Hello Kath,

Welcome to Conch-L.

As far as I know, hermit crabs do not enlarge the aperture of the shell in
which they reside, or actively modify the shell in any way.  They pick up an
empty shell "as is", stay in it until they outgrow it, then discard it and
move to a larger shell.  Of course, they do not repair any damage that
occurs to the shell, as the original inhabitant would have done, so if the
lip gets chipped, it remains that way.  My initial impression is that the
indicators you are using may not be very reliable.  I have seen many
hundreds of living hermit crabs locally, and have never noticed any evidence
of either columellar damage or aperture enlargement.  As far as the abrasion
factor goes, the smaller hermit crabs carry their shells.  They do not drag
them.  However, a crab inhabiting a large, heavy shell like Turbo
chrysostomus or T. argyrostomus might have to drag it.  I'm not sure about
that.
The holes in the Lunella cinerea, if you are sure they are not of human
origin, may have been caused either by predatory snails (Muricid or
Naticid), or by octopus.

Regards,
Paul M.