Hermit crab housing swap

November 20, 2014 • 7:29 am

This video, which you can see at Twisted Sifter by clicking the link below (I couldn’t embed it, and the BBC version on YouTube isn’t visible in the US) is right up there with yesterday’s snail-shell-using spider as one of the marvels of nature. In fact, it’s a stunning example of bizarre but orderly animal behavior, and I demand that all readers go see it now (it’s only two minutes long).

The video, shot in Belize, shows what happens when a big empty shell washes ashore and there are hermit crabs around looking for a new home. (They grow throughout their lives and must find new shells when the old ones get too small.) Sometimes fierce competition ensues for good shells, but this video shows a fantastic dominance hierarchy that forms to get every crab a new and bigger shell. A “conga line” of crabs forms, with the biggest at the front and the smaller ones following behind, ordered by size. When the biggest one enters the new, empty shell, they all swap homes at once, moving forward to the next largest shell.

I had no idea anything like this happened, but David Attenborough documents it with his usual panache. WATCH IT!

When you click on the picture below, click on the arrow on the video to watch.

Screen Shot 2014-11-19 at 1.22.06 PM

Notice how fast the swap takes: I timed the big crab’s move at four seconds. That’s because the rear end of the hermit crab is soft and vulnerable (that’s why they’re in someone else’s shell), resembling a shell-less shrimp. The crab doesn’t want that bit exposed to predators for any longer than possible.

Here, from Wikipedia, is a photo of a hermit crab outside the shell; it’s naked and pathetic. You can see, though, how the abdomen is curled up so that it handily fits inside a shell.

Pagurus_bernhardus
Pagurus bernhardus

h/t: Pär

 

61 thoughts on “Hermit crab housing swap”

    1. Yeah, except if he could afford lawyers and accountants the big crab wouldn’t just let go of the old shell, he’d make sure it provided an income stream.

          1. those crabs seemed pretty patient and didn’t seem to play too much grab-ass. I am still so amazed and impressed that that video is for real!

  7. Years ago, Armand Kuris, an ecologist who got an NSF grant to study resource allocation using hermit crabs, was criticized for wasting money to study the “housing problem in hermit crabs” by a senator from Oregon. A number of scientists wrote letters criticizing the senator for his ignorant comments. It is amazing that such cooperative behavior as shown in the video can evolve. It would be interesting to manipulate the environment of some isolated hermit crab populations that recruit locally by providing an overabundance of empty shells. Would the observed collective behavior decline in precision??

      1. “Yo, Herman! You forgot to vacuum this place! My butt’s gonna get that rash again!”.

          5. I wonder how a hermit crab with a guilty conscience would look.

            A bit red in the face, maybe.

          6. To match his red butt…

            (I’m picturing Monty Python’s policeman marching in about now saying this is all just tooo silly🐸)

          7. “And now for something completely different”. 🙂

            ( Or was that “Not The Nine O’clock News”?….I forget. )

  12. This posting reminds me of a wonderful book for children: Pagoo, which is a story of the life of a hermit crab. Full of beautiful, detailed illustrations, and little side bits of biology information. It is a treasure. Adults will like it too. I wore my copy out as a kid.
    Parents: your local library may well have this book.

  14. Thrice today did I read the title (just the title, after receiving the post in my feeds), and thrice did I parse it as “Hermit crab housing *wasp*”.

    Needless to say, I was very confused by the contents until I forced myself to read the title a fourth time, slowly… I kept expecting wasps to come knocking at the shells’ door any time… now that would be interesting animal behaviour!

    1. Wasps aside, it’s not so bizarre behaviour. If each crab’s algorithm is “stick for a while near a right-sized shell, even occupied, and be ready to move should its status changes,” you should get the queue behaviour naturally out of it. No cooperation or foresight required.

      1. What seems added here is maybe that the crabs seem to tolerate each other. A crab has a smaller crab clasping its shell from behind, and the bigger crab does not show a tendency to clobber the usurper-potential-shell-stealer. I expect they are not being social, but they are being more indifferent.

