Ross,
Your explanation of variation seems to be related to natural selection and
frequency dependent selection, and are well taken. These biological
concepts were covered in the article I co-authored on the isolation and
evolution of the land shell genus Amphidromus in Indonesia [Am.Conch. June
1997 (on Conch-Net: http://coa.acnatsci.org/conchnet/gold697.html )].
What I was interested to know is whether the biological concepts which may
influence variation in land shells would also hold true for marine species?
For instance, marine mollusks are often encrusted with marine organisms, or
periostracum which completely obscure color and pattern. Would natural
selection, or frequency dependent selection then apply to a group such as
the Cones where the periostracum obscures the color and pattern? Since
predators are not developing a search image of the color/pattern, would
genetic drift (described in the above article) be a more appropriate theory
explaining variation? -- e.g. Conus cedonulli shows extreme variation
within one population. The Amphidromus article states that genetic drift
tends to eliminate variation within a population and increases differences
between populations. Confusing!
Another instance where a species exhibits a great deal of color variation
within a population is Spondylus linguaefelis. It inhabits water deep
enough to filter out most of the warm color range that Ross refers to.
Also, the species is almost always encrusted with sponge and other
organisms, totally obscuring the color. Yet the species which lives its
life anchored in one position is found in a broad range of colors. Do
these colors have any function?
So again, the question restated is, "Do the biological concepts maintained
for intra and inter-population variability of land snails like frequency
dependent selection, genetic drift, and the founder effect hold true for
marine species? Carol mentioned that she has not come across any specific
mention in the literature. Neither have I. Does anyone know of recent
papers related to these concepts? Gary? Tim? Others? Are there any easy
answers? Lot's of questions...
Rich
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Richard L. Goldberg
Worldwide Specimen Shells
email: [log in to unmask]
homepage: http://www.erols.com/worldwide
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"The shortest distance between two points
is under construction." Noelie Alito
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At 05:32 AM 2/2/98 -0400, Mayhew wrote:
>Did anybody see which way January went?? Talk about warp-speed time
>travel!!
>
> A week or so ago, someone asked about the feature i admire the most
>about Mollusc shells: the incredible range of variation many populations
>exhibit, regarding color and patterns, especially in many of the most
>common spp, and the most successful and diverse families, such as
>Pectinidae, Neritidae, and of course Conidae. It is highly probale
>that these spp and families BECAME so successful because of their
>adaptability, which included their ability to blend in with their
>visible surroundings whenever nessessary -ie, whenever predatory
>pressure based upon color and/or pattern occurs. I believe that many
>spp maintain a genetic "pool"
>of visually-related variation, based partly upon past episodes of
>visual predation which enhanced the relative survival rate of certain
>colors and pattern-types, and partly upon simple genetic drift, which
>often produces morphs of limited or no survival value (as in shocking
>orange Nucella lapillus L., or violet Neptunea lyrata decemcostata-
>beautiful from our perspective, but not corresponding to any feature in
>their environment that i have ever seen!). The more varied a given
>species' pool of visual possibilities, the better their chances of
>surviving in different portions of their total niche, if and when
>visual predation becomes intense locally or regionally.
>A "local" example would be Littorina obtusata L.. In situations where
>crabs can easily pick them off their Fucus habitat, they are
>marvellously camoflaged, being mainly green and pale yellow, these
>being the colors of the blades and bladders of the algae they live on.
>In other localities, where crabs are less able to get at them, they are
>orange, black-striped, and a good variety of morphs or quite scarce in
>the crab-harrased populations.
> Some of the variation we humans easily notice in sunlight, with eyes
>capable of differentiating many thousands of colors, hues, and pattern
>subteties, may be due to the differing visual charactaristics of the
>bakground, the medium (air vs water of varying depth) the shell is
>viewed in, as well as the visual capabilities of the relevant
>predator(s): Water filters out certain wavelengths, and most marine
>predators cannot distinguish as wide a range of colors as we can- some
>indeed are color-blind, and rely upon form, pattern, refleciveness,
>etc., to discern their lunches from surrounding inedible materials.
>So, what may look like very different colors and hues to our discerning
>eyes, may be nearly identical to the predator(s) which help determine
>their relative frequencies by eating the inhabitants of the shells
>they can more easily find. Likewise, some patterns may be just as
>difficult for predators to discern as others which seem to us, under
>very different circumstances, to be radically dissimilar (Neritidae
>would seem to be masters at producing "visually-equivilant" patterns!!).
> Any other theories, or comments and critisisms on the ideas above,
>would be most welcome! -Ross Mayhew
>(Schooner Specimen Theories)
>
>At 11:00 PM 1/27/98 -0600, Goldberg wrote:
>Here's a topic for discussion. Color variation within a species has always
>interested me; both aesthetically and scientifically. Marine species like
>Pecten and Spondylus are prime examples. What are the accepted theories
>for particular marine species varying widely in color? Is it adaptive
>coloring? Genetic? Spondylus and Pectens are quite often encrusted or
>coated with marine growths which obscure the external coloring. So
>adaptive coloring seems a less likely theory. A number of Murex, Chitons
>and other groups have species that also exhibit a wide range of colors.
>Color variation in land shells is often an adaptive characteristic. What
>about marine shells? Anyone takers?
>
>Rich
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>Richard L. Goldberg
>Worldwide Specimen Shells
>email: [log in to unmask]
>homepage: http://www.erols.com/worldwide
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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