Do Hermit Crabs Line Up to Exchange Shells?

On April 2 2022, Twitter user @cottoncandaddy stated they just learned “hermit crabs who need new shells” will “gather … and trade shells”:

In full, the tweet read:

I just learned that if a hermit crab finds a new shell that is too big it will wait for other hermit crabs who need new shells to gather and then they will organize themselves by size and trade shells and I am pissed that the crabs have a better housing market than we do

On April 3 2022, @cottoncandaddy commented on their own tweet, adding:

this was a joke analysis until people were like “yeah well my hermit crabs who all shared a tank and didn’t have any other shells besides the ones the other hermit crabs were using all killed each other and died trying to make tiny shells work” and now I think you need it lol

Another reply in the thread included a similar tweet from January 2022:

As the initial tweet became more popular, screenshots were shared to r/antiwork, r/BrandNewSentence, r/GreenAndPleasant, and r/TIHI (thanks, I hate it):

As for whether hermit crabs actually “line up to exchange shells” in the wild, BBC Earth published two separate videos on the subject to YouTube. In September 2015, the first of two videos was shared; the second (in July 2020) was for BBC Earth Kids, and its description explained:

These clever hermit crabs have organised themselves so they can all swap shells! Hermit crabs often live together in large groups of over 100, giving them plenty of chances to swap shells with each other as they grow.

A BBC clip of hermit crabs exchanging shells was shared to r/oddlyterrifying in November 2021:

A June 2012 Scientific American article (behind a paywall) featured a preview detailing a fairly rapid exchange of shells between hermit crabs, but not as a group behavior; it is possible the rest of the article referenced the behavior described in the viral tweet:

One early morning in June of 1986, I waded into a shallow tide pool on Long Island, squatted on a plastic milk crate and dropped an empty snail shell into the water. In a few minutes a small hermit crab skittered toward the shell, probed the opening with its claws to measure the size of the interior space and rotated the spiral casing several times to look for holes. Almost quicker than I could follow, the crab pulled itself out of its old refuge and thrust its vulnerable abdomen into the snail shell I had dropped. Satisfied with the exchange, the animal strolled away, leaving its previous, smaller shell behind. A few minutes later another hermit crab discovered the first one’s discarded dwelling and, after the same inspection ritual, scuttled away with its newfound lodging. About 10 minutes later a third crab found the second’s old home and claimed its prize, abandoning a small shell with a large hole …

Wikipedia’s entry on Hermit Crabs featured a section in which the behavior described in the tweet was referenced (alongside credible citations), but also described “fights over empty shells” when available shells were scarce:

… Hermit crabs kept together may fight or kill a competitor to gain access to the shell they favour. However, if the crabs vary significantly in size, the occurrence of fights over empty shells will decrease or remain nonexistent. Hermit crabs with undersized shells cannot grow as fast as those with well-fitting shells, and are more likely to be eaten if they cannot retract completely into the shell.

As the hermit crab grows in size, it must find a larger shell and abandon the previous one. Several hermit crab species, both terrestrial and marine, have been observed forming a vacancy chain to exchange shells. When an individual crab finds a new empty shell it will leave its own shell and inspect the vacant shell for size. If the shell is found to be too large, the crab goes back to its own shell and then waits by the vacant shell for up to 8 hours. As new crabs arrive they also inspect the shell and, if it is too big, wait with the others, forming a group of up to 20 individuals, holding onto each other in a line from the largest to the smallest crab. As soon as a crab arrives that is the right size for the vacant shell and claims it, leaving its old shell vacant, then all the crabs in the queue swiftly exchange shells in sequence, each one moving up to the next size. Hermit crabs often “gang up” on one of their species with what they perceive to be a better shell, and pry its shell away from it before competing for it until one takes it over.

One cited source was a 2010 article in the journal Behavioral Ecology covering hermit crab shell exchange practices. It described various settings in which shell exchanges were observed, reporting in part:

Across the 20 stations at which vacant shells were provided, we observed a total of 16 shell vacancy chains over 24 [hours]. Vacancy chains were observed at all 9 stations started with medium-sized vacant shells, consisting of 7 synchronous and 6 asynchronous vacancy chains. At 44% (4 of 9) of these stations, both chain types were observed over the 24-h period. Vacancy chains were observed at 3 of the 11 (27%) stations started with large vacant shells, consisting of 1 synchronous and 2 asynchronous vacancy chains. At the remaining large-shell stations, no shell switching was observed, likely due to the lower abundance of appropriately sized C. clypeatus in this population.

These field observations also revealed 3 unique shell acquisition behaviors that C. clypeatus hermit crabs exhibited only in social contexts and which appeared to be associated with shell vacancy chains. The first is a novel behavior that we term “waiting”: after investigation of a vacant shell that was too large, hermit crabs would remain near (within 50 cm) the shell rather than moving away immediately: crab waiting times ranged from several minutes to >1 h, and up to 20 waiters at a time were present near the empty shell. Crabs exhibited waiting behavior at 55% (6 of 11) of stations with large vacant shells and at 100% (9 of 9) of stations with medium vacant shells. Waiters were observed at all stations where synchronous vacancy chains eventually occurred.

Piggybacking behavior, in which 2 or more crabs form a line with each crab grasping the shell of another crab from behind, was observed at 78% (7 of 9) stations with large vacant shells and at 18% (2 of 11) with medium vacant shells. These piggyback lines formed when several waiting crabs accumulated near a vacant shell that was too large for any of the crabs present. Individual crabs frequently moved in and out of such lines and appeared to jockey for position by aggressive cheliped pushing (described by Hazlett 1966). Thus, behavioral interactions during piggybacking may help establish a dominance hierarchy and may eventually lead to queues (lines of crabs leading away from a vacant shell in decreasing size order, with each crab holding onto the preceding crab’s shell). Piggyback lines often transformed into queues after the arrival of crabs that were appropriately sized for the vacant shell. Queues were observed at 6 stations, and these contained between 2 and 8 crabs. In all queues, the largest crab grasped the aperture of the vacant shell, followed by a line of crabs of decreasing sizes.

A viral tweet asserted that hermit crabs line up to exchange shells by size in the wild (which the observer mused was superior to housing distribution models among humans). Researchers have observed “piggyback lines” and “vacancy chains” among hermit crabs, which gathered to exchange shells in groups. As such, the claim hermit crabs collaborate to distribute shells was true — although the practice was not always a peaceful consensus.