A hermit crab’s shell and hard exoskeleton act as a protective cover from enemies of all sizes. The part of a crab which we rarely see is its abdomen, a delicate stem which stays protected inside of the shell until the crab is ready to transfer homes. Its abdomen is so sensitive that he will immediately start to burn if exposed to the sun for too long. Crabby needs to find a new home quickly before he becomes vulnerable, and it is critical to find one that is just the right size.
Unlike other types of crabs, the hermit crab cannot make its own shell so it tends to live off of discarded shells or other items found in the water or on land. Over its lifetime, the crab will grow and shed its exoskeleton several times. As the new hard skin grows below the surface, little by little the old skin breaks apart and is shed. As the crab grows, the shell becomes a bit too tight to live in comfortably. Eventually, like all of us, the crab decides he’s outgrown his current home and the hunt begins.
House hunting for a new shell can prove difficult as finding the perfect size home can be scarce and competitive.
Hermit crabs are incredibly organized, they begin to gather and search as shells wash ashore. The home swap process begins when the crabs start to size up the empty shell and determine if its a good fit. It’s usually too big or too small so they analyze their competition and start to measure the other crabs.
Once everyone is measured, they organize themselves into a single file line which is arranged by overall height and width. Largest in the front, smallest in the back. Each crab has the same intention, they need a new home and need to exchange their shell. The strategy is for the largest crab to hop into the new shell, and the next one takes his newly vacant shell, and so forth.
Often as the smallest crab tries to take his turn, a bystander who didn't wait in the line, now looks to sneak in. The little crab needs to act quickly or he risks not getting a shell that he desperately needs. One of the two will end up without a new shell and will be left with its abdomen exposed.
And so the search continues… often they get desperate enough that if beach trash looks like it will be big enough he will make the switch.
Crabs that are in water tend to lease their shell surface area to other water critters like anemones. At the time of shell exchanging, its essential to take their roomies with them. Sea anemones don't just stick around for the free ride, they protect the crab from enemies by stinging them with their tentacles. As the crab scoots around the seafloor he tends to kick up a lot of sand or dirt and in doing so makes food more accessible for the anemone. They develop a dependent relationship and at the time of the big move, the two friends make their move together. It's almost impossible for an anemone to detach itself from a surface, but he trusts the crab so much that he is the only one worthy enough to move the anemone. The crab peels them all off his shell and carefully places them on his new shell and waits patiently until the anemone has made himself at home.
And they live happily ever after… until they need to move again.