Originally written by Lisa Loseke 2003
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.
Postmolt 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.
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.