What Happens When a Hermit Crab Outgrows Its Shell?

Most crustaceans are covered head-to-tail with a hard, durable exoskeleton, but not the hermit crab. This scavenging crab has no protection for its soft abdomen, and they must find discarded shells from whelks or other mollusks in order to protect themselves from predators and to stay hydrated when the blistering sun is at its hottest. As a hermit crab grows, it needs a larger shell -- and that’s when life gets difficult. A good shell can be hard to find, and there are many other crabs also looking to upgrade. In an amazing display of cooperation, hermit crabs have been observed lining up in size order to swap for a new shell. A hermit crab in need of a larger home will try on a hand-me-down shell discarded by a bigger, newly-outfitted crab, and, in turn, leave its old shell for a smaller crab to inhabit.

A different kind of shell game:

• Hermit crabs cannot grow very well if they are living in a shell that is too small. They are also liable to be eaten, since they cannot retract completely into their shells.
• To get into a shell, a hermit crab squeezes in backwards, anchoring with its four back legs. Four other legs are used for walking, and the front two legs with pincers, known as the chelipeds, are used to grab prey and fend off foes.
• If a suitable shell can’t be found, a hermit crab might don other objects, such as hulls from large tree nuts, or even pieces of beach litter. On beaches where people collect shells, finding new digs can be a challenge for a growing hermit crab.

Frequently Asked Questions

How do hermit crabs find new shells when they outgrow their current ones?

Hermit crabs search for new shells by exploring their environment, often inspecting several potential shells before finding one that fits. They assess the size, weight, and condition of the shell. If a suitable shell is found, they quickly switch homes, as their soft abdomens need protection from predators and environmental factors.

What risks do hermit crabs face when changing shells?

During the shell change, hermit crabs are vulnerable to predators due to their exposed soft bodies. They also risk encountering aggressive competition from other hermit crabs for the same shell. Additionally, if a new shell is not found in time, they can suffer from stress and physical damage.

Can hermit crabs live without a shell?

Hermit crabs cannot live without a shell for long. Their shells provide essential protection for their delicate exoskeletons and help prevent desiccation. Without a shell, hermit crabs are exposed to predators and environmental hazards, which can quickly lead to their demise.

Do hermit crabs ever share or exchange shells?

Hermit crabs have been observed participating in 'vacancy chains,' where multiple crabs line up from largest to smallest, each waiting to move into a larger shell as it becomes available. This cooperative behavior helps ensure more crabs find suitable homes with less conflict and competition.

How often do hermit crabs need to change their shells?

The frequency of shell changes in hermit crabs depends on their growth rate, which is influenced by factors like diet and environmental conditions. Young, rapidly growing crabs may change shells several times a year, while adults with slower growth rates may only need to change shells once a year or less.

What can humans do to help hermit crabs with their shell needs?

Humans can assist hermit crabs by providing a variety of clean, appropriately-sized shells in their habitats. It's important to avoid painted or chemically treated shells, as these can be toxic to the crabs. Ensuring a stress-free environment with plenty of hiding spots also helps facilitate safe shell exchanges.