by Vanessa Pike-Russell
One Size Does
Not Fit All
For
each crab I have a few shells which are just a little bit
bigger than their existing shell. When the crab 
moults (approximately
once a year) they need a choice of shells to move into. The
change in body size may be small but it will be important
that your growing crab needs a choice of shells. They will
be very crabby if they do not have bigger shells for bigger
bodies.
Upsizing Before
A Moult
Sometimes
hermit crabs will change into a bigger shell just before they
moult. Often these hermit crabs have spent a long time in
the salt water pond and their abdomen have swelled just as
our feet do when on long car or bus/coach journeys. You may
have felt your feet feel
swollen and cramped within your shoes. That must be how your
hermit crab must feel when they have drank a lot of water
from the fresh water dish or their salt pond. They need to
go through this process so that they can burst their exo,
then slip free. I guess it is a little bit like when you are
young and your feet grow so fast that you often have them
split or come apart at the seams.
What To Look For
When Selecting Suitable Seashells
When
selecting shells, it is important to find seashells that are
suitable for your hermit crabs. The rule of thumb is to have
three shells per hermit crab, including:
- one the same
size as the one your hermit crab is currently in, both in
aperture (opening) and perhaps in width/length. It may be
the same shell species/type or one of the more popular seashells
so that you maximise the chance your hermit crab will be
happy in their new shell
- one slightly
smaller than the one your hermit crab is currently in, especially
if the shell they are wearing is sliding and they have poor
control of it, OR they are obviously tiny within a large
shell
- one slightly
bigger than the one your hermit crab is in, in case they
need more room pre/post moult or their own shell is too
small for them
You look
for any holes or cracks that could cause the hermit crab to
dehydrate from drained shell water. Hermit crabs use not only
to keep their gills moist so that they can breathe, but also
to regulate the salts within their shells. You may observe
your hermit crabs as they examine each shell with great care,
picking it up, turning it over, looking for any reason not
to chose it as their new mobile home. It is important that
if there are holes in the shell, especially in the whirl where
the shell water is kept, that you quickly find some suitable
replacement shells.
Differences in
Seashell Selection By Species
Aussie
Hermit Crabs (Coenobita variabilis) and Ecuadorian/Pacific Crabs (Coenobita compressus) are very similar
in appearance and seashell selection. Thais or Rock/Purple
shells are found in Australia and Parts of the Pacific, as
well as other areas. They are available to Aussie Hermit Crabs
and Ecuadorian land hermit crabs and are often harvested from
the wild wearing the rather plain looking shells with an interior
that is quite smooth and pearly of nacre. There are many types
of Thais shells, and they are linked to Murex shells, which
are another favourite shell of many land hermit crabs.
The Perfect Fit
It is important
to measure the cheliped of the hermit crab and match it up
with some shells that will offer a source of protection.
If
the hermit crab can curl up within the shell, using the Cheliped or grasping
claw as a little door such as in the image above, then they
will have a fighting chance against aggressive shell-fight
bullies.
Keep Your Hermit
From Becoming Crabby - Avoiding Shell Fights
Often
if there is a dearth of suitable shells, hermit crabs will
rap against the shells of their tank mates, or attempt to
drag them out of the shell by the cheliped (claw) or peripods(walking
legs). As you can see, the barracade above would make it very
hard for the aggressor to pull this hermit crab from its prized
Tapestry Turban (Turbo petholatus) shell home.
You may find
that despite the presence of many suitable shells, there will
be some hermit crabs that just refuse to budge from their
old and battered natural or pre-worn shells. This can be quite
frustrating to owners who do try to provide new shells for
them to change into, however there are many reasons for this
reluctance to move on. Often, hermit crabs will prefer a pre-worn
shell that has been modified by many hermit crabs over the
years and is as snug as an old woolen jumper that while looks
a bit daggy, is quite cumfy and practical. They might prefer
the weight or opening type of the shell they are inhabiting
to those offered, so will wait until a more ideal shell comes
along.
A Preference For
A Certain Type or Fit
I
have found that hermit crabs will often change into seashells
much like those they are in, if they like the particular 'fit'
for their abdomen. Just like we have preferences for the type
of shoes we wear, hermit crabs like a shell that fits 'just
right'. You might see hermit crabs whip their abdomens in
and out of shells while they make up their mind, OR wait around
until one of their tank mates moves out of a prized shell,
and then there is a seashell vacancy chain (domino effect)
whereby you may watch crab B slip into the discarded A, Crab
C move into the roomer discarded shell of crab B, and so on.
It is important
that if you have a hermit crab that desperately needs to change
their shells, that when you finally find a shell perfect for
their needs, that you keep the other hermit crabs from pinching
their shells. Perhaps just after a bath you could create a
little playpen out of a clean icecream tub and place some
clean, suitable shells with the hermit crab so that they have
the opportunity to change without having the misfortune of
finding the shell isn't such a good fit, but stuck in it because
their 'old faithful' shell was lost to them.
Seashell
Modification
"Hermit
crabs are a primary cause of wear to gastropod shells. Many
top-shells had crab-modifed apertures. Also, interior wear
patterns that corresponded to outer breakage areas were
observed in both crab-inhabited and abandoned top-shells.
Vermeij (1987:243) states that hermit crabs of the genus
Coenobita actually resorb the inner walls of the shells
they inhabit, enlarging the inner chamber. Desirable shells
are used by many generations of crabs, being outgrown, discarded,
and reoccupied by other crabs."1
If you have
hermit crabs such as Ecuadorians
then perhaps you just need to offer seashells similar to
those they found in the wild, or those seashells proven
to be a hit, such as Thais hamastoma which is growing in
popularity and is also known as the Mexican Rock Shell.
Think Like A Crab
Think
of how awful it would be if you found a pair of shoes on a
beach and went to try them on, but someone stole your shoes,
and you found that the shoes cramped your toes. You'd have
to walk home in shoes that were ill suited and wait until
the next good shell came along. If there were no spare shells
available, you may find that a desperate crab could take matters
into their own hands and simply fight to win ownership of
the shell of another, in order to survive. If you see shell
fights, it is imperitive that you re-assess your shell selections
and isolate the aggressive crab or it may lead to fatalities.
Footnotes:
1.
SCUDDER, S.J. AND QUITMYER, I.R(1998) Caribbean Journal of Science,
Vol. 34, No. 1-2, 41-49, 1998 Copyright 1998 College of Arts
and Sciences University of Puerto Rico, Mayag.ez Evaluation
of Evidence for Pre-Columbian Human Occupation at Great Cave,
Cayman Brac, Cayman Islands
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