Despite their name, hermit crabs are social animals that live in groups of over 100 in the wild. The name ‘hermit crab’ comes from the fact that they seek out shells that double as homes and hiding places.
In most cases, hermit crabs keep their shells to themselves. A shell is pivotal to hermit crabs, as it protects them from predators and the sun’s harsh rays. So, choosing the ideal shell is critical.
Occasionally, two hermit crabs may share a shell. However, this will only be a short-term measure, as one dominant hermit crab will eventually claim the shell for itself.
When two hermit crabs share a shell, it’s usually by accident. Hermit crabs could end up sharing a shell when an occupant is molting, and another hermit crab mistakes it for dead and claims the shell. Equally, two young and small hermit crabs may temporarily agree to share a large shell.
Marine hermit crabs allow sea anemones to attach themselves to their shells. This is a mutually beneficial symbiotic relationship, as both species offer protection to the other and share food. Sea anemones will live on a hermit crab’s shell, not within it.
Can Two Hermit Crabs Live in One Shell?
Two hermit crabs can share a shell, but it’s uncommon for them to do so.
The ideal shell for a hermit crab will fit snugly around its body, leaving just enough space to retreat inside. Such a shell will not leave space for a second hermit crab.
Despite this, hermit crabs have occasionally been observed sharing shells in the wild and in captivity. As independent animals, why would hermit crabs share shells?
Believing a Shell was Empty
The most common reason two hermit crabs share a single shell is mistaking molting for death. This can happen in the wild or captivity, but it’s more commonplace in the wild.
When hermit crabs undertake a complete molt, the process can resemble death. The hermit crab will become completely still, and the exoskeleton it sheds will start to smell. As per Ecology and Evolution, this releases a scent that attracts other hermit crabs.
If hermit crabs smell what they believe to be a dead hermit crab, they assume a shell is vacant. As this shell was previously used by a conspecific, it’ll be considered suitable for habitation. A new hermit crab may move into the shell, not realizing that a molting incumbent remains inside.
Both hermit crabs will need to be small for this arrangement to work. Even if this is the case, it’s not a sustainable living arrangement. Eventually, one of the hermit crabs will need to leave.
The new residents may realize their mistake and seek different shells. Alternatively, the molting hermit crab may decide the shell no longer meets its needs and seek an upgrade.
Alas, it’s equally possible, if not likelier, that the two hermit crabs will fight for the right to retain the shell. Hermit crabs are omnivorous and display cannibalistic tendencies. Consequently, a larger and more aggressive hermit crab may eat a smaller rival.
In this instance, the molting hermit crab is at the most risk. Even if the molt has concluded, it takes a couple of weeks for a hermit crab’s new exoskeleton to reach maximum strength. This makes the molting hermit crab vulnerable to any attack.
Lack of Individual Shells
Occasionally, two small hermit crabs share a shell because they can’t find individual options that meet their needs. Going without a shell isn’t an option, as it leaves hermit crabs completely vulnerable. Consequently, sharing shells is considered the lesser of two evils.
As explained by Crustaceana, this is more common in the wild. As the population of hermit crabs grows, the availability of suitable shells can become increasingly scarce. Two hermit crabs may agree to share a shell until this changes.
Captive hermit crabs shouldn’t feel the need to share shells. Spare shells are easily obtained by foraging on the beach and sanitizing what you find or purchasing shells at an exotic pet store.
Always keep an array of spare shells in a hermit crabs habitat outside the central eating and recreational areas but within easy access.
Occasionally, two hermit crabs may share a shell to enjoy greater protection from predators. This will only arise among young hermit crabs looking to double their chances of survival in the wild.
If the shell is oversized, sharing may be considered a worthy trade in the short term. A large, heavy shell offers superior protection to a smaller, brittle alternative. As per the Journal of Zoology, most hermit crabs prefer a heavier shell that offers enhanced fortification.
This protection must be balanced with the need to move. The combined strength of two hermit crabs makes a heavy shell easier to negotiate. A cumbersome burden can prevent the hermit crabs from engaging in other preferred activities, including climbing.
As a consequence, the arrangement won’t last long. Young hermit crabs grow rapidly, undergoing multiple molts and increases in mass in the first 18 months of life.
Before long, unless the shell is vast, it won’t be able to accommodate both hermit crabs.
What Should I Do if Two of My Hermit Crabs are Sharing a Shell?
Don’t ignore the arrangement if you spot two captive hermit crabs sharing a shell. It may be by choice at this point, but the relationship will irrevocably break down eventually.
While you can never force a hermit crab out of a shell, you should do all you can to encourage evacuation from one or both hermit crabs.
The easiest way to do this is by offering appealing shell alternatives. Place a range of new shells in your hermit crab’s habitat, and the residents will line up according to size and start trading shells until everybody has the ideal shelter.
Fights may break out over preferred shells, but for the most part, these negotiations will be amicable. If the hermit crabs are wrestling with their antennae, leave them to resolve the conflict. If they start pinching each other, separate them until they cool off.
You may also find that two hermit crabs get trapped in a single shell, as the evacuation point is too small to allow either out. Try submerging the hermit crabs in water for a few minutes, as this will make it easier for them to escape.
You may notice that neither hermit crab wants to be the first to leave the shared shell, as doing so involves conceding ownership to the other hermit crab. So, tempt the hermit crabs out using food.
Terrestrial hermit crabs are unlikely to share their shell. Mollusk shells are popular among hermit crabs, but only once vacated. It’s rare for a hermit crab to evict another species from a shell.
However, marine hermit crabs offer to share their shells with sea anemones. The sea anemone won’t hide inside the shell of a hermit crab but will dwell on the back of its shell or the chelipeds.
Zoological Science explains that hermit crabs and sea anemones have a symbiotic relationship that benefits both parties.
Sea anemones are toxic, so they deter common aquatic predators of the hermit crab, most notably fish and true crabs. By latching onto a hermit crab, sea anemones gain access to food sources that they couldn’t find by themselves.
If two crabs end up living in the same shell, it’s a matter of necessity, not a lifestyle choice.