The Hermit Crab's Shell

An available shell can set an entire line of crabs in motion. A metaphor to make internal mobility visible.

The tail of the hermit crab

Series: What nature already knew about talent · 3/6 · Internal mobility

The hermit crab lives in a house that is not its own.

It does not make its shell: it occupies that of a dead snail. And as it grows throughout its life and the shell does not, every so often it becomes too small and has to move. The problem is that empty shells of the right size are scarce. Very scarce. It is the resource that limits the life of a hermit crab, and much of its behavior revolves around finding the next one.

What they do to solve this was discovered by a sociologist, not a biologist. Ivan Chase, from Stony Brook University, had been studying how vacancies circulate among people for years when he noticed the Caribbean hermit crabs. And he saw something that felt familiar to him.

When a shell that is too large for the crab that finds it appears on the beach, that crab neither keeps it nor abandons it. It waits next to it. Soon another crab arrives, and another, and a line forms ordered by size, from the largest to the smallest, each one holding onto the shell of the one in front. They can wait for hours. When a crab of the right size for the new shell finally arrives, it gets into it, and within seconds the whole line moves: each crab moves to the shell just vacated by the one in front.

A new shell. Half a dozen crabs better housed. No one had to go out looking for anything.

Chains of vacancies

Chase called it a chain of vacancies, and the concept comes from sociology: it is what happens in the housing market when someone buys a new house and vacates the previous one, which is occupied by someone who vacates theirs, and so on. And it is exactly what happens in an organization when a position is filled.

Every vacancy creates a chain. When you promote someone, their position becomes vacant. If you fill it with someone from within, that person's position becomes vacant. Each link is an opportunity for someone to grow, and the chain only stops when a position is filled from outside or eliminated.

The difference between crabs and us is that they see the chain. The line is there, ordered, each one knows which shell they aspire to and which shell they leave. In a company, the chain is invisible. When a vacancy opens, the question that is almost always asked is 'Who do we hire?', not 'Who among our own could benefit from this shell, and what shell would they leave vacant?'.

Why we look outside before looking inside

The reason is not bad faith. It is that we do not know who fits.

We know what position each person occupies, who their boss is, and how long they have been with the company. What we do not know is what they can really do, beyond their current position. The history tells where they come from; it does not say how far they could go. And since history is all we have, only those who have already demonstrated something similar to what the next position requires move. Which is usually the most obvious path, and almost never the richest.

Meanwhile, the logistics person who studied programming on their own, the administrative person who speaks three languages, the technician who has been assembling the dashboards for the area for two years without anyone asking them to, remain in their small shell. No one has looked. And one day they leave because another company did offer them the big shell, and ours loses both the person and everything they knew about the business.

The figures for this are consistent in any study you look at: the lack of growth opportunities repeatedly appears among the top reasons why people leave. And hiring outside what you had inside costs more, takes longer, and fails more, because the outsider has to be taught the business and the insider already knows it.

Seeing the line

What makes the chain possible in crabs is that the criterion is visible and the same for everyone: size. Each crab knows if the shell is suitable for them because they can measure it, and the others can too.

In an organization, the equivalent of size is the real capacity of each person compared to what each position requires. Not their current position, nor their trajectory, nor their boss's opinion: their measured distance to other roles in the organization. When you have that, the chain appears on its own. You open a vacancy and the question changes: who among my people is closest, what exactly do they lack, and what position would they leave vacant if they move?

Sometimes the answer is that no one is close and you have to go outside. Perfect: you go outside knowing why. But many more times than one expects, the answer is that there are three people one or two skills away, that the gap can be closed with training in weeks, and that moving them opens up three more opportunities below.

One shell. Six crabs better housed.

The trap of the line

There is a detail about the hermit crab that is worth remembering. The line works because everyone waits for the one of the correct size to arrive. If a small crab tried to get into the large shell, it would not work: it would be exposed, and it would also break the chain for the others.

In companies, that also happens. Moving someone to a position for which they are not prepared, because they are loyal, because they have been there for a long time, or because it is the comfortable option, is not internal mobility: it is a bad hire with a familiar face. That is why mobility must be supported by evidence of capabilities and not by history, just like external selection. It is the same rule, looking inward.

At The Wise Skill, we build internal mobility exactly like this: who from your team could take on other responsibilities, measured against the position and not deduced from their profile, with the exact distance and what is needed to close it. But the question that matters you can ask tomorrow, the next time someone tells you 'we need to open a vacancy'.

Who among our own has been waiting by this shell for a long time?

If you are in a reorganization, an integration, or a growth phase where new roles need to be filled with the current staff, message me directly and I will tell you how we make the chain visible.

Next installment: the crow of New Caledonia, or why training is not just giving courses.