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hermit crab

Hermit Crab Shuffle


We came upon a tidal pool that had a dozen or so hermit crabs in it behaving strangely (to us). I took a video as we watched this while totally ignoring the dolphin strand feeding nearby ;-)

One day my Mom and I went for a walk at Beachwalker Park on Kiawah. We wandered down to the inlet between Kiawah and Seabrook. This is a place many come to observe the Lowcountry dolphin strand feeding. Mom and I, however, were distracted by something else.

There are many species of hermit crabs, terrestrial and aquatic. Most species have long, spirally curved soft abdomens. The vulnerable abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can retract. The crab would die without a shell, so it is vital it has one that fits and that it can carry around.

Here is what Mom and I saw.  (you might want to mute Mom and I - we don't say anything helpful)

Hermit Crab Shuffle

Hermit Crab Shuffle

Play Video

To help understand what is happening here, watch this BBC video.


The crabs are trading up their shells. As they grow, they need to get into a larger shell to accommodate their larger body. 

Lets go back and look at some screen shots from my video. This first just shows all the crabs and the variety of sizes and shell types. #1 seems to be the largest and I think he moved into this shell prior to our arrival. Shell #2, the second largest shell, is vacant in this shot.

Not chaos

As they scramble, some sort of order develops. A line develops, with the larger crab in front, a smaller crab hanging onto its shell, and an even smaller crab hanging onto that shell....

50 seconds

About 54 seconds into the video a crab decides to switch into the larger vacant shell. You can see the moment the crab risks exposing its tender body and leaves its old shell behind. 

54 seconds
54+ seconds
55 seconds
#1 leaving

And while all that was happening, #1 left the area with his new shell.

And there is a new shell available

At 1 minute 33 seconds into the video, another switch is about to happen.

1_33 pre switch
1_34 switch
1_34+ #4 getting ready

And of course, while that was happening, #2 escaped with his new shell.

#2 leaving
1_47 empty shell

And another switch.

#5 switch

After at least 5 crabs getting new shells, the only empty shell is too small for any crab to move into, so they disperse and wait for another vacancy event.

I recommend you watch the video again and see all this happening in real time.

Final empty

So it turns out this is a thing. Its called a vacancy chain. Scientists have studied this and determined that the average number of switches is 2.5 (see article). Instead of a solitary hermit crab wandering around the ocean floor looking for a shell the correct size, the crabs have figured out how to benefit 2.5 crabs when one shell is found. And our Kiawah crabs did significantly better than that. Instead of just one crab upgrading its shell, 5 crabs got into a larger shell.


Sociologists and economists identify similar ways that humans benefit from vacancy chains (Housing, jobs, cars, etc.) and make these more efficient by studying hermit crabs. I found this article in Scientific American very interesting. "Hermit Crabs Trade Up"

One question remained in my mind. How did all these crabs come to be in this one place for this event? Well, it turns out that hermit crabs are extremely attracted to the smell of a dead hermit crab! Even more so than they are attracted to a food smell (see article). 


These little creatures have developed a social behavior which allows them to efficiently exchange their precious resource, their shell, efficiently, and developed a biological ability to smell out the opportunity for it to happen.


I am continually amazed by complexity existing in nature. This discovery is just one example. If I keep looking and asking questions, what else can I learn?

hermit crab
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