Thanks for your response, Paul. Since Trochus radiatus is an intertidal
or shallow water species, I think the color red could play a role. In
this instance, there's no evidence that the hipponicid moved or didn't
move since its larval stage. I notice that many of these "riders" erode
the outer layer of the host shell as it fits itself onto the host. And
that many of them have evidently moved around a bit, judging by the
patterns left on the host shell.

This hipponicid's edges fit exactly into the erosion pit on the host. I
will try to get a decent digital image to send to anyone who'd like to
see what I'm writin' about.

David

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