Last Updated on October 8, 2023 by Joanne Harper
Hermit crabs aren’t born with shells, so they must be acquired from other animals. Size, durability, scent, and mobility are among the many criteria that hermit crabs seek in a shell.
Wild hermit crabs scavenge their shells. Usually, shells will be vacated by other hermit crabs or sea snails that died or upgraded their shells. Then, they’ll change shells many times during their lifetime.
While young, hermit crabs regularly outgrow their shells. As adults, hermit crabs seek to upgrade or exchange shells for new, more suitable shells.
Providing a range of shell options keeps hermit crabs happy, occupied, and stress-free.
Are Hermit Crabs Born with Shells?
Hermit crabs aren’t true crabs in that they’re not born with shells. Instead, they must source shells to protect their exoskeleton. A hermit crab without a shell is easy for predators to pick off.
All hermit crabs start life as zoeae. Females carry eggs, which enter the ocean when ready to hatch. These eggs explode upon contact with water, and zoeae are born. These are tiny larvae floating with plankton.
If a zoea survives for 60 days, it’ll evolve into a megalopa. This is a tiny aquatic specimen, and hermit crabs find their first shell at this stage. The megalopa life stage lasts around 30 days.
If hermit crabs survive this long, they’ll bury themselves in sand and molt. Upon emergence, a molted megalopa is a juvenile that needs a cycle of shells to accommodate its growth and survive into adulthood.
How Do Hermit Crabs Get Their First Shell?
The Journal of Experimental Marine Biology and Ecology explains that a megalopa claims its first shell in the ocean. As it’s so minuscule at this stage, it’ll usually be a shell vacated by a sea snail.
A megalopa uses this shell to protect itself once it reaches land. It’ll grow lungs and enhanced gills to adapt to terrestrial life. The megalopa will remain tiny, around the size of a fingernail.
The shell search begins in earnest after evolving into a juvenile hermit crab.
Like all young animals, juvenile hermit crabs are vulnerable. As their mothers don’t raise them, hermit crabs must learn how to survive immediately, which involves finding a vacant shell.
Juvenile hermit crabs constantly outgrow their shells. Revista Chilena de Historia Natural explains that some hermit crabs delay growth and development, giving them access to more shells.
Medium-sized shells are the hardest to find, while small shells are plentiful, as they can be claimed from gastropods. Large shells are abundant but more coveted by adult hermit crabs, and juvenile hermit crabs will rarely best an elder in conflict.
Where Do Hermit Crabs Find Their Shells?
Wild hermit crabs find new shells while scavenging, so many crustaceans choose to locate themselves in a territory with a plentiful supply of new shells.
Most hermit crabs rely on seawater to locate shells. Animal Behavior explains that the presence of water vapor enhances the senses of hermit crabs, which makes shells easier to find.
Changing shells involves leaving an existing shell to move into a new shell, making them vulnerable to predators. A hermit crab will only consider trading shells if the new home is much better.
How Do Hermit Crabs Get New Shells?
Hermit crabs leave their existing shell reluctantly when it’s safe.
The hermit crab will “try on” a new shell. If it feels comfortable, it’ll remain. If not, it’ll return to its shell before another hermit crab claims it.
There are three ways that hermit crabs source a new shell:
Scavenging for Empty Shells
Hermit crabs trawl the beachfront for food. In doing so, they’ll also seek out spare shells. If a hermit crab finds a shell more suitable for its needs than the vessel it occupies, it’ll consider changing.
This could be an empty seashell, the remnant of a dead gastropod, or the shell of a dead conspecific. The hermit crab may not like the new shell and return to its existing one.
Garbage is a cause of mortality in hermit crabs. For example, some hermit crabs climb inside plastic bottles and bottle caps and can’t escape.
Dead Hermit Crabs
When a hermit crab dies, it releases a distinctive pheromone. Ecology and Evolution explain that other hermit crabs will pick up this scent, and surviving hermit crabs will investigate.
As hermit crabs live in colonies and are social animals, they’re concerned with the continuation of their species. Ergo, releasing this ‘death scent‘ is an attempt to allow life to go on for others.
The dying hermit crab ensures that a good shell isn’t wasted.
Fighting Other Hermit Crabs
Captive hermit crabs enjoy living in groups. Despite this, they’ll become combative on occasion.
Shell rapping involves a dominant hermit crab knocking on the shell of another. Hermit crabs engage in shell rapping when one hermit crab covets the shell of another.
This may happen when it’s outgrowing its current shell, so it’ll challenge another hermit crab to a conflict. If the aggressor wins, the hermit crabs switch shells.
As explained by Behavior, sometimes both hermit crabs will be happy with their new shells.
How Often Do Hermit Crabs Change Shells?
Hermit crabs constantly change their shell in the first 1-2 years of life. Hermit crabs grow fast, with males experiencing particular growth spurts immediately preceding molting.
Females change shells less frequently because they grow more slowly. Pregnant hermit crabs carry eggs in three appendages on the abdomen.
