I suggest a name change for “hermit” crabs to be changed to “socialite” crabs, and here’s why:
As a hermit crab’s body grows, it will outgrow its current shell and must go on an expedition to find a new bigger shell. This leads to an incredible process called the “Shell Exchange”.
A hermit crab will set out in hopes of finding that new perfect shell: not too big, not too small, but just right. Once it stumbles across a vacant shell, it will leave its current shell to go inspect the size of the new one. If it happens to be too big, instead of continuing the search for that perfect sized shell, it will simply return back to its original shell and take a snooze. Slowly over the course of the day or night, other hermit crabs from far and wide will join together around the vacant shell, socializing and passing time. At some point, the crabs form a conga line, ordered from largest to smallest, patiently waiting for the one hermit crab who will set everything into action. Once a hermit crab comes along who finds the large vacant shell to be just the right size, he will claim it leaving behind his slightly smaller shell. All the crabs in the conga line now exchange shells in sequence, leaving every crab with a perfectly sized shell!
Check out this amazing Shell Exchange in action at : https://www.youtube.com/watch?v=f1dnocPQXDQ !
Hermit crabs have these shell exchange gatherings many times throughout their lives, making me believe their name should really be “socialite” crabs rather than “hermit crabs!
The only other time you might see a hermit crab out of its shell is when it feels threatened.
Hermit crabs rely tremendously on their shells for protection, as the core of their bodies are soft, unlike most of their crustacean relatives. This can lead to vigorous competition between hermit crabs because vacant shells tend to be very limited. This ends up leaving many hermit crabs with shells too small for their body size. If these hermit crabs sense predators, they are not able to retract back into their shells and therefore will abandon their shells temporarily to find better refuge.
This past week in the Biology of Invertebrates lab we worked with hairy hermit crabs to see how long it would take for the crabs to re-enter their shells once taken out. We did this by removing the crab from its shell, then placing both the crab and its shell into a small tank and timed how long until the crab re-entered its shell. My group found that the crabs felt very naked and afraid without their shells, and as soon as they found their shell in the tank they would immediately crawl back into it, as shown below:
If you have fallen in love as much as I have with these cute and clever invertebrates and want to learn more about them, check out this BBC page full of information on the “socialite” hermit crab !