Originally written by Vanessa Pike-Russell and Lisa Loseke
Arthropods (e.g., insects and crustaceans) must molt their exoskeletons periodically in order to grow; in this process the inner layers of the old cuticle are digested by a molting fluid secreted by the epidermal cells, the animal emerges from the old covering, and the new cuticle hardens.
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.
A hermit crab will shed their exoskeleton when it becomes too snug about their growing body. Hermit crabs cannot go shopping for new skin, they instead shed their exoskeleton and build up the tender tissues with fluids and with the help of chitin, they develop a hardened exoskeleton. To be able to do this, your hermit crab will need a lot of moisture. You might find your crab near the water dish a lot prior to a molt. If you were to watch your crab molt, you would see your crab stretch and twist until the exoskeleton splits, then slips out of it like a suit. Some crabs cannot do this in one piece, so you may see legs and claws strewn about.
Arthropods molt periodically in order to grow and mature. Triggered by hormones released when its growth reaches the physical limits of its exoskeleton, the molting begins (apolysis) when the cuticle separates from the epidermis due to the secretion of a molting fluid into the exuvial (cast-off skin or cuticle) space. The endocuticle (chitinous inner layer of the cuticle) is then reabsorbed and a new epicuticle (outer, shiny or waxy layer) secreted. Ecdysis is the act of shedding whatever remains of the old cuticle.
“The Y-organ (YO), or molting gland, is the source of steroid hormone production and consequent molt cycle regulation in decapod crustaceans, and is responsive to both external environmental and internal physiological signals1,2,3. Control of molting involves a complex interaction between the eyestalk neurosecretory center, which produces inhibitory neuropeptides, such as molt-inhibiting hormone (MIH) and crustacean hyperglycemic hormone (CHH), and the paired YOs in the anterior cephalothorax2,4,5,6. The YO undergoes transitions in physiological properties at critical stages of the molt cycle. During intermolt, the YO is kept in the basal state by pulsatile releases of MIH to maintain low hemolymph ecdysteroid titers7,8. A reduction in circulating MIH relieves YO repression. The activated YO hypertrophies to increase ecdysteroid synthesis; the hemolymph ecdysteroid titer increases and the animal transitions to the premolt stage1,5. Autotomy, the reflexive loss of a limb due to injury, suspends premolt for a few weeks, which allows time for a new regenerate to form and grow, and the animal molts with a complete set of walking legs5,9,10. A critical transition occurs at mid-premolt, when the animal becomes committed to molt. The committed YO increases ecdysteroid production further and becomes insensitive to MIH and CHH and a regulatory signal associated with the regenerating limb5,10,11. The animal progresses through to ecdysis without delay.”
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.
Step 1: Apolysis — separation of old exoskeleton from epidermis
Step 2: Secretion of inactive molting fluid by epidermis
Step 3: Production of cuticulin layer for new exoskeleton
Step 4: Activation of molting fluid
Step 5: Digestion and absorption of old endocuticle
Step 6: Epidermis secretes new procuticle
Step 7: Ecdysis — shedding the old exo- and epicuticle
Step 8: Expansion of new integument(covering or investing layer)
Step 9: Tanning — sclerotization(The hardening and darkening processes in the cuticle (involves the epicuticle and exocuticle with a substance called sclerotin) of new exocuticle. Now the chitin and protein are laid down and the exoskeleton will become hardened and shiny after a few weeks.