The female’s body remains smaller, so it has a higher choice of shells large enough to offer protection. For this reason, most females reproduce in their first year of life.
Regardless of the sex of a hermit crab, it’ll change shells periodically.
Hermit crabs can be trapped in a shell if they outgrow it. Equally, the shell may be damaged by conflict or wear and tear. Equally, the hermit crab may want a better shell.
When a hermit crab molts, it grows a new exoskeleton. This usually sees the hermit crab increasing in size and mass, sometimes drastically. If so, this means the hermit crab will need a new shell.
Hermit crabs often take their shells with them when burrowing to molt because they store water supplies in their shells, keeping them alive during underground molting. A molt may take 1-2 months to complete.
Molting hermit crabs should be separated from their tank mates. Arrange this before the molt begins, as a molting hermit crab shouldn’t be disturbed.
Signs that a hermit crab is preparing to molt include:
- Growing sluggish.
- Eating and drinking more than usual.
- Digging a lot.
- The exoskeleton is fading to a dull gray.
- Glazed, milky look in the eyes.
Immediately post-molting, hermit crabs lack a shell. The previous shell may be recycled but likely cracked and splintered during molting.
With no shell, a hermit crab is vulnerable, so keep it alone with a selection of new shells. Once a shell has been chosen, you can return the hermit crab to its friends.
Inter-hermit crab conflict can result in shell trading.
Monitor this practice, as hermit crabs have unique personalities, and some may be more docile and submissive than others, which opens the possibility of being bullied.
If you notice one particular hermit crab is constantly being ousted from its shell, consider rehoming it.
Sometimes, hermit crabs want a change of scenery, so they change their shells for increased mobility, protection, or boredom. So, offer an array of different shapes and sizes.
How Do Hermit Crabs Choose Their Shells?
Choosing a shell for a hermit crab is akin to picking a home for humans. Just as you’d prefer not to take the first house or apartment you see, hermit crabs take their time finding the right shell.
When a shell becomes available, hermit crabs often line up to review it, taking turns trying it on for size. There is a pecking order in any hermit crab colony, whether wild or domesticated.
If a dominant hermit crab likes a new shell, it’ll likely be claimed.
This creates a domino effect among the remaining hermit crabs. To return to our house purchasing metaphor, this is the chain of properties. The shell vacated by the dominant hermit crab is now available, and the process will begin anew until each hermit crab is satisfied.
Any permanent mobile home for hermit crabs must meet four criteria:
The scent of a shell is pivotal to hermit crabs because it’s their most important sense. If the shell smells entirely neutral, it’ll likely be rejected.
It’ll be more appealing if the shell carries the scent of another hermit crab or gastropod. Equally, the smell of saltwater may be enough to tempt a hermit crab.
Bigger isn’t always better when it comes to hermit crab shells. As discussed, a large shell may attract unwanted attention from rivals. Equally, a large, heavy shell may be cumbersome.
The size of the hole that a hermit crab enters is important, as hermit crabs seek a shell that makes them feel safe but not snug. If the hole a hermit crab enters and exits from is too small, it’ll get stuck.
A hermit crab’s shell is a source of protection from predators and the elements. As a result, it needs to be sturdy and tough enough to stave off the sun’s rays.
Hermit crabs may rap upon a shell to check how tough it is. Hermit crabs are aware of the strength of a shell. If a shell grows cracked and damaged, it’ll feel exposed and become belligerent.
Hermit crabs seek shells that afford sufficient mobility, as they must walk comfortably with a shell on their back. Failure to access food due to the weight of the shell will become distressing.
Hermit crabs will flee and hide if they feel threatened. However, a hermit crab may get stuck between the aquarium walls and obstacles if the shell is too large and cumbersome.
Should I Change My Hermit Crab’s Shells?
Hermit crabs fiercely guard their shells. If you attempt to pull a hermit crab out of its shell, it’ll strongly resist emerging. A hermit crab would rather be torn limb from limb than relinquish a shell by force.
You can offer hermit crabs a variety of shells by leaving them in an aquarium. Just dot them around the substrate, and hermit crabs will take a close look.
If interested, the hermit crab will try on a shell, usually after dark, because they’re nocturnal.
Encouraging Hermit Crabs to Change Shells
If you’re concerned that the shells of hermit crabs are too small or otherwise unsuitable, do your utmost to encourage change. Offer a wide selection of shells so that there’s lots of variety.
Moisten the inside of the shells with boiled seawater. Marine and Freshwater Behavior and Physiology stated that hermit crabs are attracted to boiled shells.
Don’t take the alternatives away if hermit crabs haven’t changed shells overnight. The hermit crabs may be happy with their current shells, and there’s no way of knowing when this will change. Existing shells may crack, be outgrown, or cause conflict.
So, remove the shells and moisten them roughly once per week. If there’s no interest, switch them up for some alternatives to see if they’re more appealing.