As hermit crabs are not born with shells, they must be acquired from other animals. Size, durability, scent, and mobility are among the many important criteria that hermit crabs look for in a shell.
Wild hermit crabs scavenge their shells. Usually, these shells will be vacated by another crab or sea snail that died or traded up to a bigger shell. They will change their shells many times during their lifetime.
While young, hermit crabs regularly outgrow their shells. As adults, hermit crabs look to upgrade or exchange shells for a new shell that is more suitable. Providing different options keeps pet hermit crabs happy.
Are Hermit Crabs Born with Shells?
Hermit crabs are not true crabs, in that they are not born with shells. Instead, they must source shells to protect their exoskeleton. A hermit crab without a shell is easy pickings for any predator.
All hermit crabs start life as zoeae. Females carry eggs, which enter the ocean when ready to hatch. These eggs explode on contact with water, and the zoeae are born. These are tiny larvae that float with plankton.
If a zoea survives for around 60 days, it will evolve into a megalopa. This is a tiny aquatic specimen, and at this stage, hermit crabs find their first shell. The megalopa life stage of a hermit crab lasts around 30 days.
If hermit crabs last this long, they will bury themselves in sand and molt. Upon emerging, a molted megalopa is a juvenile hermit crab. It needs a constant cycle to shells to keep up with growth to survive into adulthood.
How Do Hermit Crabs Get Their First Shell?
As explained by the Journal of Experimental Marine Biology and Ecology, a megalopa claims its first shell in the ocean. As the animal is so minuscule at this stage, this will usually be a shell vacated by a sea snail.
A megalopa uses this shell to protect itself once it reaches land. The megalopa will grow lungs and enhanced gills so it can adapt to terrestrial life. The megalopa will remain tiny – around the size of a fingernail. If it needs a new shell, it may settle for a bottle top.
After evolving into a juvenile hermit crab, the shell search begins in earnest. Like all young animals, juvenile hermit crabs are vulnerable. As they are not raised by their mothers, hermit crabs learn how to survive immediately. This involves finding a sturdy vacant shell.
Juvenile hermit crabs constantly outgrow their shells. As explained by Revista Chilena de Historia Natural, some hermit crabs deliberately delay growth and development for this reason. The smaller the hermit crab is, the more shells it will have to choose from.
Medium-sized shells are the hardest to find. Small shells are plentiful, as they can be claimed from gastropods. Large shells are also abundant. Large shells are coveted by adult hermit crabs, though. Juvenile hermit crabs will rarely best an elder in conflict.
Where Do Hermit Crabs Find Their Shells?
Hermit crabs in the wild find new shells while scavenging. The hunt for new shells is a major part of a hermit crab’s routine. Many choose to locate themselves in a territory with a plentiful supply of new shells.
Most hermit crabs rely on seawater to locate shells. As explained by Animal Behavior, the presence of water vapor enhances the senses of hermit crabs. This makes shells easier to locate. Without access to water, hermit crabs grow disoriented and confused.
Changing shells is a double-edged sword for hermit crabs. To move into a new shell, they must leave their existing mobile shelter. This leaves them vulnerable to predators. A hermit crab will only consider trading shells if the new home appears appealing.
Hermit crabs captured by seafront shop owners for sale will already have shells. Hermit crabs do not survive long without this protection. Shop owners paint these shells in a range of colors. This is a sales technique designed to appeal to shoppers.
Hermit crabs are indifferent to the aesthetics of shells. Painting shells can be dangerous, in fact. Hermit crabs can find themselves glued inside a shell by wet paint or breathe in toxic paint fumes.
How Do Hermit Crabs Get New Shells?
Hermit crabs leave their existing shell reluctantly, and only when it is safe to do so. They will only leave their shells when no predators are nearby and the sun is not high.
The hermit crab will “try on” a new shell. If it feels comfortable, it will stay put. If not, it will return to its previous shell before another hermit crab can claim it. There are 3 ways that a hermit crab will source a new shell.