“Typically premolt animals enter their burrows with their abdomens markedly swollen by food reserves… After molting the animal eats its exuviae, which contribute organic materials and calcium salts needed for the new skeleton… Very little information is available in regard to molting of Coenobita. Coenobita clypeatus is reported to hide during the process most of which occurs in the shell (de Wilde, 1973). There is a noticeable reduction in activity for several days prior to the molt and after ecdysis the exuviae are positioned just in front of the mouth of the shell (A.W. Harvey, pers. comm.). During calcification the new soft skeleton of the chelae and other walking legs is molded to fit the shape of the shell. If the animal increases markedly in size it may no longer fit neatly within the old shell and a rapid trade up in shell size may be necessary to avoid water loss and predators. There is no information available on calcium balance or storage through the molt or on growth increments of Coenobita. Coenobita clypeatus grows up to 500 g if large-enough shells are available” (Greenaway, P. 2003 p. 21)
Land Hermit Crabs that are eating foods high in calcium, fiber, chitin and foods high in nutrients their bodies need will often have a much higher molting rate; which slows with age or lack of larger seashells. If a crab is in a seashell, which is snug with no alternatives, they will not molt as readily as one with a vast selection.
Exercise is known to increase hunger, and thus will affect the rate of molting. In the wild, land hermit crabs have been known to walk many miles a night, and graze on foods along the way. A hermit crab can be safely exercised in the tank with a plastic hamster wheel.
Scientist Mike Oesterling of the Virginia Institute of Marine Science has noted this in Blue Crabs.
“In the summer months, food availability has a major affect on shedding activity. If a crab does not satisfy the physiological need to shed (increased muscle tissue, body cavity ‘cramping’, etc.), it will not enter the molting cycle. In other words, if it doesn’t get adequate nutrition it’s not going to grow.” (Oesterling, M. 2003)
Hermit Crabs are social animals, and as such, there is usually a ‘pecking order’ among groups or colonies. As with many animals and organisms, when there is a scarcity of resources you will see a ‘pecking order’ among hermit crabs. The resources most important to hermit crabs being protein and calcium-rich foods and varied diet; hiding spots; space to dig down to molt; different sizes of seashells; water; and salt water.
If a crab is ‘top crab’ than it would get the most food, like with puppies and seagulls. We see this on a small scale within the crabarium, where hermit crabs vie for position in the food bowl or a favourite hiding spot. I have often watched my jumbo hermit crabs fighting for access to the salt-water bowl or Treat dish. It is not unusual for them to fill the bowl completely and keep other hermit crabs away, defending their right to eat first.
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.
“Land Crabs store large quantities of lipids in the hepatopancreas, perhaps representing an adaptation to the variability of terrestrial environments. Unfortunately, few comparative data are available. Charles Darwin (cited in Reyne, 1939) remarked on the fact that over a liter of oil could be rendered from a large B. latro. The hepatopancreas of this animal contains up to 83% lipid (Lawrence, 1970; Storch, Janssen, and Cases, 1982), becoming particularly fat prior to molt (Wiens, 1962). Land crabs may rely heavily on “lipid economy”. Lipid biosynthesis increases markedly prior to ecdysis (O’Conor and Gilbert, 1968) concurrent with the degradation of muscle (particularly the chelae) that permits extracting the limbs through narrow joints in the old exoskeleton (Skinner, 1966b). Subsequent regeneration of muscle, and growth of new muscle tissue, will require nitrogen sources if based on stored lipids” (Wolcott, T. G. 1988. p 90)
Autotomy and Regeneration
“Crabs possess the ability to autotomise their appendages when trying to escape the grip of a predator. The appendages, which detach at preformed breakage planes, are able to regenerate, and require several molts to reach normal size (Weis 1978; Barnes 1986). Because the new cuticle is lost with the autotomised appendage, regeneration only occurs after a complete molting cycle has passed. At this point, the new limb continues to grow beneath the existing but it is doubled over in a folded position (Lee and Weis 1980). At the next molt, the newly generated limb may only appear as a bud or a stump, as it has not had the physical space within which to attain normal size. The new limb continues to grow in a folded position under the hardening exoskeleton until the next molt (Hobbs 1991). This process is repeated until the new limb attains its normal size.” (Charmaine Andrea Huet, 2000)
Photo Credit: Carol of Crabworks. Photograph of the exoskeleton of a C. clypeatus land hermit crab taken in 1999 and used with permission. Maryanne Ponte, Vanessa Pike-Russell
University of Massachusetts Amhurst: Biology 497H – Tropical Field Biology.
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