Health: Molting

This forum is where you discuss issues relating to molting hermit crabs, including pre- and post-molting issues. If you are having a molting emergency please post in the Emergency Forum.
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Health: Molting

Post by HCADirectors » Wed Mar 13, 2013 1:49 pm

My Crab Is Molting Above Ground!

This article is only about normal healthy molts. For information on what to do in the event of a dangerous Surface Molt, please visit the Emergency Forum.

What is molting?

Molting is the process by which hermit crabs grow - they shed their exoskeleton and grow a new one to replace it. Since their exoskeleton is hard and cannot expand as they grow larger, molting is essential to a crab's health and well being. Hermit crabs molt underground, and they do not come back up to the surface until they are completely finished. They hide their tunnels by back filling them, and create sturdy little caves usually near the bottom of their enclosure. The entire molting process can vary greatly in the time spent underground, and also in how much time passes until they molt again, and this variation is based on both the size of the hermit crab and on the individual crab.


Molting is a stressful time, for the crab and the owner. Often new owners are worried their crab is sick or needs their help, but digging up your crab will only cause it unnecessary stress and could possibly injure it or cause it to stop the molting process out of fear. Remember, your hermit crab has been molting his whole life successfully - it knows what it is doing, and it can do it without help. The hardest, but most important thing to learn about "crabbing" is to trust your crabs’ instincts!

My New Hermit Buried Immediately, Is It Molting?

Probably! New hermit crabs will often molt as soon as they can to repair any damage or injury sustained during transport, so your new crab may suddenly disappear and won't reappear for several weeks or months. The crab will take a significant amount of time to rest, destress and relax before and after the actual molt - it takes a lot of energy! This means that crabs molting for the first time in a new tank, or crabs who have been stressed out, may take an extra long time to destress before they perform the actual molt. However, new crabs can also disappear for weeks at a time and not molt before returning to the surface.

My tank stinks!
There have been several members lately who are asking about awful smells coming from their tank when they have a crab underground. Many are describing what they smell as rotting or ammonia, and are worried that it may be a sign of death. In the past, members have also described the molting odor as a chemical or medicinal smell like iodine, or like the scent that comes from ladybugs or fireflies. While deaths can and do occur while molting, never dig to find out if that's what happened! If the crab is molting, digging it up can harm and possibly kill it. While a dead crab may be upsetting to think about, leaving it where it is will not harm your tank or your other hermit crabs. It is best to wait out the smell and and the molt and see what happens without intervening. The majority of the time the crab comes up okay in a few weeks or months.

Why deep substrate is important

When crabs molt, there is a period of time when they're very soft and vulnerable. During this time, they're completely defenseless, and are not able to protect themselves from other crabs. On top of that, they have this delicious "molting smell" that to other hermit crabs smells like fresh meat and an easy meal. When a crab is about to molt, it might test out various places in the tank before settling on a spot. Then the crab will dig down as deep as it can, back filling the tunnel behind it, and will form a sturdy molting cave. Having deeper substrate gives the molting crab more room to hide in, and more substrate between it and the surface to mask its molting smell which keeps it safe from other hungry crabs. The substrate should be at least 3 times deeper than your largest crab is tall, so that it can dig a sufficient molting tunnel and cave.

Molting Sacs, and Why Nutrition Is So important!

When hermit crabs are about to molt they stock up on water and nutrients and they store these reserves as fluid in a molting sac, much like how a bear retains fat for winter. The molting sac looks like a pencil-eraser sized growth that is attached to their soft abdomens on their left side. Depending on the species and the color of the crab, the molt sac can range from clear to grey to orange and sometimes other shades as well. They use a substantial amount of energy to molt and lose a lot of nutrients when they shed their old exoskeleton, which is why it is so important they eat that shed exoskeleton to take back some of those lost nutrients. To encourage successful molts, feed your crabs a high protein diet, and supplement with calcium. Protein will give the crab the energy and nutrients it needs to molt, and calcium helps the crab develop his new exoskeleton. There are many other key elements in their diets, but these two are the most important for molting. Protein is found in meat, such as shrimp, beef or poultry, as well as bugs, bee pollen, crab carapace and nuts. Calcium is found in cuttlebones, egg shells, oyster shells, and also in insects and crustaceans that have a hard exoskeleton.

How Long Does It Take for a Crab to Molt, and How Often Will They?