Scavenging for Empty Shells
Hermit crabs trawl the beachfront for food. In doing so, they will also seek out spare shells. If a hermit crab finds a shell more suitable for its needs than the vessel it occupies, it will consider changing.
This could be an empty seashell, the remanent of a dead gastropod, or, as we’ll discuss in a moment, the shell of a dead conspecific. The hermit crab may not like the new shell and return to its existing one. Some hermit crabs will remain in a new mobile home, though.
Scavenging hermit crabs can grow confused. Litter is a significant cause of mortality in wild hermit crabs. These crustaceans climb inside empty plastic bottles, for example, and are unable to get out again.
Dead Hermit Crabs
When a hermit crab dies, it releases a distinctive pheromone. As explained by Ecology and Evolution, this scent will be picked up by other hermit crabs. These surviving crabs will immediately come to investigate.
This is not hermit crabs being ghoulish, though this species does resort to cannibalism to survive if necessary. Instead, the release of the pheromone announces the availability of a shell.
As hermit crabs live in colonies and are social, they are concerned with the continuation of their species. Ergo, the release of this ‘death scent‘ is an attempt to allowing life to go on for others. The dying hermit crab is ensuring that a perfectly good shell does not go to waste.
Fighting Other Hermit Crabs
Captive hermit crabs enjoy living in groups. Despite this, hermit crabs will come into conflict on occasion. This is will usually take the form of shell rapping. This 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 feels it is outgrowing its current shell. It will challenge this second crab to a conflict. If the aggressor wins, the hermit crabs switch shells.
As explained by Behavior, not all shell rapping among hermit crabs results in a winner and a loser. It is possible that both hermit crabs will be happy with the outcome. If the evicted hermit crab is content with the trade, all parties involved are pleased with the outcome.
How Often Do Hermit Crabs Change Shell?
Hermit crabs in the first year or two of life change shell constantly. Hermit crabs, especially males, grow quickly. Males experience particular growth spurts immediately preceding molting.
Females change shells less frequently. This is because female hermit crabs grow slower. This is a natural evolution.
Pregnant hermit crabs carry eggs in three appendages on the abdomen. The female’s body remains smaller so it has a greater choice of shells large enough to offer protection. For this reason, most females reproduce in their first year of life.
Whatever the sex of a hermit crab, it will change shells periodically. This may be an act of necessity. 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 just want a change.
When a hermit crab molts, it grows a new exoskeleton. This usually sees the hermit crab increasing in size and mass – sometimes drastically so. This means the hermit crab will need a new shell.
Hermit crabs will often take their shells with them when burrowing to molt. This is because hermit crabs store water supplies in their shell. This keeps them alive during the underground molting process. A molt may take several weeks or even months.
Molting hermit crabs should be separated from tankmates. Arrange this before the molt begins, as a molting hermit crab should never be disturbed. Signs that a hermit crab is preparing to molt include:
- Growing sluggish and lazy
- Eating and drinking more than usual
- Digging a lot
- Exoskeleton 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 it likely cracked and splintered during molting. With no shell, a hermit crab is vulnerable. Keep it alone with a selection of new shells. Once a shell has been chosen, return the hermit crab to its friends.
As discussed above, inter-hermit crab conflict can result in shell trading. In theory, this not a bad thing. As long as all hermit crabs have shells, that’s all that matters.
All the same, do keep an eye on this practice. All hermit crabs have a unique personality. Some may be more docile and submissive than others. That makes it possible for hermit crabs to be bullied.
If you notice one particular hermit crab is constantly being ousted from its shell, consider rehoming it. If the same thing happens again, keep the hermit crab alone. While most hermit crabs prefer company, it can be dangerous and stressful to be regularly evicted.
Sometimes, hermit crabs just want a change of scene. They may change their shell for increased mobility, greater protection, or just boredom. Hermit crabs like variety in their lives.
