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How Does A Hermit Crab Molt Its Exoskeleton?

(Last Updated On: October 13, 2022)

Molting is an essential part of any hermit crab’s life cycle. As hermit crabs age and increase in size, they outgrow their existing exoskeletons.

Hermit crab molting is broken into four stages: proecdysis, ecdysis, metecdysis, and anecdysis. Once the molt begins, the exoskeleton will break, and the hermit crab will step out. Over the next 4-8 weeks, a new exoskeleton will grow and harden.

If a hermit crab has lost or shed limbs, these will regrow during a molt. Hermit crabs are also more fertile after molting, making them more open to mating.

Juvenile hermit crabs molt more often than adults, as they have more growing to do. Once hermit crabs reach adulthood, molting occurs roughly every 18 months. Novice hermit crab owners often confuse molting with death, so it’s necessary to understand the signs.

Why Do Hermit Crabs Molt?

The primary purpose of hermit crabs molting is to increase physical mass. Hermit crabs grow too large for their existing exoskeleton. Molting is the act of shedding this exoskeleton and growing a replacement.

A larger hermit crab is likelier to protect itself and its colony from predators. Marine Biology explains how female hermit crabs can carry larger clutches of eggs post-molt, while molting also increases fecundity.

During a molt, hermit crabs regrow lost limbs, including legs and chelipeds. Consequently, some hermit crabs start to molt due to extreme physical distress.

In most cases, the hermit crab will wait for a scheduled molt to replace any shed limbs.

How Often Do Hermit Crabs Molt?

During their first year of life, hermit crabs molt multiple times. This is so they can grow up and reach adulthood, with each molt increasing its size.

Once hermit crabs reach adulthood, molting slows to roughly every 18 months.

Signs a Hermit Crab is Going to Molt

The appearance and behavior of hermit crabs change before they molt. The most common signs of molting include:

  • Growing lethargic and lazy, losing interest in play or exercise.
  • Eating more than usual so the hermit crab’s existing exoskeleton has enough fat.
  • Spending more time bathing in saline water and seeming to fill a shell with fresh drinking water.
  • Eyes growing dull and glassy.
  • Skin fading to a dull gray, losing all previous luster and bright coloring.
  •  Constantly digging and scratching at the substrate as though looking to dig a tunnel.

These symptoms will lead directly into proecdysis, the first stage of a hermit crab molting cycle.

how often do hermit crabs molt?

How Does a Hermit Crab Molt?

The complete hermit crab molting cycle consists of four stages:

1/ Proecdysis

Proecdysis is sometimes known as a pre-molt.

Hermit crabs enter proecdysis before submerging themselves under the substrate commencing the process in earnest. During proecdysis, a new exoskeleton begins to build.

You’ll notice this if your hermit crab has shed limbs, and new buds will become visible where the legs or chelipeds once were. Biological Bulletin explains that multiple lost limbs can delay the onset of molting until all are ready to regrow.

The hermit crab won’t shed its exoskeleton during proecdysis. Any calcium that remains in the exoskeleton is absorbed into the bloodstream, aiding the strength of a new exoskeleton.

The hermit crab will look for salt at this stage, making shedding easier—offer salty snacks like chopped nuts and easy access to saline bathing water. The hermit crab will spend a lot of time in saltwater at this stage, filling the shell with a reserve of water.

2/ Ecdysis

Ecdysis is the active process of shedding an existing exoskeleton.

Blood pressure will increase at this stage due to the excess water and salt the hermit crab has absorbed. This higher blood pressure helps the hermit crab crack the exoskeleton.

This second stage of molting will always take place under the substrate. If a hermit crab lacks an exoskeleton, it has no protection from predators of the sun’s rays.

Once the exoskeleton breaks open, the hermit crab will slowly and steadily wriggle its way out. This is necessary to allow the new exoskeleton to grow and harden.

Despite this, the hermit crab is unlikely to leave its shell during ecdysis.

3/ Metecdysis

The new exoskeleton is fully formed at this point, but it’s yet to completely harden, which will take another week or two. The hermit crab will likely emerge from the substrate and start eating and drinking again.

Do Hermit Crabs Eat Their Exoskeleton?

Most hermit crabs will eat their shed exoskeleton following metecdysis.

This provides the hermit crab with essential nutrients, including any remnants of calcium. If a hermit crab can eat its exoskeleton, it won’t need to risk emerging to the surface for food.