        1. I suspect you’d see a different story where two (or more) same-sized crabs found only one right-sized move-upper shell.

        2. I think even the toleration can be explained in terms of self-interest. Once you’ve claimed dibs on the shell you want, it’s a bad risk to let go of it in order to scrape someone else off the shell you’re about to give up.

  15. A hermit crab will sometimes forcibly rip another from a shell it wants, occasionally fatally. More commonly a ritualized fight ensues, resulting in the loser vacating the shell before being harmed. This “conga line” apparently reduces the risk of violence even further.

    @#12
    I was totally just plotting to find Pagoo the other day, having not read it in nearly 25 years. Holling Clancy Holling’s books are all worth reading.

  16. Thanks, I did get to see it here in Canada, & it was rather amazing. I’d not heard of anything like this before. Patricia

  17. This raises a very important question for creationists. Where did hermit crabs stick their butts BEFORE the Fall, when there were no shells emptied by their owner’s death? Hmmmm?

    2. They probably had their own shell but lost it all because of that woman with that damn talking snake & the apple!

      Once a proud crab, majestic in its shell, it is now a bum with an embarrassing bum.

    3. This is a brilliant question, and one I’ll be sure to remember and use. Thanksgiving is just around thee corner, and I have been treated to creationist rants around the table on more than one occasion.

      Thanks for the tip!

  18. “…(I)t’s naked and pathetic.”

    This made me burst into laughter.

    Fascinating video, of course. It’s always interesting to see different forms of cooperative behavior arise from “selfish’ goals which serve the acting party.

  19. Hmmmm…. I just wonder if this could be perhaps an example of a group selection argument? If for example a shell was just “a bit” too big it would make sense for a crab to “defect” and “jump the queue” ahead of even bigger crabs to grab an “almost perfect” new home. If this doesn’t take place then crabs are acting for the “welfare of the group” in a sort of way.

    This behaviour is elegant, but the strategy is dead simple:

    Do While own_shell=small AND shell_ahead=bigger AND good_fit;
    If shell_ahead=occupied,wait,
    Else move_into_shell
    End If;
    End;

    Beautiful… ain’t evolution great!

    1. I don’t think there is any need to invoke group selection to explain this behaviour. Each hermit crab is acting in its own self interest to move up into a larger shell to allow it to grow. If they don’t jump up the queue to get even bigger shells than the minimum size required that is surely better explained by the likelihood that, if they did, they would be competing at a disadvantage with a larger crab, rather than that they are working for the welfare of the group to ensure optimum allocation of shells to crabs. The neat hierarchy is simply brought about by the ‘rule’ that big crabs beat smaller ones in a contest.

      The apparent coordination of the behaviour is brought about by the fact that suitable shells are relatively rare so when a new one becomes available it creates an opportunity for a whole mob of crabs to up-size their accommodation. Anyone who doesn’t join in doesn’t get a new shell.

      I agree that evolution is beautiful!

      1. That’s a good explanation Jonathan which does away with any group selection possibility. It’s an assessing sort of strategy with a willingness to “wait around”. It would be nice if this could all be put to the test, say by imposing a phoney small crab in the front of the queue to see variations in behaviour.
        I can certainly vouch for the fact that this algorithm is NOT working in ski-lift queues where shorter skiers often butt in front of me.

      2. In my experience, when there are multiple shells available of different sizes, individuals will not choose one that is bigger than they need. So there also might be factors such as being better protected in a snugger shell, and/or not having to lug around any more weight than needed at any given size.

        BTW, when they’re in the right-sized shell, they are very hard to remove by force.

  20. Aw, that was marvelous! I’d heard of this behavior before but never seen a movie of it. I had no idea so many crabs could be involved!

    Hermit crabs at one time were one of my favorite pets. They don’t have much upstairs but their behavior is fascinating.

  21. Awesome video!

    And, while I was there, I was able to this to this one; the relationship is, uh, something about sexual selection and costly displays here…

    1. Gross! Those guys certainly don’t do it for me (and I don’t mind beards). Are those Cheetos in the one guy’s beard?

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