Hermit crabs need shells to survive in the wild and captivity. No hermit crab will retain the same shell for life because they need to upgrade to larger shells as they increase in size. When a hermit crab leaves its shell, it’s known as shell evacuation.
Most hermit crabs won’t evacuate a shell that still meets their needs. If the shell is large enough to accommodate the hermit crab and offers sufficient strength and protection from predators and the sun’s rays, it’ll guard its shell jealously.
If the hermit crab is scheduled to undertake a molt, it’ll increase in size and mass. The hermit crab will evacuate its shell, so it doesn’t become trapped once the molt is complete. Once the process is complete, it’ll seek out a new vessel.
If a shell has been cracked or otherwise damaged, a hermit crab will likely leave and seek a safer, more robust alternative. Equally, hermit crabs evacuate shells that are too large and heavy to maneuver or when a rival hermit crab continually challenges it over the shell.
Hermit crab evacuation doesn’t need to be considered an emergency. As long as it has access to an alternative shell that meets its needs, it’ll quickly find a replacement.
How Long Can a Hermit Crab Survive Without a Shell?
Shells are a necessity for hermit crabs. The exoskeleton of a hermit crab isn’t sturdy enough to resist impact injuries if it falls from the roof of its tank and can’t shield it from the sun’s UV rays.
If a hermit crab evacuates its shell, it must immediately find a replacement. Keep spare shells for pet hermit crabs, as any evacuation needs to be a short-term measure.
Why Do Hermit Crabs Leave Their Shells?
Hermit crabs don’t decide to leave their shells lightly. If a hermit crab vacates a shell, you need to understand why it has happened.
Hermit crabs require an environment that mirrors their natural habitat to survive and thrive. Core components of this include:
- Ambient temperature of at around 80OF.
- Humidity levels of 80% to avoid suffocation.
- No less than six inches of substrate for a hermit crab to bury itself within.
- Water to bathe in and drink.
If these needs aren’t met, hermit crabs grow increasingly uncomfortable. If a hermit crab feels too hot, unable to breathe, or otherwise distressed, it may evacuate its shell, hoping to improve its conditions.
Discomfort in the Shell
If the conditions within a tank meet the needs of a hermit crab and other residents feel comfortable in the environment, check the shell’s interior post-evacuation.
Sand may have got within the shell, rubbing against the skin of the hermit crab. Alternatively, if it stored food within, it could have spoiled and begun spreading bacteria or fungi.
Check for any signs of parasites. The Journal of Experimental Marine Biology and Ecology confirms that countless mites and other pests can enter a shell, especially if insects make their way into a habitat.
When a hermit crab molts its exoskeleton, it’ll usually increase in size. This evolution in mass is most notable for the first 18 months of a hermit crab’s life when it molts most frequently.
Most evacuate during the process because a shell is usually too small for a hermit crab following molting. The hermit crab won’t evacuate completely before commencing a molt.
You may notice a hermit crab out of its shell in its water dish, especially if this is saline water. This allows the hermit crab to douse its gills, keeping them damp and making breathing considerably easier.
Many hermit crabs will initially enter the water wearing its shell, filling the vessel with water. This is especially likely pre-molt, as it ensures it retains sufficient moisture while hiding under the substrate.
If the hermit crab leaves the shell to absorb itself in water, this isn’t a matter of immediate concern. Monitor the hermit crab, and remove it from submersion if more than 20 minutes pass. Terrestrial hermit crabs can’t breathe underwater indefinitely, so they’ll drown.
Shell No Longer Fit for Purpose
If a shell is cracked or broken, it’s longer fit for purpose. In such an instance, the hermit crab will almost immediately look to trade the shell for a replacement. The longer it remains in a substandard shell, the likelier it’ll face danger.
Also, if a hermit crab has a too solid and heavy shell, it may not be usable. Hermit crabs have impressive proportional strength but can’t maneuver in an excessively cumbersome shell.
Hermit crabs don’t just hide in their shells all day, as they like to explore their surroundings and climb. If a shell is too heavy to accommodate these instincts, it’ll likely evacuate in favor of a lighter alternative.
Competition from Rival Hermit Crabs
Hermit crabs usually live side-by-side without much conflict, but disputes can arise. Hermit crabs will fight over territory, the right to mate with a female, and occasionally shells.
Proceedings of the Royal Society explains how a hermit crab may instigate a fight over a shell, coveting the shelter that belongs to a rival, but change its mind once the conflict begins. Some hermit crabs will keep attacking until the argument is settled.
If the incumbent hermit crab has no interest in fighting for the right to its shell, it’ll hide inside until the aggressor loses interest. Some hermit crabs will evacuate and battle, looking to settle the dispute and continue living in peace.
How Does a Hermit Crab Switch Shells?
When a hermit crab is ready to change shells, it becomes a group activity. One evacuation starts a chain that involves the entire colony, the members of which line up according to size. The initial hermit crab crawls out of its shell and into a new, more suitable replacement.
Once the hermit crab that evacuated has a new shell, its old one is available for use by other hermit crabs, assuming it’s not damaged. The largest, dominant hermit crab will evacuate its existing shell and try on the now-vacant replacement.
If this hermit crab likes the shell, the process continues. The next one in line will try on the most recently evacuated shell, and so the chain continues. Typically, exchanging shells following an evacuation is a peaceful and mutually beneficial experience for all parties.
Why Did My Hermit Crab Come Out of its Shell and Die?
All living things must die eventually, and hermit crabs are no exception. Nobody can be certain if hermit crabs perceive their mortality, but it’s common for them to evacuate a shell before death.
When hermit crabs die, they release an unmistakable odor akin to rotten fish. Ecology and Evolution explain how this aroma attracts conspecifics.
Part of this is due to the omnivorous, scavenging nature of hermit crabs, as these animals have no concerns about consuming dead comrades. Equally, a dead hermit crab means a shell is now lying vacant.
Hermit crabs will line up to try on this new shell for size. As with many small animals, the species’ survival is pivotal to hermit crabs. By vacating a shell before death, a hermit crab enhances the odds of survival for another colony member.
Hermit crab evacuation should be considered on par with moving house. Leaving a shell isn’t done without thought, but it’s sometimes essential.
If a hermit crab in your care evacuates its shell, ensure it chooses a replacement quickly and that all others in the tank are safely and contentedly secured in new shells.