Crabs Trade Shells in the Strangest Way | BBC Earth

BBC Earth
25 Sept 201504:16


TLDRHermit crabs constantly search for new, larger seashells to inhabit as they grow, gathering in groups when a suitable empty shell washes ashore. They cooperate surprisingly by lining up from largest to smallest crab and orderly exchanging shells to each get a properly fitting new home. The crabs patiently wait for just the right additions to the chain in order to trigger the intricate housing swap in an amazing display of collective organization and intelligence just to survive.


  • 😱 Hermit crabs use empty seashells as mobile homes, needing to upgrade to a bigger shell as they grow.
  • 😅 When a new shell washes ashore, crabs gather to try to upgrade, arranging themselves by size.
  • 👉 The crabs line up to exchange shells, each moving into the shell vacated by the crab ahead.
  • 😊 The chain reaction allows each crab to move into a fitting shell, triggered when the biggest crab moves.
  • 🤔 None of the crabs can make a move until the chain is fully formed with the right sized crabs.
  • 😮 A small crab finally takes its place to complete the chain, allowing the shell swapping to start.
  • ⚠️ A crab left without a shell risks death by cooking under the intense sun.
  • 😥 Despite the shell swapping, one crab ends up with just as small a home as before.
  • 😫 The final shell even has a hole, but is still better protection than no shell.
  • 🕵️‍♂️ The crabs demonstrate remarkable cooperation and order to ensure as many as possible get better shells.

Q & A

  • Why do hermit crabs need to find new seashells periodically?

    -As a hermit crab grows over time, its shell becomes tighter, so eventually the crab needs to move into a bigger shell to have enough room.

  • Why is it difficult for a hermit crab to find a suitable new shell?

    -Suitable empty seashells are scarce, so when a new shell washes ashore, many crabs will gather to try to claim it, even if it is too big for them.

  • How do the gathered crabs decide who gets a new shell that washes ashore?

    -The crabs arrange themselves into an orderly queue from biggest to smallest. This allows them to exchange shells in an organized way so each crab gets a appropriately sized shell.

  • Why can't the crabs exchange shells as soon as they line up?

    -None of the crabs can move into a new shell until the chain of crabs is complete, so they wait for the right sized crab to arrive and take its place in the queue.

  • What happens when a crab finally moves into a new shell?

    -This triggers a chain reaction where each crab hurries to move into the shell vacated by the crab ahead of it in the line, allowing for an orderly exchange.

  • What happens if a crab ends up without a shell after the swapping?

    -Being exposed without a shell is fatal for a crab due to the intense tropical sun. So even a damaged shell is better than no shell at all.

  • Why don't the smaller crabs wait for a shell that properly fits them?

    -The smaller crabs settle for a shell that is too big because suitable shells are so rare. Having a shell that is too big is better than the risk of being left with no shell.

  • What social behavior do the crabs display in this situation?

    -The crabs display cooperative, organized behavior by lining up according to size and waiting their turn to exchange shells in an orderly fashion that benefits the whole group.

  • How does this scene illustrate the challenges hermit crabs face in their environment?

    -It shows that finding adequate shelter is an ongoing struggle, as they periodically outgrow their shells. Waiting for scarce resources forces social coordination.

  • What might happen if a crab refuses to participate in the shell exchange queue?

    -If a crab does not join the queue, it likely won't get access to any of the shells. And if it tries to take a shell out of turn, the other crabs could prevent it since they outnumber solitary crabs.



🦀 Hermit Crabs Line Up to Exchange Shells

This paragraph describes how hermit crabs outgrow their shells over time and must find new, larger shells to live in. When a suitably large empty shell washes ashore, a group of smaller crabs that need bigger shells waits for other crabs to arrive and arrange themselves into a size order queue. They then systematically exchange shells from largest to smallest crab until each has a better fitting shell.



💡Hermit crab

Hermit crabs are crustaceans that lack a hard shell of their own, so they use abandoned shells, like seashells, to protect their soft abdomen. The video shows how hermit crabs gather together and arrange themselves to exchange shells when they outgrow their current shells. This behavior allows them to upgrade to a better-fitting, protective shell.

💡Mobile home

The hermit crabs use seashells as mobile homes that they carry around for shelter and protection. As the crabs grow over time, they need to find larger seashells that fit their bodies, so the shells function like portable houses.

