Dear Olivier and others who are interested in shell colors.

> Any idea ?

I am also no chemist, but has been exploring to understand
the nature of shell colors as there are many mysteries about them.

It is reported that the blue color of the skeletal Alcyonarian coral,
Heliopora, is derived from Biliverdin (combined with a protein?)
The blue-green colors of insects are likewise BBP that is
bilin binding protein, derived from heme or porphyrin structure
(chlorophyll as well?), and the blue colors of marine pelagic
fish might show the same origin .

One example of related references is found at:

http://www.jenner.ac.uk/Lipocalin/REVIEW/review7.htm

The shell colors of Haliotis and Turbo vary between brown to
green according to the kind of diet alga: those fed green algae
become light green and those fed red or brown algae tend to
show darker brownish shell colors. The source material for
pigments might be modified or degradated to produce
variable final products that have different structures which
in turn show different colors or hues.

The shell colors of Ruditapes philippinarum are very variable
but they turn into dull brown when heated, while the varied
shell colors of Donax spp. and Meretrix spp. are stable to
boiling temperature. There must be significant differences in the
chemical make up of colors between these heat-resistant and
non-resistant.

The purple color is prevalent in many shells and is the dominant
and basic one in Donax spp., so that I am most interested.
I hope to find more information about this topic, because I am
conducting experimental crosses between different color
morphs in donacids and some other bivalves.

Cheers,

Masashi Yamaguchi
Faculty of Science
Univ. of the Ryukyus
Nishihara, Okinawa
903-0213  Japan