Last Updated on October 21, 2023 by Joanne Harper
Shell rot is a disease associated with marine hermit crabs that rarely impacts captive animals.
Despite the name, shell rot has nothing to do with the hermit crab’s shell. It refers to bacteria that attack the chitin, leading to holes in the exoskeleton.
You can identify shell rot on a hermit crab by assessing the legs and, to a lesser extent, the claws and back. Shell rot manifests as green patches that resemble mold.
As the disease progresses, these patches will darken. Then, holes will appear in the exoskeleton.
Wild hermit crabs experience shell rot due to an inappropriate diet. Oyster shells also cause shell rot because they’re often infected with bacteria.
Captive hermit crabs develop shell rot in dirty, contaminated water. Outbreaks of shell rot can be contagious, so affected hermit crabs should be moved to an isolation tank.
The only way a hermit crab can recover is to molt and regrow a new exoskeleton.
What Is The Difference Between a Healthy Shell and Shell Rot?
Shell rot is a bacterial disease that adversely impacts the exoskeleton.
A hermit crab’s exoskeleton is coated with chitin, a firm, semitransparent polymer. The function of chitin is to protect a hermit crab’s limbs and body from damage.
If a hermit crab has developed shell rot, bacteria attach to the exoskeleton and eat away at the chitin.
Initially, shell rot symptoms result in green patches on the hermit crab’s legs, claws, or back, as though it’s growing moldy.
As bacteria wear away the chitin, these patches turn from green to dark green to black. Eventually, holes will appear in the exoskeleton.
How Do Hermit Crabs Get Shell Rot?
Shell rot affects about 10% of all wild marine hermit crabs.
They get shell rot from food infected with bacteria, usually oyster shells. This means you should check for signs of shell rot before buying a new pet hermit crabs and introducing them to your colony.
Always boil the shells if you introduce oyster shells into a captive hermit crab enclosure as decorations, which will kill the harmful bacteria. Bacteria in the water of a hermit crab tank can also cause shell rot.
Consider changing and replacing up to 50% of the water in the hermit crab enclosure weekly. This simple routine will reduce the risk of the water becoming unsanitary.
Terrestrial hermit crabs seldom develop shell rot, especially in captivity, as they have almost no interaction with marine hermit crabs. Regularly replace drinking and bathing water in the tank.
If you notice anything visually untoward about the water or substrate in a hermit crab enclosure, conduct a deep clean and change everything.
Is Shell Rot in Hermit Crabs Serious?
The longer shell rot is allowed to take hold, the larger the holes in the hermit crab’s exoskeleton. These holes will allow more bacteria to affect the body, eventually proving fatal.
Shell rot is most concerning if you notice it affects the hermit crab’s body.
As well as allowing more bacteria, this reduces a hermit crab’s defense against threats in the tank. This can lead to stress, possibly with life-threatening consequences.
Is Shell Rot Contagious?
Shell rot can be contagious to other crustaceans. Upon noticing symptoms, isolate the affected hermit crab to a separate tank. Take this step before the hermit crab begins molting.
If a hermit crab sheds an impacted limb, remove this leg or claw from the tank before it’s eaten. The hermit crab with shell rot should still be isolated, as other limbs may be affected but not yet visible.
You must determine the cause of shell rot in hermit crabs. Even if other hermit crabs in the same tank aren’t showing signs of illness, they could be affected for the same reason.
Temporarily relocate all animals from the tank and deep clean it, including changing the substrate. Replace all water sources before returning the hermit crabs you believe are healthy.
It’s uncertain if the bacteria involved with shell rot will spread to humans. Always wear gloves when handling an infected hermit crab, and keep them away from the face and mouth.
Can Shell Rot be Avoided?
Shell rot can be avoided by maintaining a clean and sanitary living environment for hermit crabs and preventing bacterial exposure.
This involves boiling or baking toys and decorations and removing uneaten food before it rots.
Can You Reverse Shell Rot in Hermit Crabs?
There’s no treatment or cure for shell rot in hermit crabs.
While protein and calcium-rich foods promote strong and healthy exoskeleton growth, they don’t prevent bacterial growth. Also, antibacterial medications are usually toxic to hermit crabs.
Shell rot is usually irreversible, but you may be able to remove the harmful bacteria if you immediately respond to the symptoms. If the disease has taken hold, the hermit crab must molt.
How Do You Clean Shell Rot?
If you identify shell rot early enough, you may be able to clean the affected areas of the hermit crab’s limbs or body. To achieve this, you’ll need the following cleaning formula:
- 2 cups of non-saline water (dechlorinated.)
- 3 tablespoons of ground calendula.
- 1 tablespoon of myrrh powder.
- 1 tablespoon of chamomile flower petals. Don’t grind to create a powder.
Once you have the ingredients for your wash, set up an isolation tank for the hermit crab. This should be lighter than you may usually provide, though not so bright that it distresses your pet.
Boil the water and apply the other ingredients, allowing them to sit for roughly 20 minutes. After this, strain the chamomile petals, stir, and leave the water to cool.
Once the wash is ready to be applied, dab at the affected areas twice a day for 3 days.
If you don’t see results after 3 days, you likely won’t. The shell rot will continue progressing and will only be resolved by shedding the impacted exoskeleton during a molt.
If the patches of shell rot show signs of clearing, maintain this routine for up to a week. If you have intervened early enough, the problem will diminish.
Will Shell Rot Clear Up By Itself?
A hermit crab can overcome shell rot by molting.
The molting process involves shedding the entirety of an existing exoskeleton and growing a replacement, which will theoretically be devoid of bacteria.
Mature hermit crabs usually molt every 18 months but may commence the process sooner. You can also encourage a hermit crab to molt with the following steps:
- Set up an isolation tank for the molting hermit crab. This shouldn’t contain toys, climbing frames, or other animals—just food, water, and around 6 inches of substrate.
- Place the hermit crab in this isolation tank, ensuring it settles.
- Dim the lights and cover the tank if necessary. This encourages the production of crustecdysone, a hormone that tells hermit crabs it is time to molt.
- Leave your pet alone, only making occasional health checks. Avoid handling or direct interaction.
Under these circumstances, a hermit crab is likelier to burrow itself under the substrate and molt.
If you don’t see your pet hermit crab for several days or weeks, this is almost certainly what’s happening. Don’t fear the worst, assuming the hermit crab has died.
How Do You Know if Shell Rot is Healing?
Observing the hermit crab after a molt is the only way to tell if shell rot is healing. When a pet hermit crab re-emerges, it won’t have a shell and will have 8 new limbs.
If the legs, claws, and back contain no dark patches, it is usually safe to assume that a hermit crab has recovered from shell rot by molting and is no longer in peril.
Once the hermit crab has found a new shell, it can be returned to a primary aquarium.
How Long Does Shell Rot Take to Heal?
In the right circumstances, shell rot will be cured after a single molt. Of course, every hermit crab has a different timetable for molting. Some finish molting in weeks, while others take months.
If the shell rot is severe, a hermit crab must complete at least 2 molting cycles before it recovers. The hermit crab must continue living in isolation until it molts a second time.
Don’t fill the isolation tank with entertainment or add other hermit crabs because shell rot will remain contagious. The more bare and dimly lit the tank, the more likely it is to molt again.
Regularly check on the hermit crab during this time because it may grow lonely if left alone for too long. Don’t allow a pet hermit crab to face stress and isolation while attempting to treat shell rot.
Shell rot is a severe condition for marine hermit crabs. However, it can be avoided with quality care and water maintenance. Shell rot recovery can happen when a hermit crab molts its exoskeleton.