💡Shell exchange

When a group of hermit crabs gathers around a new, larger shell, they arrange themselves into an orderly queue or line based on size. This allows them to efficiently exchange shells, with each crab moving into the shell vacated by the next largest crab ahead of it in line.

💡Chain reaction

The shell exchange occurs as a chain reaction - when the first, largest crab moves into the new large shell, it triggers all the subsequent smaller crabs to move forward in the line and exchange shells in sequence. This chain allows the group to efficiently upgrade their shell sizes.

💡Death sentence

Being left without any shell to take shelter in is described as a "death sentence" for a hermit crab. Their soft bodies are vulnerable without the protection of a shell, and they can easily die or "cook" under the intense tropical sun.

💡Cooperative behavior

The video showcases an extraordinary example of cooperative, or collaborative, behavior among organisms. The crabs demonstrate coordinated teamwork by standing in line and waiting turns to get better shells, maximizing the outcome for the entire group.

💡Size hierarchy

The crabs organize themselves based on size order. The largest is at the front of the queue, then each subsequent smaller crab lines up behind it. This size hierarchy allows each crab to upgrade to the next size shell available.


Despite the cooperative queued behavior, the crabs also demonstrate opportunism. When a slightly larger crab arrives late, it pushes itself to the front of the line to get the best shell first.

💡Zero-sum game

There is an implicit competition amongst the crabs for the limited number of available shells. Since shells are scarce, the shell selection process resembles a zero-sum game - one crab getting an acceptable shell often directly prevents another crab from acquiring that shell for itself.

💡Upgrade vs downgrade

The overarching drive for each crab is to upgrade to a better, larger shell relative to its current shell size. However, the last small crab ends up only getting a worse, damaged shell full of holes, exemplifying a downgrade but still better than no shell.


Hermit crabs use abandoned seashells as mobile homes

As a crab grows, its shell becomes an ever tighter fit, so eventually it needs to move to a bigger one

Suitable empty shells are scarce, so crabs gather when a new shell washes ashore

The crabs line up by size to exchange shells, with the biggest at the front and smallest at the back

The crabs wait for the right sized crab to come and complete the chain so they can swap shells

The latest arrival crab pushes to the front to start the shell swapping chain reaction

Each crab hurries to move into the shell vacated by the one ahead of it in line

A new arrival muscles in at the end, leaving the smallest crab without a shell

Being left without a shell is fatal since naked crabs cook quickly under the hot sun

After all the swapping, there is still one unused shell left, but it has a hole

Even a damaged shell is better protection than being homeless for a hermit crab

Crab grows over time so needs to move into ever larger shells

Crabs lack their own protective shells so rely on finding empty ones

Crabs have an orderly system to fairly exchange shells according to size needs

Strongest crab starts the chain reaction of sequential shell swapping



Hermit crabs use abandoned seashells as mobile homes.


As a crab grows, its shell becomes an ever tighter fit.


So, eventually, a crab needs to move in to a bigger one.


Suitable empty shells are few and far between...


And this one is far too spacious for this crab.


But instead of continuing its search, this small crab settles down to wait.


Other crabs, also looking to upgrade to a bigger home,


soon gather whenever a shell washes ashore.


Unfortunately, the new shell is too big for them, too.


Despite that, the gathering crabs begin to measure each other up,


and then they do something really rather extraordinary.


They arrange themselves into an orderly queue.


The biggest at the front, smallest at the back.


They're lining up with one aim, to exchange properties.


But none of the crabs can make a move, because the chain is not yet complete.


They're all waiting for the right sized crab to come along.


And this is it.


The latest arrival pushes to the head of the queue.


At first, the chain falls apart.


As it reforms, the small crab finally makes its move...


and takes its place at the end of the line.


After a close inspection,


the big crab moves in.


This triggers a chain-reaction.


Each crab hurriedly moves into the shell


vacated by the crab ahead of it in the line.


Just as the small crab attempts to move into its new home...


a fresh arrival muscles in.


To be left without a shell is a death sentence.


A naked crab will quickly cook under the intense tropical sun.


But, after all the swapping, there is still one shell left.


Sadly, it's no bigger than this small crab's original shell.


And worse, it's got a hole in it.


But even a bad shell is better than being homeless.