Hi Paul,
Just got a bit scared - if I'm not looking at hermit crab damage, my
interpretations of shells from a number of sites has just gone out the
window! Yes, I agree - dissolution must be a slow process, however (just to
throw in another red herring) it might not necessarily be non-adaptive. Is
it possible that the CaCO being dissolved by the crab is taken up by the
crab to fortify it's own structure (chelipeds etc). This is certainly the
case with many terrestrial snails which are attracted to marine shell
mineral deposits. I'm out of my domain here and may be talking absolute
rubbish - just an idea tis all.
The reason why I do not favour abrasive processes such as beach-rolling
is due to the positioning of the damage. Frequently the outer surface of
the shell still has definite patterning and sometimes even colour (these
shells are about 3000 years old), however the columellar pad and the
columella itself will be extensively altered - or absent. Beach-rolling
tends to produce a much more homogeneous effect with the outer surface of
the shell displaying the most damage.
With regards to neritids, it's all go on the hermit crab front! The only
source of information I can find on hermit crab/shell interactions (and
specifically neritids) is contained within an archaeological PhD thesis
(Carucci, J. 1992. Cultural and Natural Patterning in Prehistoric Marine
Foodshell from Palau, Micronesia) and the references contained therein.
Carucci cites one particular Japanese study (Kinosita and Okajima 1968)
which focused on the hermit crab genus Coenobita. It was observed that all
hermited Nerita striata under study (n=484) had lost their columella. They
also point out (missed this one before) that it is only terrestrial and
semi-terrestrial genera that cause this type of damage. There does appear
to be a relationship between particular hermit crab species and the shells
they choose to inhabit. Apparently Coenobita favour Nerita, Turbo and Thais
(the three major taxa that I am finding damaged in my assemblages). In
Carucci's study of Palauan shell, he found that 43% of the hermited shells
were neritids. Naticids are another targetted taxon.
Wow! Thanks for all this input/feedback. I'm really having to engage my
brain!!!
Kath.
<br><br><br><html><DIV>Katherine Szabo </DIV>Archaeology and Natural History
<DIV></DIV>Research School of Pacific and Asian Studies
<DIV></DIV>Australian National University
<DIV></DIV>
<DIV></DIV>Canberra ACT 0200
<DIV></DIV>Australia
<DIV></DIV>Ph:61-2-6125 2235
<DIV></DIV></html>
>From: Paul Monfils <[log in to unmask]>
>Reply-To: Conchologists of America List <[log in to unmask]>
>To: [log in to unmask]
>Subject: Re: In defence of hermit crabs
>Date: Thu, 11 Oct 2001 22:30:19 -0400
>
>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.
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