Nice summary Paul
A further note on polar bears - their fur is actually translucent and hollow
like a straw.  Their skin is black!  The combination is a strategy to gather
warmth from the sun. This is not really shell-related, I guess (at least they
swim a lot in the ocean so it's sort of ocean-related, and shells are in oceans
too, so that's my pathetic link to being shell-related).

Nora (natural history fan, lacking a bit..OK a lot  in the shell department)
Calgary, Alberta

"Monfils, Paul" wrote:

> 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.