Crabs and Molting Process

Crabs and Molting Process

Molting is an inherent cyclic process that occurs in all crustaceans, as a crab grows its hard exoskeleton (shell) becomes too small for its body and it must grow a new one. The process of replacing the exoskeleton is called molting.

The molting cycle is an extremely complex process that is normally divided into 4 main stages: inter-molt, pre-molt, ecdysis (shedding), and post-molt. In addition, it is a very stressful and dangerous process for the crab that can take up to several hours and requires a lot of energy.

So, if you are an aquarist (or a crab farmer) it is absolutely critical that you have the correct information on what is happening with your pet, what can affect the molting process, and, of course, do’s and don’t’s.

In this article, you will know details of each molting stage and I will answer the most popular questions regarding crab molting. 

Crabs’ Molting Cycle

Molting the Rainbow Crab (Cardisoma Armatum)
Rainbow Crab

Crabs do not have skin and internal skeleton-like human beings and most other mammals do. Instead of skin and bones, crabs are equipped with an exoskeleton – the external skeleton that exists on the outside of an organism.

The exoskeleton is a hard, external covering or shell that provides both structural support and protection from predators (like a knight’s armor).  

On the one hand, it greatly benefits the crabs because it enhances leverage for muscular movements and protects internal organs. On the other hand, the main disadvantage of the external skeleton is that it restricts the growth of the animal.

The exoskeleton is hard and rigid. So, it does not grow – it cannot grow. Therefore, crabs must shed their old exoskeletons to make room for new ones. Unfortunately, it is not that easy.

In crabs, the molting cycle is a highly complex, asynchronous (a stepwise) process and consists of 4 stages:

  1. Proecdysis(Pre-molting Stage)
  2. Ecdysis (Molting Process)
  3. Metecdysis (Post-molting Stage)
  4. Anecdysis(Inter-molting Stage)

Interesting fact: Similar to other crustaceans, in crabs, the molting process is controlled by environmental and endocrine hormones, which are located in their eyestalks.

Complexity

Biologists recognized the molting cycle process is one of the most complex processes in crustaceans. The crustacean molt cycle affects the status of a number of physiological processes, including the interaction with environmental stressors and even disease agents.

According to some studies, more than 50 different genes are differentially expressed across the molting cycle.

Recent studies have shown that many organs and systems are involved in molting regulation, including the hepatopancreas and the immune system.

Note: The hepatopancreas (variously called the liver, pancreas, midgut gland, gastric gland, digestive gland, etc.) is an important organ involved in the process of crustacean molting, and plays a vital role in energy storage and breakdown, nutrient accumulation, and metabolism.

In crabs, the molting is accompanied by structural changes and material metabolism in various tissues of the body, among them muscles and the carapace undergo the most dramatic changes during the molting cycle.

1. Pre-molting Stage (Proecdysis)

The pre-molt stage is a critical phase in preparation for the ecdysis (shedding) stage. During this stage crabs start:

  • intensively absorb nutrients (especially calcium) from the food and environment,
  • accumulate energy reserves in the hemolymph for the next molt,
  • reabsorb calcium from the old skeleton making the shell more flexible.
  • secrete enzymes under its shell that start the separation process (of the new thin layer from the old one).

Unlike crayfish, most crab species do not have gastroliths (little stones located on both sides of the stomach wall) to store the calcium. Instead, they have the mineral granules of hepatopancreas that function as storage forms of calcium and phosphate during the inter-molt and pre-molt periods.

Note: Calcium (Calcium carbonate) is essential for crustaceans because this is the main component of their shell. Depending on the species, it can range from 40 – 70%.

As the crab approaches the pre-molt stage, the old exoskeleton undergoes partial degradation and reabsorption of the old exoskeleton, and the initial deposition of a new cuticle occur during pre-molt.

By doing so, crabs achieve 2 goals:

  1. get more calcium for the new exoskeleton,
  2. reabsorption of calcium also serves to weaken the current exoskeleton in preparation for shedding. Otherwise, crabs may have some problems with breaking the old shell at all.
PREMOLT refers to the stage whereby a crab is about to molt. Depending on the size and species of the crab, the pre-molt phase lasts a few weeks to several months. Large and old crabs require more time and often molt only to restore lost limbs.

 2. Ecdysis (Molting Process)

The pre-molt stage terminates with ecdysis – the shedding of the old shell. For that, the crab’s body releases specific hormones to initiate the molting process.

