In many freshwater shells the color and patterns are in the periostracum,
not the shell itself. This is the opposite of most molluscs. Peel off the
periostracum of a freshwater mussel and you have a white piece of chalk.
The inner nacre may be colored, but the outer patterns and color are
strictly in the periostracum. This seems to me to be a rather important
distinction between freshwater and marine/terrestrial molluscs, but what it
means in the grand phylogenetic scheme I haven't determined.

Since these animals are usually buried, or covered with algae, marl, or
insect cases, there can be no camouflage value. And from what would they be
camouflaged? Few animals eat them as adults, and those probably find them
by touch. The periostracum certainly protects the underlying shell from
dissolution but who needs a color pattern? Let's face it, the color is
there solely for people to enjoy -- or perhaps some non-adaptive trait
linked to an important gene. Nah. It's got to be the people angle.

Are there any marine molluscs in which the color patterns lie in the
periostracum?


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 G Thomas Watters, PhD
 Ohio Biological Survey &
 Aquatic Ecology Laboratory
 The Ohio State University
 1315 Kinnear Road
 Columbus, OH 43212 USA
 v: 614-292-6170  f: 614-292-0181
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