A shell is the most important thing in a hermit crab’s life, so they choose their shells carefully, selecting one of the ideal size and fit. It’s rare for a healthy, living hermit crab to fall out of its shell.
When a hermit crab finds a shell, it’ll place its 4 rearmost legs and abdomen in the shell. Then, it hooks small appendages from the abdomen, called uropods, into the shell to ensure it won’t fall off.
If a hermit crab has abandoned its shell, it’s likely by choice. This could be because the shell was too large and heavy to walk and climb or because it was causing pain or discomfort.
Hermit crabs remove their shells while molting, but as this is a vulnerable time for them, they usually bury their shell under the substrate during the process.
Hermit crabs vacate shells when upgrading to a replacement. So, ensure the hermit crab is making this choice of its own accord and isn’t being forced out of its shell against its will by a conspecific.
How Do Hermit Crabs Stay in Their Shell?
Hermit crabs rarely leave their shell by choice, carefully choosing a vessel that is the right fit. A perfect hermit crab shell size will be large enough to offer protection but not so big that it can fall out.
When a hermit crab identifies a shell, it’ll try it on for size. The hermit crab will slide into the shell and push its abdomen against the inner wall, along with the 2 rearmost pairs of legs.
If the shell appears to be a good fit, the hermit crab will hook appendages at the end of the abdomen, known as uropods, inside the inner wall of the shell.
A hermit crab’s uropods are too small to see with the naked eye but vital to retaining a shell.
Once a hermit crab has identified a suitable shell, it’ll usually guard it. It’s rare for a hermit crab to accidentally slip out of a shell, so it must vacate by choice or be forced out by a rival.
Is it Normal for A Hermit Crab To Be Out of Its Shell?
If a hermit crab doesn’t have a shell, its soft and vulnerable abdomen is exposed. This leaves it at risk of attention from predators and adverse weather conditions, especially in the wild.
It’s unlikely that the shell slipped off the hermit crab’s back; something happened to necessitate shell evacuation. Understand why this has happened, and help the hermit crab to find a new shell.
Why Is My Hermit Crab Out of its Shell?
No hermit crab should be left without a shell for long.
Shells protect hermit crabs from heat, shield the soft underbelly, and assist with moisture retention around the gills, which staves off suffocation.
Here are the main reasons a hermit crab is out of its shell:
Molting
Hermit crabs need to molt their exoskeleton periodically, growing a replacement. This happens several times in the first year of a hermit crab’s life, then every 18 months once it reaches maturity.
When a hermit crab molts, it invariably increases in size. This is why older hermit crabs are larger than younger conspecifics. As the hermit crab grows, it’ll be too large for its former shell.
The shell must be vacated before the molt commences, or the hermit crab could become trapped inside a shell that can’t accommodate its new mass.
Most hermit crabs molt while burrowing under the substrate, taking their shells with them.
Before molting, a hermit crab fills its shell with water, which ensures it has a source of hydration during the molt, which takes several weeks or months.
Hermit crabs are vulnerable while molting. It’s easy to mistake molting for death. If a conspecific makes this error, the molting crab risks being cannibalized by a tankmate.
The Journal of Experimental Marine Biology and Ecology explains how hermit crabs are attracted to the scent of death of their own kind, which means a shell is likely to be vacant.
As natural scavengers, hermit crabs may also devour what appears to be a corpse.
Upgrading To A New Shell
Hermit crabs are always looking for a better shell. As shells are critical to hermit crabs, they must have the best shelter possible.
There are various explanations for a hermit crab to willingly evacuate a shell:
Inappropriate Size
Hermit crabs like their shells to be snug to minimize the risk of the shell slipping off their back. Equally, a hermit crab needs a light shell to encourage mobility.
If a shell is too large and heavy to easily maneuver the surface of a substrate or climb, the incumbent hermit crab will seek a replacement that offers a superior fit.
While two hermit crabs occasionally share a large shell for safety, they prefer to retain independence and maintain one shell each. This is the meaning behind the name “hermit crab,” despite them living in large colonies, and ‘Hermit’ refers to living alone in a shell.
Damage
If necessary, hermit crabs will use anything as a shell, including bottle caps or plastic bottles.
Abandoned sea snail shells are the most popular form of vessels for hermit crabs, as these are typically an appropriate size and abundant on beachfronts.
Sea snails have tougher shells than their land-based counterparts, which is an evolutionary defense against the bite force of their common predators.
The impact of gravity is less keenly felt underwater, so sea snails can move with a heavier shell.
This means a sea snail’s shell can withstand the sun’s heat and light attacks from small predators, protecting the hermit crab.
If the shell is cracked or damaged, the hermit crab will look for a replacement to enhance its safety.
Pain or Discomfort
Animal Behavior explains how hermit crabs can feel pain and may evacuate a shell due to an uncomfortable experience. Some hermit crabs will hold onto a shell if they consider the quality of the vessel superior to the painful interaction.
If a hermit crab is infested with parasites like mites, it’s likelier to evacuate its shell. Mites can make a hermit crab’s life miserable, attacking the eyes, abdomen, and legs.
Shell Exchange
A new shell will be tried on for size by the different hermit crabs, and they’ll decide who is entitled to wear the new shell.
The dominant hermit crab in a colony will be the first to try on the shell. The decision is made if this hermit crab considers the shell a perfect fit.
If this hermit crab isn’t interested, the next one in line will try it on for size.
The shell exchange doesn’t end when the new shell has been claimed. As per Oecologia, hermit crabs prefer to wear shells previously occupied by a conspecific.
As a result, the exchange loop starts again with this shell that has just been vacated.
This process will continue amicably until each hermit crab in the colony is happy with its shell, whether retaining the existing vessel or upgrading to a replacement.
Shell Theft
Shell theft may arise in a colony where one hermit crab acts as an aggressor and attempts to claim an occupied shell. This begins with a challenge known as shell rapping.
Behavioral Ecology explains that the intensity of shell rapping is a notice of intent from the aggressor.
Forceful shell rapping suggests the aggressor is determined to claim the shell by any means, while gentle shell rapping is considered a more amicable challenge.
Fighting Over Shells
The shell owner has a choice of whether to respond to the challenge.
If the hermit crab accepts the challenge, they’ll engage in antennae wrestling. Then, the winner of this encounter will claim the shell.
If the shell incumbent has no interest in defending its home, it’ll hide within the shell.
Some hermit crabs will accept this, grow weary of rapping, and move on, and others will become increasingly aggressive and forcibly attempt to evict the hermit crab.
Hermit crabs are usually quite docile and can settle their differences amicably. They’ll defend their shells with their lives, so a forced eviction could lead to a fight.
If you see 2 hermit crabs fighting with the chelipeds rather than antennae wrestling, especially attempting to sever the eye stalks, separate them and place one in an isolation tank.
According to Ethology, hermit crabs remember antagonist interactions, so monitor them carefully when reintroducing them to the same habitat.
Hermit crabs choose their shells carefully and rarely accidentally fall out of these vessels.