So, how molting process works in crabs?

To put it simply, during pre-molt stage crabs develop the second layer of the exoskeleton; this second layer is very soft and elastic. When the second layer is ready, the crab starts pumping up its body with water (air for the terrestrial crabs).

Water can enter the body both by ingestion and by absorption through the external surface. This process usually takes a few hours before shedding and increases rapidly during the molting process itself.

Interesting fact: To increase the hydrostatic pressure crustaceans often stop urinating.

Crabs and Molting Process - breaking pointThe main goal of the water uptake is to hit a critical number, so the old exoskeleton can crack at the breaking point that is located at the back of the crab’s carapace.

When the old shell is broken at the breaking point, the crab can withdraw from it.

Note: This principle also applies to land crab as well. It is just even more complicated for them because they need to obtain the quantity of water needed for the post-molt increase in size, which must occur rapidly before running the risk of desiccation.

Note: Ecdysis or molting process is the shortest stage out of four. Depending on the crab species and how old it is, it usually lasts from a few minutes to several hours.

3. Metecdysis (Post-molting Stage)

Post-molt is a critical phase in the molting cycle of crab.

This is a period in which it is recovering from molting and during which its exoskeleton is hardened.

During early post-molting stage crabs need to hide because they are too soft to protect themselves and thus may be more vulnerable to bacteria, viruses, or predators.

This is by far the most dangerous stage for the crab.

Interesting fact: As with all crustaceans, crabs do not have an adaptive immune system. Instead, they rely upon an innate immune system to avoid exogenous stresses.

There are two main goals at this stage:

  1. Right after molting, crabs continue to absorb a significant amount of water in order to stretch their body and increase in size.
  2. Crabs start remineralizing the new cuticle with stored calcium and external supplies. These nutrients help to produce a substance called chitin synthetase, which is essential to creating and hardening the new exoskeleton of the crab.
After molting, the crab’s new shell is soft, making it more vulnerable to predation and diseases.

 The new exoskeleton generally hardens after about a few hours or days, depending on the species.

4. Anecdysis (Inter-molting Stage)

The inter-molting stage is the last and the longest stage. Basically, it is a period when the crab is resting between the molting cycles. During the inter-molt, they feed normally.

As the crab becomes bigger, this stage gradually increases in time as well. That is why small and young crabs molt more frequently than large ones.

After molting, the metabolic demand (to harden the exoskeleton) for calcium is particularly great. Crabs use calcium to produce a new and healthy shell.  I highly recommend reading my article “How to Supplement Shrimp and Snails with Calcium”.

Crab and Molting Signs

Are there any signs that the crab is about to molt? Actually, there are some signs that can tell help us. So, let’s take a look at them.

1. The difference in crab’s behavior

If you keep the crabs in the tank, you will notice that every crab has a unique personality. They prefer to eat a certain amount of food; they like some places more than others; or they act a certain way.

So, when they are about to molt their behavior will change. For example:

  • Eating

Changes in Their eating habits change drastically before molting. At first, the crab can start eating more than usual. However, after some time, you will see a sharp decrease in appetite. They start eating less until eventually stop eating altogether (often a few days before molting)

  • A decrease in activity

Lethargic behavior is another sign of upcoming molting. The crab will be less active before molting. A crab getting ready to molt will often become very slow and sluggish.

  • Hiding

Crabs are nocturnal animals. During the daytime, they usually prefer to stay hidden, even though some of them will venture out in search of food any time of the day.
However, before molting, the crab would prefer to remain elusive and hidden from sight. In the aquariums, they start spending more time in secluded areas away from their tank mates. They understand the risk of being exposed during this vulnerable stage of their life-cycle.

2. Difference in crab’s appearance

  • The dull or pale coloration of the exoskeleton

This is the earliest sign that pre-molt has begun is the detachment of the epidermis from the cuticle (separation of new shell and old one sometimes can be better seen where the joints meet). Lightening tone of the exoskeleton. When it happens the color of the crab becomes dimmer or duller.

  • Cloudy eyes

Another molting sign is cloudy eyes. The crab will have whitish, cloudy, or hazy-looking eyes.

3. Limb regeneration

Any lost appendages will begin to regenerate as limb buds. If your crab lost a limb (a leg, claw, etc.,), you will see a growing nub where the limb should be. As the time for molting approaches, it will swell and become more defined.

