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The molting process is a central, and nearly continuous, part
of a crab's life. A crab may spend 90% of its time getting ready
to molt, molting, or recovering from a molt. There are many dangers
to molting including predation, difficulty in movement as muscles
have no ridged points of attachment, desiccation, and the risk
of an unsuccessful attempt to exit the old exoskeleton. Eighty
to 90% of arthropod deaths are related to molting.
The Molt Cycle
There are four phases of the molt cycle: intermolt,
premolt, molt,
and postmolt. During the intermolt,
the exoskeleton is fully formed and the animal accumulates calcium
and energy stores. It is the longest phase and constitutes the
time between molts.
Premolt starts when the old exoskeleton begins to separate from
the epidermis (skin), and the new exoskeleton begins to form below.
Calcium and other nutrient are reabsorbed from the old exoskeleton
at this time and stored in the tissue of the crab. This serves
the dual purpose of softening the old exoskeleton and recycling
the calcium for the new exoskeleton. The muscles in the pinchers
and limbs then shrink in anticipation for when they are to be
pulled out of the narrow joints of the old exoskeleton during
the molt.
Molting occurs as the old exoskeleton cracks and the crab pulls
out of it backwards. The new exoskeleton continues to form and
is pale and soft. Bloating with water is responsible for the increase
in size after a molt. In the case of land crabs who may not have
access to water directly after molting, this water comes either
from the shell water (which they carry around with them in their
shell), and/or from water accumulated in the blood and water sacs
during preecdysis. This water pressure is used to stretch the
new soft exoskeleton into a larger form. After some rest, the
crab eats its old exoskeleton as a source of calcium and other
nutrients.
Postmoltt
occurs as the new exoskeleton hardens through the two processes
of sclerotization (tanning) and calcification. Sclerotization
is the chemical process where proteins form chemical bonds between
each other to form a more rigid structure. Calcification is the
process of putting calcium into the exoskeleton. Also in this
phase the muscles grow back to their natural size and the excess
water is lost, leaving room for further growth throughout the
intermolt.
The Importance of Water
Because water pressure is the driving force behind the expansion
of the new exoskeleton, it is very important that hermit crabs
live in a very humid environment and have access to water that
is deep enough to fill their shells. Also, hermit crabs make their
blood saltier during a molt to have the water gain necessary for
the expansion. Thus a salt water pond is essential for the regulation
of this process as well.
Bibliography
Ruppert E. E. and Barnes. 1994. Invertebrate Zoology 6th ed.
Saunders College Publishing, Philadelphia.
Stevenson J. R. 1985. Dynamics of the Integument. Pp. 2-43 in
D. E. Bliss and L. H. Mantel. The Biology of Crustacea Vol. 9:
Integument, Pigments, and Hormonal Processes. Academic Press,
Inc. New York.
The Crab Street Journal: Articles:: All About Molting
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