Dan Yoshimoto's question, "Why [does Xenophora] 'decorate' with certain
species and not another?" is in a grand old tradition that dates back as
long as people have observed animal behavior. Xenophora is not the only
animal that shows selectivity in choosing items and cementing them onto its
home; many species of polychaete worms are also selective in the same way.
Some worms select flat objects and cement them as if they were bricks; some
pick sharp objects and arrange them so they protrude; the range of behavior
is very great. Likewise with crawfish. In the nineteenth century, the big
question was whether this building activity demonstrated intelligence at
some level (I have references if anyone wants them, though they're in
hard-to-find journals). I'm not sure whether any definitive conclusion was
ever reached. The answer depends on how you define intelligence, e.g.,
whether you include automated behavior as intelligence.
 
I'd like to hear some ideas on Dan's question. Also, from our evolutionary
wing: How did Xenophora's behavior ever get started?
 
Andrew K. Rindsberg
Geological Survey of Alabama