What is Autotomy?
Comes from the latin words autos for "self" and tomos
for "cut
Autotomy can be described as self-cutting, Websters dictionary
describes autotomy as a "reflex separation of a part (as
an appendage) from the body: division of the body into two or
more pieces." Hermit Crabs can autotomize (drop) and regenerate
(regrow) their limbs from juvenile to adult stages. The break
occurs along a fracture plane located at the appendage's base.
Why do land hermit crabs drop limbs?
The rate at which the limbs regenerate depends upon the molt
cycle (Morgan, 1900; Zeleny, 1908; Bliss, 1960; Skinner 1962,
1985). During aggressive encounters, a crab will often choose
to flee and autotomize (self-amputate) the limb being held so
that they can escape. Other reasons why a land hermit crab may
self-amputate is in response to stress, ill health, to reduce
blood loss from a wound, or as a response to the presence of bacteria
or pests. (Cooper, 98). There is a thin grove on each crab appendage
close to where it joins the body. This is the fracture plane,
along with an internal membrane. Separation occurs instantly through
Stress from fluctuating temperatures
Hermit crabs are stressed by changes in environmental factors,
it is too hot or cold, humid or dry. Some times when hermit crabs
lose limbs it is a sign of stress or ill health. It is important
to keep your crabarium as close to the environment they are used
to in the wild. That means recreating the tropics which as we
know means warmth and moisture.
Another reason crabs loose a limb or a
cheliped is when a crab moults and does not shed their entire
exoskeleton in one piece, but instead section by section, over
a number of days. Generally if they survive the moult they grow
their limbs back again (regeneration) and can be happy and healthy.
There are cases where one hermit crab will
act aggressively towards another hermit crab. It could be territorial
or over a desired shell. In the wild a hermit crab will "throw"
a claw or leg if another hermit crab tries to pull them out of
their shell. This is a responsive behaviour and their limbs are
built in a way that they are able to "drop" or "throw" a limb
easily so they may survive an attack. This is called Autotomy.
When one crab likes another's shell, say
Crab A likes Crab B's shell, Crab A will go up to Crab B's shell,
knock its shell ( that of Crab A) against the other crab's shell
(Crab B), causing the crab in the desired shell (Crab B) to come
out and have a look at what is going on. Now the first crab will
try to pull the second crab out of its shell by a cheliped or
other limb. The second crab will normally drop his cheliped(grasping
claw) or leg/s and retreat inside his shell, using his remaining
cheliped to protect himself. Preferring to loose a limb instead
of loosing a shell.
Illness from contaminated living conditions
There are many stores that do not meet
the needs of the land hermit crabs they sell. If a tank is overcrowded,
unclean and there lack of fresh water. It is important that you
regularly clean the tank and remove any signs of contaminated
foodstuffs or mouldy substrate.
If your hermit crab tank has a mite infestation
you will need to get rid of them ASAP! Visit the
MITES page for more information
Why doesn't the crab bleed to death?
The crab does not lose blood because it instantly clots. A second
membrane helps in closing the wound, creating a nub-like covering
which develops into a limb bud.
What happens next? Can they regrow the lost limb?
A nub-like covering developing into a limb bud, which is at first
transparent and slowly unfolds. After one or more moults the bud
(or 'gel limb' as it has been described) soon regenerates to the
way it looked before the limb loss and will return to full size
after a few moults. Sometimes this can trigger a number of moults
one after the other as the crab struggles to regenerate all limbs
successfully. Sensory neurons must grow and re-establish the appropriate
connections with the neural network of the ventral nerve cord
if the new limb is to exhibit its original function (Cooper, 98).
(Fig. 1. and Fig. 2 from Cooper, 1998)
References:
Anatomy for Vetinary Technicians. Purdue University School of
Vetinary Medicine
http://www.vet.purdue.edu/~lamarch/term.htm
Bergmann, M., Taylor, A.C. & Moore, P.G. 2001. Physiological
stress in decapod crustaceans (Munida rugosa and Liocarcinus depurator)
discarded in the Clyde Sea Norway lobster fishery. J Exp Mar Biol
Ecol, 259: 215-229.
Cooper, R.L. (1998) DEVELOPMENT OF SENSORY PROCESSES DURING LIMB
REGENERATION IN ADULT CRAYFISH. The Journal of Experimental Biology
201, 1745–1752 (1998)
MORGAN, T. H. (1900). Further experiments on regeneration of
the appendages of the hermit crab. Anat. Anz. 17, 1–9.
William H. Amos (2003). Breaking Off. Hidden Worlds Back. Saturday
February 1, 2003
URL: http://www.caledonianrecord.com/pages/hidden_worlds/story/521f67cc0
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