Hermit Crabs: Nature's House Hunters

When I was younger, my family went to Long Beach Island every summer, and every year we bought hermit crabs at a shell shop. Though none of them lived very long in our tank, most lasted long enough to change their shells at least once. That's right, hermit crabs don't keep the same shells their entire life. In fact, they aren't born with a shell at all!



If you've ever been in a pet store or a shell shop, you've seen hermit crabs with brightly painted shells, but those aren't really theirs. Hermit crabs are actually unable to make their own protective shell, so they've adapted by finding empty shells on the ocean floor to tuck themselves into. When a hermit crab outgrows its shell, it just finds a new one. The shells they find usually belonged to gastropods, which are snails and other animals with similar shells.

Look at the strange, soft body of the hermit crab that's usually inside the shell

But what happens when there aren't enough shells? Shells are a limiting resource for hermit crab populations, and the crabs compete for them. A hermit crab will fight and even kill a competitor to gain access to the shell it wants. One time I had that happen with some of the crabs I was keeping as pets! Sometimes a group of hermit crabs will gang up on another crab they think has a better shell, and then this group will fight each other for it until the strongest crab wins.

If hermit crabs are different enough in size, they will work together to find shells that fit. Different species of both marine and terrestrial hermit crabs form vacancy chains to exchange shells. If a crab finds an empty shell that is too big, it waits by the shell for more crabs to show up. If the new shell is still too big for all of these crabs, they hold onto each other in a line from the shell in size order. As soon as a crab arrives that's the right size, the biggest crab in the chain claims the newly vacant shell and shells are exchanged down the line. It has to be seen to be believed so be sure to watch the video (linked to above).

These animals are so dependent on each other and on other species to survive, and it's working just fine for their species. Maybe you'll think about these complex relationships the next time you see those colorful hermit crabs in a pet store!