Keep in mind that all of these signs not always occur. In addition, there are always a few odd individuals who be comfortable just by staying out on the open substrate or eating until the time comes.

Shortest Molt Cycle Duration and Limb Regeneration

Losing legs or claws is pretty normal in the crab’s world. Crustaceans often exhibit autotomy – the reflexive voluntary removal of body parts by the animal caused either by stress or a predator threat.

Due to the fact that any limb loss may adversely affect functions such as feeding, mating, and defending, limb regeneration is urgently required.

Due to the fact that crabs can regenerate limbs only via molting, it has been observed that the molting cycle is generally decreased by limb loss.

Interesting facts: Biologists compared 3 treatments:

  • control (normal molting),
  • partial limb autotomy,
  • full limb autotomy. To ensure that feeding would not be interrupted owing to the lack of appendages in the full limb autotomy group, trash fish pieces were placed directly in front of the mouth opening of each crab.

Results of the experiment showed that the molting percentage was the lowest in the control group (~50 %), high in the partial limb autotomy group (~80%), and the highest in the full limb autotomy group (more than 90%).

Note: Based on this knowledge, limb autotomy is often used by many soft-shell crab production farms to induce molting in crabs.

The problem though is that most of a crab’s energy is directed to the promotion of limb regeneration instead of growth. The full limb autotomy group had the lowest growth rate. Even more, most of the crabs subjected to partial or full limb autotomy either could not fully regenerate their limbs in time prior to molting or the regenerated limbs were smaller in size.

In crabs, limb autotomy induces faster molting and results in a higher molting percentage. At the same time, however, it results in a significantly smaller body size.

Crabs, Molting, and Hiding Places

If you notice the signs that a molt is impending, make sure that they have enough food (before they stop eating) and hiding places in your tank, so they will be less stressful. The proper environment is important for a successful molt.

This is very important, especially for a community tank! The problem is that in this phase, crabs are soft, weak, and cannot fight back.

Though the actual molting usually takes up to few hours, the crab’s body will also be soft and vulnerable for some time before the new exoskeleton completely hardens.

Therefore, if you keep crab in a community tank with fish or other crabs, you will have to provide them with lots and lots of hiding places.

Handling Molted Crab

We all want our pets to be happy and healthy. However, I have to repeat – as the old exoskeleton is shed, the crab’s body will be soft and extremely vulnerable.

For this reason alone you should NEVER HANDLE a molting crab, or right after it molts.

Molt Death Syndrome

Molt death syndrome is a condition when crabs are unable to extract themselves successfully from the old exoskeleton during the molt.

It has been commonly linked to inappropriate nutrition, water parameters, and pathogenic infections.

Unfortunately, crabs that get stuck while molting nearly always die. In the best-case scenario, the result can be mutilation.

Note: Do not think that you can manually help the crab to get out of its shell. Over the years in this hobby, I have never heard that any of their tries were successful. This is not worth doing. As a matter of fact, but crab will have better chances of surviving if you do not try to save it!

In order to help, it would be better to create all the conditions for a favorable outcome of molting. 

What should I do with the old exoskeleton?

Crabs and Molting Process - open carapaceNothing. Do not remove the old exoskeleton from your aquarium.

Many animals that shed their skin later eat their molt to regain energy lost during the molting process and crabs are not exceptions.

What to do After a Crab Molts?
Nothing. Leave your crab alone. Your crab is absolutely helpless until the new exoskeleton hardens. Give it at least a few days.

How many times do crabs molt?

Most crab species undergo molting up to 10 – 20 times during their life cycle, including during larval development.

Young growing crabs will molt more frequently (up to once a few weeks) than older crabs (who may only molt every few months).

Please note that the above is just a rough gauge.

The number and durations of molts are not fixed and depend on the environment, particularly temperature, the availability of food, and the husbandry provided. In addition, the actual molting intervals are likely to be different from species to species.

In Conclusion

Growth in crab species is a stepwise process associated with molting. This is an essential and ongoing physiological process occurring in the life-history of all crabs. During molting, the old exoskeleton (shell) is periodically removed for replacement by a new one.

The molt cycle stages have been defined as post-molt, inter-molt, pre-molt, and ecdysis (molting).

Molt is a critical developmental process. It is also a slow and delicate one. Be sure not to bother the crab while this is happening.

Related articles:

Crab External Anatomy
Crab Internal Anatomy

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