The basic conclusion is that certain hermit crabs may produce the patterns of damage that you have observed, but others leave little trace.  I have seen other mention of the columellar notch created by certain hermit crabs, but do not have references handy.  Hermit crabs are a very large group and are paraphyletic, i.e., some abandoned their shells and took up ordinary crab habits again.  The king crab of Alaska is perhaps the most spectacular example; perhaps they had trouble finding shells big enough for them?  As a result of this diversity, different hermit crabs may be expected to treat their shells differently.

In the fossil record, a major way of detecting hermit crab presence is the pattern of epibionts.  In some cases, a hydrozoan or other encrusting animal or algae will grow over the shell when a hermit crab occupies it.  There are a couple of hints that this took place with a hermit crab rather than an empty shell or a live one.  If the shell lives buried (e.g., naticids), or the epibiont is growing in places normally occupied by the live mollusk, then the epibiont growth occurred after the death of the mollusk.  Flat slippers (Crepidula plana and related species) often live alongside hermit crabs, though I have found one in a dead scallop shell.  Hermit crab occupation is a common but not the only way in which a dead shell can be kept exposed to the seawater, suitable for epibionts.  If the crab drags the shell, there may be a bare or thin patch on the bottom side of the shell.  In some cases, a symbiotic relationship develops, and the epibiont grows along with the hermit crab, giving it a self-expanding shell.  These typically produce a D-shaped aperture, and the original snail is entirely encrusted by bryozoans or coral.

    Dr. David Campbell
    Old Seashells
    46860 Hilton Dr #1113
    Lexington Park MD 20653 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droigate Spa




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