Hermit crabs are a group of Anomura crustaceans. Despite the close resemblance, they are not true crabs. They do not grow the shells they call home. Rather, they scavenge and take them from the mollusks that make them. When they feel in danger, they hide away in their shells for protection and comfort. Their reclusive nature has earned them the “hermit” title, and finding the right shell is a huge part of a hermit crab’s life.
Hermit crabs need shells to protect themselves from the outside world. They are born with extremely soft, delicate bodies, so they need to acquire hard shells to shield themselves from predators and sharp objects. Hermies also need shells to maintain the optimal amount of moisture around their bodies, lest they dehydrate. Shells even protect them from temperatures that are too hot or cold.
Even if there are no predators in a pet hermit crab’s environment, it rarely survives for long without the protection of its shell. The stress of being vulnerable can be deadly. There are special rules in hermit crab groups that determine which hermie gets which shell. These crustaceans swap shells multiple times throughout their life. After all, shells determine mating success in groups and their ability to grow in size.
Do Hermit Crabs Grow Their Own Shells?
Hermit crabs do not grow their own shells. Although you may never see a hermie without one, the shell is not a natural part of its biology. Instead, every hermit crab has a natural instinct to find and take the shells of other creatures, adopting them as their own.
As babies (or zoea), hermit crabs are completely free of shells and have the appearance of larvae. They go through multiple stages of growth before they’re prepared to leave saltwater and travel on land. During the middle stage, they are known as magelops. They will begin to develop their legs, and in the later phase of this, they will start looking for a protective shell to keep them safe.
At the next stage, hermit crabs become juveniles. At this point, terrestrial hermit crabs will make landfall. They will continue scavenging and stealing shells to swap out with their old ones. After all, hermit crabs cannot grow inside of a home that’s too small. They need to upgrade as they develop and expand in size.
The exoskeleton that most crustaceans are born with is enough to protect them. The exoskeleton hermit crabs have, however, is not. Hermit crabs have soft, vulnerable bodies. The lower half of their exoskeleton isn’t tough enough to protect them from danger. Because of this, they must seek out shells that are left behind (or, sometimes, currently in use) by another creature.
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 as well as other chemicals and minerals as they grow. The mollusks then secrete calcium carbonate from their mantle. This is an organ that covers the entirety of their body. Held together by organic material, the calcium hardens outside of their bodies and forms a protective shell. As time goes on, the shell gets bigger and bigger.
When a mollusk dies, it leaves its shell (which does not disintegrate) behind. Hermit crabs have soft bodies like mollusks do but are unable to secrete calcium carbonate like they can. Instead, they take ownership of the shell that was left behind.
If a hermit crab is particularly desperate, though, or has its eye on one exact shell, it may be less polite. Hermit crabs have been known to drag out or kill snails to take possession of their shells.
How Do Hermit Crabs Get Their First Shell?
Hermit crabs have many ways of acquiring their first shell. The most common method, however, is to inherit one from a bigger hermie.
When it’s time for a hermit crab to move to a bigger shell, it will leave its old one behind. A group of hermies will then form a line behind it. One by one, they will 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.
With that said, hermit crabs aren’t always willing to form an orderly line. If one hermit is struggling 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 will intimidate the other hermie by continuously tapping on its shell. If the owner of the desired shell is not in the mood to fight, it has to leave the comfort of its home. It will then brave the ocean in a vulnerable state.
However, shells are such an integral part of a hermit crab’s survival. Because of this, many hermits would rather fight for their home and have a chance at keeping the shell.
Taking Shells From Mollusks
Hermit crabs may obtain their first shell by simply scavenging for one. When mollusks die, their decomposing bodies give off a strong odor. Hermit crabs follow the scent and can locate the unoccupied shells.
Looking for a shell is not an ideal scenario for a hermit crab. That’s especially true for a young one that’s looking for its very first home. Young hermit crabs have to compete against each other when scavenging for a home. Those that aren’t fast or strong enough to get a shell right away either have to stay vulnerable for much longer or inhabit a broken shell.
Are Hermit Crabs Attached to Their Shells?