If a molting hermit crab doesn’t eat its exoskeleton, you can offer it to other hermit crabs in an enclosure. Wait for the exoskeleton to become accessible – don’t dig it up and disturb a molting hermit crab.

If the exoskeleton smells of rotten fish, dispose of it.

4/ Anecdysis

During anecdysis, the new exoskeleton continues to harden up and solidify, primarily through the consumption of calcium and protein.

Anecdysis continues until the next molt, which could be as long as 18 months in adult hermit crabs.

How to Help A Hermit Crab Molt

Understanding how to care for a molting hermit crab is a crucial component of care. There are three ways hermit crabs can benefit from your help while molting:

Isolate the Hermit Crab

As social as hermit crabs are, molting is a private and personal experience, which means a hermit crab will prefer to be alone until the molt is complete. If you can, set up a second tank exclusively for use by molting hermit crabs.

This tank doesn’t need to be as big as your primary habitat, but it needs at least six inches of substrate so the hermit crab can burrow during molting. Ensure the habitat meets the 80/80 rule of a temperature of 80OF and 80% humidity.

Place this second habitat somewhere quiet and leave the hermit crab to complete its molt without disturbance. If you can’t set up a second habitat, the hermit crab will molt in a primary tank.

Leave the Hermit Crab Alone

Never disturb a molting hermit crab. Don’t worry if your hermit crab doesn’t surface for food or water. It has its old exoskeleton for nourishment and likely filled its shell with drinking water.

If your hermit crab molts in its primary enclosure with conspecifics, quarantine it if you can. Block access to your hermit crab so others can’t climb over it while under the sand or eat the exoskeleton before the molting hermit crab can do so.

Keep the habitat and surrounding area as calm as possible during a molt.

Offer New Shells

As discussed, hermit crabs grow in mass after a molt. The shell your hermit crab wore at the onset of the molt may no longer fit. Ensure your hermit crab can change shells.

Provide a selection of different shells for your hermit crab and allow it to explore them.

Behavioral Ecology and Sociobiology confirm that hermit crabs always prefer a solid, intact shell. Options should match existing shells in shape.

Avoid leaving these shells in plain view of the other hermit crabs in a habitat. This may spark a vacancy chain, with every hermit crab in your care considering a new shell.

Give your molted hermit crab first refusal on any new shell.

hermit crab molting cycle

How Long Does a Hermit Crab Molt?

There’s no hard-and-fast rule about how long a hermit crab molt takes to conclude. Some hermit crabs molt in four weeks, while others will remain fully submerged under the substrate for twice this long.

Don’t rush a hermit crab’s molt; accept that it takes as long as it needs. Don’t dig up the hermit crab after eight weeks to check progress, as this will cause distress.

It’s common for novice hermit crab owners to confuse molting with death. This can be a worrying time, especially if your hermit crab has been hiding for a while, but you’ll need to be patient.

Look carefully at the substrate in a hermit crab tank each morning. You may notice tracks and disturbances to the sand that suggests the hermit crab has briefly been surfacing overnight while you were asleep. Wait 12 weeks before growing concerned that your hermit crab didn’t survive its molt.

Consider gently digging around the substrate and testing for a reaction at this stage. If the hermit crab is half out of its shell, the previous exoskeleton remains uneaten, and the hermit crab gives off a foul odor of rotten fish, it may have died.

How to Tell When a Hermit Crab Has Done Molting

When a hermit crab concludes molting, it’ll display a complete reversal of behaviors demonstrated in the proecdysis stage. It’ll emerge from the substrate with bright eyes, a deep shade of color in the skin, and eat and drink heartily.

It takes 1-2 weeks for a new exoskeleton to fully harden after molting. Consequently, a hermit crab molted in an isolation tank shouldn’t immediately be returned to a shared habitat.

As the hermit crab that molted will wear a new shell, it may attract envious glances from others. The hermit crab will be nervous about being challenged for this new shell until it has fully grown and hardened its exoskeleton.

Hermit crabs dislike being alone for too long, so consider bringing a second conspecific into the isolation tank. This second hermit crab can provide company until it’s ready to rejoin its colony.

Molting is a natural process that all hermit crabs must undertake. Do all you can to make your hermit crabs comfortable while they shed exoskeletons and grow replacements.