Factor this into any selection of shells for your hermit crabs. Offer an array of different shapes and sizes. As we mentioned previously, hermit crabs do not care about the colors or designs of shells. Accessibility of shells is pivotal, though.
How Do Hermit Crabs Choose Their Shells?
Many factors influence shell selection in hermit crabs. Choosing a shell for a hermit crab is akin to picking a home for humans. Just as you would prefer not to take the first house or apartment you see, hermit crabs take their time finding the perfect shell.
When a shell becomes available, hermit crabs often line up to review it. They will take it in turns to try the shell 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 will 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. The process will begin anew until every crab is satisfied.
To return to our initial question, though – what do hermit crabs look for in a shell? Any permanent mobile home for hermit crabs must meet four key criteria.
The scent of a shell is pivotal to hermit crabs. This sense is the most dominant in a hermit crab’s arsenal. If the shell smells entirely neutral, it will likely be rejected.
If the shell carries the scent of another crab or even a gastropod, it will be more appealing. Equally, the scent of saltwater may be enough to tempt the hermit crab.
The size of a hermit crab’s shell is arguably the primary concern. Bigger is not always better. 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 all-important, though. Hermit crabs seek a shell that makes them feel safe but not snug. If the hole that a crab enters and exits a shell from is too small, the hermit crab can get stuck. This will create feelings of panic.
A hermit crab’s shell is a source of protection from predators and the elements. As a result, it needs to be sufficiently sturdy. It must also be tough enough to stave off the rays of the sun.
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, hermit crabs feel exposed and become increasingly belligerent.
Hermit crabs seek shells that afford sufficient mobility. Hermit crabs need to be able to walk comfortably with a shell upon their back. Failure to access food due to the weight of the shell will become distressing.
Hermit crabs can also become trapped if their shell is immobilizing. Hermit crabs will flee and hide if they feel threatened. If the shell is too cumbersome, they may get stuck between aquarium walls and obstacles. This will be factored into a choice of shell.
Should I Change My Hermit Crab’s Shells?
You can purchase a range of shells online or from an exotic pet specialist. Any merchant that handles hermit crabs regularly will have a selection of suitable shells. You can usually browse by size, shape, and style.
Never force hermit crabs to change shell against their will. Hermit crabs ferociously guard their shells. If you try to pull a hermit crab out of its shell, it will resist. Hermit crabs would rather be torn limb from limb than relinquish a shell by force.
You can certainly offer your hermit crabs a variety of shells. Leave these in an aquarium. Just dot them around the substrate. The hermit crabs will investigate at their leisure. If interested, the hermit crabs will try on these shells. This will typically happen after dark.
There will come a time when hermit crabs need to switch shells. This could be through choice or necessity. The important thing is that hermit crabs call the shots. Take a hands-off approach to hermit crab shells.
Encouraging Hermit Crabs to Change Shells
If you are concerned that the shells of your hermit crabs are too small or otherwise unsuitable, try to encourage change. Offer a wide selection of shells. Hermit crabs enjoy having a variety.
Moisten the inside of the shells with boiled seawater to make them more appealing to hermit crabs. Marine and Freshwater Behavior and Physiology confirm that hermit crabs are attracted to boiled shells. Never use tap water as it contains chlorine and copper, both of which are fatal for hermit crabs.
If your hermit crabs have not changed shell overnight, do not take the alternatives away. The hermit crabs may be happy with their current shells. There is no way of knowing when this will change, though. Existing shells may crack, be outgrown, or cause conflict.
With this in mind, remove the shells and remoisten them roughly once a week. Keep repeating this process. Eventually, your hermit crabs will grow curious enough to investigate.
Hermit crabs like to find shells by themselves and choose when to switch home. In the wild, this will be a natural process. In captivity, you can offer an array of shells to choose from. In doing so, you’ll keep your hermit crabs happy and engaged.