An excellent example for the marine mollusks was published last year:
 
Whiteley, D. A. and D. F. Owen. 1997. Massive polymorphism and natural
selection in _Donacilla cornea_ (Poli, 1791) (Bivalvia: Mesodesmatidae).
 
They found that at some sites the frequency of various shell colors matched
the frequencies of colors of pebbles in the substrate, suggesting natural
selection for camouflage. At other sites some common (and some rare) colors
didn't match the background, suggesting that frequency dependent selection
played a role.
 
 
Also see my article in American Conchologist vol. 25(1):25-26 (March 1997),
"Why do shells have their colors?" I suggest that in some cases the
explanation is not visual--shell pigments can serve a structural function
also, strengthening the shell.
 
Gary
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Gary Rosenberg, Ph.D.                     [log in to unmask]
Malacology & Invertebrate Paleontology    gopher://erato.acnatsci.org
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