The time it takes for a crab to molt and how often depends on the size, species and overall health of the crab.

Any crab – regardless of size or species - can take as little as a few days to a maximum of 8 months, and possibly longer.
That said, average molting times are:
Micro/Teeny - 1 week (but can stay underground for up to 4 weeks), every month
Small - 2-3 weeks, every 1 to 3 months
Medium - 3 to 6 weeks, every 2 to 5 months
Large - 4 to 8 weeks, every 4 to 10 months
Extra Large - 6 weeks to 8 weeks, every 10 to 12 months
Jumbo - 8 weeks to 16 weeks, every 12 to 24 months

Regenerating Limbs & Gel Limbs

Hermit crabs can regrow lost limbs and partial limbs! As the new exoskeleton begins to form beneath the old one, small and clear gel-like limbs will begin to form where the old limb used to be. Usually these limbs will grow for days or even weeks before the crab molts, but sometimes they will not be visible before the crab buries itself to begin the molting process. These gel limbs start off clear, or can be a pale grey, orange or pink and sometimes they'll begin to darken when the crab is about to molt. When the crab resurfaces after it’s molt, it will have a skinny version of the old limb which many times will be a different color than the rest of its body. It may take a few molts for the limb to catch up in size, but it will be fully functioning. Most crabs can get around just fine with less limbs, partial limbs, or even many smaller limbs, so don't worry if your new crab is missing a leg or claw .

Pre-Molt Signs

When a hermit crab is about to molt, you may notice an increase in food and water consumption. Sometimes they'll drink an excessive amount of salt water, and seek out more high protein foods than usual. They may begin digging holes all over the tank and even stay in them periodically to test them out. If the crab is expecting a growth spurt it might change into a shell that seems too large right before it buries. Other crabs choose a shell that may be a bit tighter that what they usually have on. Sometimes the crab will start to slow down and may appear lethargic. Often when a crab is about to molt, its old exoskeleton will appear dull and ashy and its toenails and hair (called setae) will be worn down. If they have good care while in captivity there may be no warning signs at all.

The Molting Process

Nearly all their time spent underground is in preparation of and then in recuperation from the molt. The actual process of molting only takes a few hours, but they are extremely vulnerable in the time leading up to, during, and after the molt, which is why they stay buried.

First, they spend time finding a good spot to make a little cave around them so they can spin and move and have enough space to get their old skin off without bumping into the walls. The crab will tunnel around and move from where it started digging on the surface, but once the crab finds the perfect spot it will stay in one place until it is ready to return to the surface. To test the safety of the location they have chosen, they often sit there for days or weeks making sure it's the right spot. The crab will also spend time gearing up for the actual act of shedding its own skin.

When a crab molts, they normally shed their old skin in less than an hour. It can take a day or more to harden before they are able to start eating their shed exoskeleton. Hermit crabs shed the hardened skin from the entire front half of their bodies in one solid piece, along with the hard tip of their tail – a fresh molt will look exactly like a living hermit, minus the abdomen. The freshly molted crab will be extremely pale and will also be covered in a slimy wetness that was used to lubricate it out of its own skin. They are extremely soft and gummy during this period and can receive severe injuries that can kill them or cause disfiguration that can last through several molts. They’ll remain leathery and vulnerable to injury for several more days.

Finally, the act of eating their old exoskeleton can take them days or weeks. And even then they're still not ready to come up! Most crabs will often sit for a couple more weeks, finishing up hardening and expanding their new body before they are okay enough to be back on the surface around other crabs.

Some crabs may even choose to return to the surface early to eat and drink and store up nutrients, and then return back underground for several more weeks or days. This is completely normal.

Post molt signs:

Crabs who have just molted will normally eat and drink excessively to gain back all the nutrients they lost during the molting process. They’ll also have bright, colourful exoskeletons as well as long, black, sharp toenails and sometimes they'll be very, very hairy! This hair is called setae and it covers their bodies like whiskers, allowing them to feel their surroundings. A molted crab may have new limbs if it lost limbs before his molt, and it may appear much healthier and more active. The crab could also appear bigger, as much as three times as big, and they may change colours drastically.

I Think My Crab Shrunk!

For several weeks after a molt a hermit crab can appear to be smaller than before, although as time passes it will normally return to the same size as prior to the molt, or slightly larger. There have also been cases of crabs permanently shrinking from a molt, and we are still unsure why this happens.

Updated: October 2014