Crabs Trade Shells in the Strangest Way | BBC Earth

Summary

Hermit 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.

Takeaways

• 😱 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.