Hermit crabs are a group of Anomura crustaceans. Despite their close physical resemblance, they’re not true crabs, so they don’t grow their shells.
Instead, they scavenge shells from marine gastropods. When they feel in danger, they hide in their shells. Their reclusive nature has earned them the “hermit” title.
Hermit crabs need shells to protect themselves. They’re born with soft, delicate bodies, so they must acquire hard shells to shield themselves from predators, extreme weather, and wear and tear.
Also, hermit crabs need shells to maintain moisture around their bodies, lest they dehydrate.
Do Hermit Crabs Grow Their Own Shells?
Although you’ll rarely see hermit crabs without a shell, it isn’t a natural part of their biology. Hermit crabs find and take the shells of marine gastropods, adopting them as their own.
As babies (zoea), hermit crabs are free of shells and have the appearance of larvae. They go through multiple stages of growth before they’re prepared to leave the saltwater and travel on land.
During the middle stage, they’re known as magelops. They’ll begin to develop legs, and in the later phase, they’ll seek a protective shell to protect them from harm.
In the next stage, hermit crabs become juveniles. At this point, terrestrial hermit crabs will land; they’ll continue scavenging and seeking shells to swap out with their old shells.
Hermit crabs can’t grow inside a shell that’s too small; they need to upgrade as they increase in size.
The exoskeleton that most crustaceans are born with is enough to protect them. However, the exoskeleton of hermit crabs isn’t, as they have soft, vulnerable bodies.
The lower half of their exoskeleton isn’t tough enough to protect them from harm. For this reason, they must seek out shells left behind (or, sometimes, currently in use) by other creatures.
Where Do Hermit Crab Shells Come From?
Hermit crab shells primarily come from marine gastropods.
This is a particular class of mollusk that can create shells naturally. Mollusks can be divided into snails, clams, and other marine species in the phylum Mollusca.
Their outer skin absorbs the ocean’s salt, chemicals, and minerals as they grow. The mollusks then secrete calcium carbonate from their mantle.
This is an organ that covers their entire body. Held together by organic material, the calcium hardens outside their bodies and forms a protective shell. As time progresses, the shell gets bigger.
When a mollusk dies, it leaves behind its shell (which doesn’t disintegrate). Hermit crabs have soft bodies but can’t secrete calcium carbonate. Instead, they take ownership of the left-behind shell.
How Do Hermit Crabs Get Their First Shell?
When it’s time for a hermit crab to move to a bigger shell, it’ll leave its old shell behind.
A group of hermit crabs will form a line to decide who gets it. One by one, they’ll examine the shell to see if it’s to their liking.
Once a hermit crab has chosen the abandoned shell as its new home, the others in line decide who gets the second hermit crab’s old shell.
Hermit crabs aren’t always willing to form an orderly line. If one hermit crab struggles to find an appropriate shell in time, or there aren’t many options on its beach, it will fight for it.
According to Behavioral Ecology and Sociobiology, hermit crabs can evict others from their shells.
When a hermit crab is interested in an occupied shell, it’ll intimidate the other hermit crab by continuously tapping on its shell.
If the owner of the desired shell is disinterested in fighting, it’ll have to leave the comfort of its home.
However, shells are an integral part of a hermit crab’s survival. For this reason, many hermit crabs would rather fight for their home and have a chance at keeping their shell.
Taking Shells from Mollusks
Hermit crabs may obtain their first shell by scavenging for one. When mollusks die, their decomposing bodies give off a strong odor, and Hermit crabs follow the scent and locate unoccupied shells.
Looking for a shell isn’t an ideal scenario for a hermit crab, especially when it’s young. Those that aren’t fast or strong enough to get a shell will be vulnerable for longer or inhabit a broken shell.
Are Hermit Crabs Attached to Their Shells?
Hermit crabs aren’t physically attached to their shells, so they can move in and out of their home whenever they choose. However, they only do so when necessary.
Like most creatures with exoskeletons, as hermit crabs grow, they molt. This involves shedding their outer skin; their new exoskeleton grows underneath the old one and hardens hours after molting.
As a hermit crab develops, it outgrows its shell and needs a new one. However, even if the hermit crab needs a bigger shell, it won’t molt and move out until it has found an adequate replacement.
According to the Journal of Experimental Biology, hermit crabs would rather stay in their shells than move to more adequate shells due to the risks associated with the transition.
In the experiment, researchers presented hermit crabs with multiple shells. They noticed that the shells that blended into their environment were the most preferred by hermit crabs.
The researchers then presented the hermit crabs with more shells that matched the environment better. They expected the hermit crabs to move into the newer shells but were surprised to find the hermit crabs were disinterested.
Researchers theorized that their disinterest is because hermit crabs are vulnerable outside their shells. They would rather avoid the risk of switching to a better shell if the shell they chose is adequate.
How Do Hermit Crabs Hold On To Their Shells?
Hermit crabs’ shells are created by mollusks and tailored for their bodies. Despite this, hermit crabs have no problem fitting into, moving around, and living in their discarded shells.
Hermit crabs hold on to their shells using their uropod, a small appendage located at the back of a hermit crab’s abdomen. The uropod grips the shell from the inside and allows the hermit crab to move around with the shell.
The uropod is a much bigger appendage than a hermit crab’s legs. It needs to be that way so that it can carry shells that almost always dwarf the hermit crabs by comparison.
A hermit crab’s ability to carry its large shell impacts its strength. In the above study, researchers also explored what determines a hermit crab’s strength.
They separated the test subjects into two categories:
- Hermit crabs with broken shells
- Hermit crabs with intact shells
When observing the hermit crabs as they fought for shells, researchers noted that hermits that occupied intact shells had more strength than those that occupied broken shells.
It’s theorized that this is because hermit crabs with intact shells carry more weight. Hermit crabs with incomplete shells develop less strength because their damaged homes weigh less.
How Long Can a Hermit Crab Live Without a Shell?
A hermit crab’s ability to live without a shell depends on its environment. Hermit crabs only survive in environments with specific parameters, such as:
If the conditions aren’t right, the hermit crab will die, even if it has a shell. However, if the conditions are bad and the hermit crab has no shell, it’ll die from exposure a lot faster.
Hermit crabs rely on their shells to avoid getting cold and stay hydrated by keeping water in their shells.
Also, hermit crabs will die without a shell because their bodies are too soft. Dragging their soft abdomens across the ground will increase the risk of injury and infection.
Although there are no studies on a hermit crab’s emotional well-being when shell-less, their erratic, fearful behavior when forcefully removed from their shell shows the stress they experience.
Do Hermit Crabs Need Extra Shells?
When keeping hermit crabs as pets, you need a choice of shells available. Hermit crabs never truly stop molting and growing, which means they’re always searching for superior shells to occupy.
When young, hermit crabs molt several times a year. However, as they age, they molt every 18 months. So, it’s best to have various shells available for three reasons:
- Prevents fighting
- Promotes molting
- Safety reasons
When hermit crabs can’t find an empty shell to move into, they’ll fight the owner of an occupied shell. You can prevent hermit crabs from fighting by providing suitable alternatives.
Giving them extra shells will also promote molting. When hermit crabs can’t find a good shell to move into, they’d rather delay their molt than find themselves shell-less.
This is problematic because hermit crabs need to molt to grow and shed old, damaged skins. Over time, a hermit crab’s shell can crack and become worn.
Providing extra shells allows hermit crabs to inhabit intact shells and stay safe.