Hermit crabs are not physically attached to their shells. They can move in and out of their home whenever they choose. However, they only do so when they absolutely must.
Like most creatures with exoskeletons, as hermit crabs grow, they have to molt. This involves shedding their outer skin. Their new exoskeleton grows just underneath the old one and hardens hours after molting. As a hermit crab develops, it outgrows its shell and needs to find a new one. However, even if the hermit crab needs a bigger shell, it will not 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 shell than move to a more adequate one. That’s because of the risks that come with changing shells.
In the experiment, researchers presented hermit crabs with multiple shells. They noticed that the ones which blended into the environment were the most preferred by hermies. The researchers then presented the hermit crabs with more shells that matched the environment even better. They expected the hermit crabs to move into the newer shells but were surprised to find the hermit crabs refusing to do so.
Researchers theorized that it’s because hermit crabs are so vulnerable when outside their shells. They would rather not risk changing into a better shell if the one they chose is adequate. As such, it doesn’t matter if hermit crabs are not physically attached to their shells and won’t be severely injured if separated from them. It’s dangerous to force them out, as the stress they experience when vulnerable can be deadly.
How Do Hermit Crabs Hold On To Their Shells?
The shells that hermit crabs use are not tailored for them. They are built by mollusks and tailored for the bodies of said mollusks. Despite this, hermit crabs have no problem fitting into, moving around, and living in these shells.
Hermit crabs hold on to their shells using their uropod. The uropod is 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. If you look up pictures of hermit crabs inhabiting glass shells, you can see how the uropod spirals alongside the shell’s interior.
The uropod is a much bigger appendage than the rest of the hermit crab’s legs. It needs to be, so it can carry shells that almost always dwarf the hermit crabs by comparison. In fact, a hermit crab’s ability to carry its large shell directly impacts its overall 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 which occupy intact shells have better strength than those that occupy broken shells. It’s theorized that this is because hermit crabs with intact shells have to carry more weight. Hermit crabs with incomplete shells develop poor strength because their damaged homes weigh far less.
How Long Can a Hermit Crab Live Without a Shell?
A hermit crab’s ability to live without a shell depends greatly on its environment. Hermit crabs only survive in environments with specific parameters, such as:
- A temperature of 70 to 80 degrees Fahrenheit
- A water salinity level of 1.020 and 1.030
- Humidity between 60% and 80%
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 will die from exposure a lot faster. That’s because hermit crabs rely on their shells to keep from getting too cold. They also stay hydrated by keeping water in their shells.
Hermit crabs will also die without a shell because their bodies are too soft. Having to drag their soft abdomens across the ground will increase the risk of injury and infection. Not to mention, hermit crabs have evolved to view shells as an absolute must-have item for survival.
The stress hermit crabs go through when shell-less is more than we can even imagine. Although there haven’t been many studies on a hermit crab’s emotional well-being when shell-less, theories can be made. Based on their erratic, frightened behavior when forcefully removed from their homes, it’s safe to say that the stress they experience contributes to premature death.
Do Hermit Crabs Need Extra Shells?
When keeping hermit crabs as pets, it’s important to have multiple shells available for them to choose from. Hermit crabs never truly stop molting and growing. That means they never stop searching for new shells to occupy.
When they are young, they molt several times a year. However, as they age, they molt every 12 to 18 months. That makes it best to always have multiple shells they can choose from. This serves 3 purposes:
- It keeps the hermit crabs from fighting
- It promotes molting
- It keeps them safe
When hermit crabs cannot find an empty shell to move into, they will fight the owner of an occupied shell. You can prevent your hermit crabs from fighting to the death by keeping a lot of shells at their disposal.
Giving them extra shells will also promote molting. When hermit crabs cannot find a good shell to move into, they would rather delay their molt than find themselves shell-less. This is extremely dangerous because hermit crabs need to molt in order to grow and shed old or damaged skins.
Over time, a hermit crab’s shell can get broken or cracked. A hermit crab can injure its exposed body with a broken shell’s shard or with an item in the tank. Having extra shells on hand will allow the hermit crab to inhabit an intact shell and stay safe.