When the Terrarium Started Drawing on the Glass

Complexity doesn’t wait to be invited.

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I opened a terrarium recently that had been sealed for five or six years.

It wasn’t pretty.

It wasn’t curated.

It was the ecological equivalent of a forgotten lunchbox.

But it was alive.

I rebuilt it gently, added fresh moss and a couple of small plants, and put it back on its bathroom shelf to get on with its life.

A few days later, the glass started doing something.

Not fogging.

Not moulding in the usual “oh no” way.

Drawing.

Pale branching lines appeared and spread slowly across the inside like frost or lightning that had decided to take a scenic route.

This wasn’t roots.

It wasn’t cracks.

It was slime mould.

A Thing That Refuses to Be Categorised

Slime mould is one of those organisms that seems designed to irritate tidy people.

It isn’t a plant.

It isn’t a fungus.

It isn’t an animal.

It’s basically a single cell that sometimes decides to become a team.

What looks like a delicate web is actually a moving network that flows around looking for bacteria and organic leftovers to snack on.

It doesn’t grow politely upwards like moss. It oozes sideways like it has somewhere to be.

Watching it feels less like watching growth and more like watching something problem-solve. Which is unsettling, considering it does not have a brain.

Why It Showed Up Now

Opening a sealed terrarium is like rearranging the furniture in a haunted house.

Everything wakes up.

Oxygen shifts.

Moisture redistributes.

Dormant microbes stretch.

For something that feeds on bacteria and decay, this is an invitation, not a problem.

Bathrooms don’t hurt either. Warm, humid, low light. The kind of environment where strange things can get on with their business without supervision.

Its appearance wasn’t a sign of failure.

If anything, it meant the system had matured enough to host something weird

Is It Going to Destroy Everything?

No.

Slime mould isn’t interested in your moss. It’s not here to take over, sabotage your setup, or turn your terrarium into a cautionary tale.

It’s here for the microscopic mess.

It feeds on bacteria, fungal spores, and the general background debris that quietly accumulates in any living system. The things you don’t see but that are always there, breaking down, recycling, keeping the whole thing moving.

From its perspective, your terrarium isn’t a display. It’s a buffet.

It shows up, has a wander, samples what it needs, and moves on once the easy food is gone.

It doesn’t burrow into plants. It doesn’t attack moss.

It doesn’t start chewing through your carefully placed landscape like a tiny ecological vandal.

If anything, its presence suggests that there’s enough biological activity for something higher up the chain to make an appearance.

It’s less an invasion and more a brief inspection.

A short visit from something that specialises in tidying up the invisible.

And when it’s finished, it usually retreats just as quietly as it arrived, leaving little evidence beyond the faint memory of those branching lines on the glass.

The Disappearing Act

Slime mould prefers low light. Bright exposure tends to make it retreat.

If it vanishes from the glass, it hasn’t died dramatically overnight or staged a quiet collapse while you weren’t looking.

It’s just moved.

Most likely back into the substrate, or into a part of the system where it can continue its work without the spotlight.

Closed ecosystems are full of things that would rather operate quietly in the background, carrying on with their small, necessary roles whether we notice them or not.

Not a Problem, Just a Presence

In laboratory settings, slime mould has solved mazes and mapped efficient transport routes.

In a bathroom terrarium, it mostly just reminds you that life does not require supervision to be complicated.

You set up a system. You step back.

And something turns up that you never planned for, never invited, and definitely didn’t design. Not everything that appears is a problem.

Sometimes it’s just nature passing through, leaving a faint, branching signature to say:

This place is alive enough for me.