Pardon me, are you still using that shell?

One animal that is common in tidepools the world over are hermit crabs.  Peering into a tidepool you might see a snail that appears to be scurrying around sideways and then you'll see the little eyes and legs peeping out.  A hermit crab!  Unlike other crabs, hermit crabs have a soft, curled up abdomen that must be protected and hermit crabs depend on using snail shells as their little house.  Crabs (and all other arthropods) must shed their shell in a process called molting in order to grow and hermit crabs undergo an additional danger because as they grow they need to find new shells.  A hermit crab is always on the lookout for a new and better shell.  What are the chances of finding a shell of exactly the right size?  Since every hermit crab is looking a slightly bigger shell when one hermit crab changes shells then this will vacate a shell that is suitable for another, slightly smaller, hermit crab.  In the wild hermit crabs have been observed participating in an amazing behavior called a vacancy chain in which they will line up in order of size in order to take advantage of a new shell.  How does this come about?  For one thing the smell of dead or dying snails or other hermit crabs have been shown to attract hermit crabs resulting in several hermit crabs congregating and competing for new shells.  Hermit crabs will fight for shells by pushing each other with their claws.  Rotjan et al. (2010) reported in Behavioral Ecology that when hermit crabs are living in high density they will exhibit several interesting behaviors that lead to the establishment of a line organized by size to facilitate an orderly change into new shells.  For example a hermit crab that encounters a shell that is too large for it to inhabit comfortably will wait within the area of that shell for up to an hour.  This behavior increases the chances that other hermit crabs will show up allowing the waiting hermit crab to acquire a shell of the appropriate size.  In addition, in the presence of a large shell multiple hermit crabs will participate in a behavior called "piggybacking."  A crab will grab onto the back of crab slightly larger than itself.  Once a crab large enough take advantage of the new shell arrives then the hermit crabs form a line in order of size.  When the largest crab moves into the new shell then the next largest crab moves into the now vacant shell and so on down the line until the very smallest shell is left behind.  I find this to be a fascinating and surprisingly efficient way for everyone to get a new, bigger house!