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.
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