Moulting... the Crisis

So what is moulting?

Moulting is when an animal sheds its old skin and then grows a new skin.

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.

[Images of Wumba by Vanessa, January 2002]

.

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 moult. If you were to watch your crab moult, 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.

.

[ Images used with permission.©Copyright 1999 Carol of CrabWorks ]

Once your crab has slipped free of that constricting exoskeleton, he 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 moult periodically in order to grow and mature. Triggered by hormones released when its growth reaches the physical limits of its exoskeleton, the moulting begins (apolysis) when the cuticle separates from the epidermis due to the secretion of a moulting 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 the post-moult photos.

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 70-80oF (21-27oC).

Autotomy and Regernation:

"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 moulting 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 moult, 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 moult (Hobbs 1991). This process is repeated until the new limb attains its normal size." (CHARMAINE ANDREA HUET, 2000)

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