Hi Kath,

No vindication necessary! - this is just an exchange of ideas :-)  Hope
you didn't feel attacked!
I guess my problem is that I can't see the adaptive advantage of
expending all the energy required to enlarge the interior of a shell,
when it would be so much simpler, and so much more energy efficient, to
just switch to a larger shell - especially considering that the switch
is inevitable, and the only thing accomplished by enlargement would be a
short postponement of the event.  The crab would essentially be
enlarging the shell in preparation for abandoning it, rather than
inhabiting it.
The type of damage you propose must surely be a very slow process,
whether produced by chemical or physical action, especially on something
as solid as a nerite shell!  If such action does occur, it seems it
would have to be chemical in nature, as the chitinous exoskeleton of a
crustacean is much softer than the shell of a nerite or other gastropod,
and probably could not scratch the shell, much less physically grind
away an appreciable part of it.  Given how slow such a process must be,
I can't imagine that a hermit crab lives in any one shell long enough to
produce much cumulative damage.  (Is there any data on how long a crab
inhabits one shell, on the average?)
You mentioned that the type of aperture damage you observe "seems an
unlikely wear pattern in relation to beachrolling and other intertidal
action".  Perhaps this could be tested by putting some nerite (or other)
shells in a mineral tumbler along with water and sand?
Another question, for my own information - do hermit crabs regularly use
nerite shells?  It seems like nerites don't have much of an inner spiral
to them?  Are there by any chance specific hermit crabs which are
specially adapted to inhabit nerites?

Regards,
Paul M.