FAQ What is molting?

Orginally written by Vanessa Pike-Russell

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.

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The growth cycle of a land hermit crab is based on a process known as molting (or molting), often triggered by the amount a hermit crab eats and drinks. Hermit crabs have rigid exoskeleton which cover the eyes, claws (chelipeds), legs (peripods) and parts of the shield and posterior carapace. These areas do not grow as the crab grows, and need to be shed.

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.

Shed Exoskeleton of Coenobita Clypeatus

FAQ What is molting? Photo Credit Carol Ormes Shed Exoskeleton of Coenobita Clypeatus

Molted Exoskeleton of Crab Kate, one of two land hermit crabs owned by Carol of CrabWorks. Images used with permission.Copyright 1999-2005 Carol of CrabWorks

Once your crab has slipped free of that constricting exoskeleton they will either retreat into the safety of a large shell or bury down into the sand or other fine substrate to hide away for a time. There are some cases where a hermit crab would do neither of these and choose to moult above the substrate and is visible throughout the moulting period. It all depends on the crab and how safe he feels within his crabarium and the type of substrate offered.

Arthropods molt periodically in order to grow and mature. Triggered by hormones released when its growth reaches the physical limits of itsexoskeleton, 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.

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 like Wumba in these post-molt photos.

Wumba’s molting photos:

This photo depicts a freshly molted crab having just pulled free from it’s shed exo:

Sierra freshly molted by Jen Stedner

Sierra freshly molted by Caroline

One this painful part of the process is over, your crab will now need to recover in the least stressful of environments. The temperature and humidity should be kept in the ideal range of 75-85F.

“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 moulded 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. It would depend on location as to the amount of exercise and grazing a hermit crab will do, but we have to be aware that a hermit crab stuck in a tank will not be as strong and healthy as one which is allowed out of the tank.

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.

Hermit Crabs grow through molting. If you notice a hermit crab pre and post molt you will see very little difference, but over ten or twenty years it is quite significant. Another way to tell age is to look at the thickness of antennae and the little ‘teeth’ on the cheliped/grasping claw.

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 exo 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.

Hermit crabs need to shed their exoskeleton every now and then, this allows them to grow and regenerate any missing limbs. You might have experienced the wonder and surprise at seeing a snake shed his skin. The shed skin looks like a duplicate of the snake, but it is only the cast off skin that didn’t grow with the snake. When a hermit crab grows its exoskeleton (skin) doesn’t. Imagine a pair of tight-fitting shoes. When your feet grow, your shoes do not. You need to go and get some new shoes which will fit.

“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 increasesmarkedly 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)

Hermit crab gel limb regeneration photos by Vanessa Pike-Russell

More articles on molting:
Is my hermit crab dead or molting?
Regulation of Crustacean Molting: A Multi-Hormonal System
On molting by Jad Johnson
All about molting by Lisa Loeske
CrabLoverDon on molting
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
and
University of Massachusetts Amhurst: Biology 497H – Tropical Field Biology.
St. John, USVI March 16, 2001 to March 25, 2001 Photo Gallery
URL: http://www.bio.umass.edu/biology/troptrip3/

Information references:

Charmaine Andrea Huet. Spatial Distribution Of Brachyuran Crabs In Sarawak With Emphasis On Fiddler Crabs (Genus UCA) As Biomonitors Of Heavy Metal Pollution. Institute of Biodiversity and Environmental Conservation UNIVERSITY MALAYSIA SARAWAK 2000
http://www.webcastmy.com.my/unimasresearchgateway/thesis/thesis_0062/chap1.htm

Dunham, D. W., and S. L. Gilchrist. 1988. Behavior. Pp. 97-138 in Biology of the Land Crabs, W. W. Burggren and B. R. McMahon, eds. Cambridge: Cambridge University Press.

Fletcher,W.J. and Amos, M. 1994 Stock Assessment of Coconut Crabs. ACIAR Monograph No.29 32p
Mike Oesterling of the Virginia Institute of Marine Science. Quote relates to blue crabs.
URL: http://www.blue-crab.org/fullmoon.htm

Fletcher, W.J., Brown, I.W., Fielder, D.R., and Obed, A. 1991b. molting and growth characteristics. Pp. 35-60 in: Brown,I.W., and Fielder, D.R. (eds), The coconut crab: aspects of Birgus latro biology and ecology in Vanuatu. Canberra, Aciar Monographs 8.

Fox, S. (2000) Hermit Crabs : A Complete Owner’s Guide. pp. 27. Barrons Books : NY

Greenaway, P. 2003. Terrestrial adaptations in the Anomura (Crustacea: Decapoda).
In: Lemaitre, R., and Tudge, C.C. (eds), Biology of the Anomura. Proceedings of a symposium at the Fifth International Crustacean Congress, Melbourne, Australia, 9-13 July 2001. Memoirs of Museum Victoria 60(1): 13-26.

Greenaway, P. 1985. Calcium balance and molting in the Crustacea.
Biological Reviews 60: 425-454. Herreid, C.F. 1969b. Integument permeability of crabs and adaptation

Grubb, P. 1971. Ecology of terrestrial decapod crustaceans on Aldabra.
Philosophical Transactions of the Royal Society of London B 260:
411-416.

Held, E.E. 1965. molting behaviour of Birgus latro. Nature (London)
200: 799-800.

Hobbs, H. H., III. 1991. Decapoda. In Ecology and Classification of North American Freshwater Invertebrates. J. H. Thorp, and A. P. Covich (eds.). Academic Press, New York, NY, p. 823-858.

Osterling, M. molting and the Full Moon. Online article [URL http://www.blue-crab.org/fullmoon.htm”>

Wolcott, T. G. 1988. Ecology. Pp. 55-96 in: Biology of Land Crabs (W. Burggren and B. McMahon, Eds.), Cambridge University Press, New York.