individuals in a given locality are not albinistic, even though they are all
subjected to the same environmental factors.  Of course it could be argued
that certain individuals might have genetic predispositions to certain
environmental stimuli; but a more likely explanation for localized
concentrations of albinism (or other genetic traits) is inbreeding within
the localized population.  Albinistic mammals and birds don't last long in
nature, and are consequently very rare, due to two factors - greater
visibility to predators and susceptibility to skin and especially eye damage
from ultraviolet radiation.  In molluscs, these factors are minimalized
since (1) an aquatic habitat filters out harmful UV rays (more or less,
depending on depth), and most mollusc shells are opaque, even when
unpigmented, and (2) most of their predators do not depend on eyesight to
find them.  Therefore, albinism could quickly become commonplace in a
confined, inbreeding population of molluscs, even if it is a recessive
trait.
We should keep in mind that "albinistic" means more than just "white".  It
means lacking pigmentation by virtue of a specific genetic cause.  Polar
bears are white.  They are not albino.  They do not have a genetic inability
to produce melanin, as shown by the presence of melanin in their eyes,
mouths, inner ears, and internal organs.  The same is true of most white
domestic cats and dogs.  However, it is possible to have a true albino cat
or dog, or presumably an albino polar bear, in which there is no melanin
anywhere, because they are incapable of producing it.  In other words, when
a normally white species produces an albino individual, it is not as obvious
as when an albino appears in a normally pigmented species.  So, when a shell
dealer offers an "albino" specimen, the terminology is useful for describing
what the shell looks like, but it doesn't necessarily describe an actual
albino.  This is especially true of a species like Cypraea tigris, which
shows a range of color from pure white to pure black.  If an all-white C.
tigris is an albino, what is a specimen which has only two or three spots on
an otherwise white shell?  Dealers commonly refer to such sparsely-spotted
specimens as "semi-albino" or "near albino", which again is decriptively
useful, but probably not technically accurate.  On the other hand, an
all-white Strombus with a colored aperture might well deserve the
designation "semi-albino", or to be more accurate, "selectively albino".  In
other words, its loss of pigmentation may well be due to true genetic
albinism, but not all of its pigments are affected.
Lastly, someone inquired about the difference between "albino" and
"albinistic".  The difference is strictly grammatical - they do not refer to
different conditions.  "Albino", though we commonly use it as an adjective,
is technically a noun.  "Albinism" means a genetically mediated inability to
produce normal pigmentation.  "Albino" means an individual which exhibits
albinism.  "Albinistic" is the associated adjective.  So, an albinistic
individual is an albino, and vice versa.  And, technically we should speak
of "albinistic shells", not "albino shells".  But common usage often
prevails.
Paul M.