A hermit crab's shell is more than his home: it's critical for protecting his soft abdomen and maintaining a proper hydration level. In the wild, a hermit crab relies on other mollusks' cast-offs to provide his housing. It's up to the caretaker to provide a pet hermit crab with an appropriate shell and suitable molting conditions.
Molting to Grow
As a hermit crab grows, he sheds his exoskeleton and trades in his old shell for a new shell that will fit his larger frame. Known as molting, it's a very stressful, potentially dangerous process during which the crab is vulnerable; he can't move because he has no muscle control and his new exoskeleton is very soft. Because of this vulnerability, hermit crabs usually burrow underground to molt for protection, insulation and darkness, which is necessary to start the molting process known as ecdysis.
Signs a hermit crab is ready to molt include:
- cloudy eyes
- soaking in the water dish or consuming large amounts of water
- ashy appearance to the exoskeleton
- dampening the habitat's substrate by spilling water
When you sense it's time for your crab to molt, you may want to provide him an isolation tank so he can molt in peace. If you choose to provide him a molting tank, it should have between 6 and 12 inches of moist sand and enough space to hold food and water dishes. If your crab's regular habitat has suitable substrate -- moist and deep enough -- he can also molt among his tankmates. Give your crab a variety of shells to choose from after he's shed his exoskeleton and give him privacy, only tending to his food and water as necessary. Molting time varies depending on the crab's size and can be as short as a couple of weeks for a small crab or as long as two or three months for a very large crab.
When your hermit crab is molting, leave him be unless he is being threatened by his tankmates. Digging up a molting crab adds additional stress, resulting in a potentially life-threatening situation for the crab.
The Right Shell
You prefer to choose your living quarters and your wardrobe and it's the same for your hermit crab. His shell must suit him; it should be the proper size and shape to protect him, with a little room for growing until he's ready to move into a larger shell. Shape is important, too; in the wild, hermit crabs make do with whatever is available to them, including miniature liquor bottles, bamboo tubes and plastic bottle lids. As the juvenile crab's body grows, his abdomen is shaped according to his accommodations.
Sizing It Up
A hermit crab's shell should be large enough for him to completely withdraw inside, using his large claw as a barrier over the opening. Check out your crab's large claw and look for shells with an opening a bit larger than his claw. Your crab should be able to seal himself inside the shell with his large claw and left walking leg.
According to HermitCrabs.com, the perfect shell has no holes and fits the crab snugly, allowing him to withdraw completely into the shell.
Give Him Choices
Just because you fancy a particular shell doesn't mean it's right for your hermit crab. Provide your crab with a variety of shells to choose from. He'll know which will fit him best and is most comfortable for him. Choose differently shaped shells with similar-sized openings to what he currently has and a few that are slightly larger.
When you provide your hermit crab shells to choose from, take a pass on painted shells. Though they can be pretty to look at, they can mean death for a hermit crab. Some shells are painted with toxic materials while others use non-toxic paint but rely on a protective clear coat to maintain the finish. Over time, the habitat's sandy substrate can wear on the paint and finish, causing it to chip and be ingested by the crab. Look for pretty, natural shell finishes if you want to dress your crab a little more formally.