Path length

Put a scraping of mudflat or a bit of sand under a microscope and give it a minute to settle.  Some of the things in there will be moving.  Not drifting - moving, in lines, changing direction, occasionally reversing and going back the way they came.

Some of those things are diatoms, and diatoms have no limbs.  No flagella, no cilia, no visible apparatus of any kind.  Each one is sealed inside a box of silica it built itself.  It looks about as capable of locomotion as a pill.

The way it works is genuinely strange.  One whole family of diatoms has a slit running along the shell, and the cell pushes adhesive out through that slit, sticks it to whatever surface is underneath, and then hauls on it from the inside using the same kind of motor proteins that move things around inside our own cells.  The cell isn’t swimming.  It’s gripping the floor and dragging itself along its own glue.  Behind it, it leaves a trail of the stuff, which is one of the reasons anyone worked this out.

Three papers this year, and I think they line up into something.

The first compares five species living on surfaces and asks whether the shape of the slit predicts how the organism moves.  It does.  Different geometries of slit go with different characteristic movements, closely enough that you can look at the hardware and make a decent guess about the behaviour.

That is a satisfying result and I want to be honest about its limits.  Five species is five points, and a correlation across five species tells you nothing about what happens within one - whether individuals of the same species with slightly different slits move differently, which is the version of the question I’d actually want answered.  It also says nothing about what sets the shape of the slit in the first place.  There’s no genetics in it at all.  It’s a structure-to-behaviour link, which is more than description, but it stops well short of mechanism.

The second paper is the one that changed how I think about the first.

Working with a single species, it shows that the cell controls the curvature of its path by switching which part of the slit is actually in contact with the surface - one branch or two - and that it does this dynamically, while moving.  So a turn isn’t a consequence of the shape it happens to have.  It’s a thing the cell does.  There’s a fixed layer and a live layer, and the fixed layer sets what’s possible rather than what happens.

Caveats here too, and they’re the usual ones.  One species.  Laboratory glass, which is flat, clean, rigid and nothing whatsoever like a mudflat.  There’s a decent chance that some of what looks like decision-making on glass is different on a surface with texture and chemistry and other organisms already living on it.  I’d want to see it on something rough before I lean on it.

But even provisionally, the two-layer shape is worth carrying around.  Hardware that constrains, software that chooses.  I suspect I’ve been guilty of looking at conserved structures and treating them as explanations, when a structure usually explains the envelope and not the trajectory.

The third paper is where it gets properly odd.

Arctic sea ice is not solid.  It’s shot through with brine channels, threads of very cold, very salty liquid running between the crystals, and there are diatoms living in there.  This year someone watched them gliding inside the ice - not surviving in it, moving through it - at temperatures that set a new record low for movement in any cell with a nucleus.

The part I can’t stop thinking about is that this isn’t a gradient.  Arctic species do it.  Temperate species don’t.  Not slower, not less well - they don’t do it.

That distinction matters more than it might look.  If temperate species glided at low temperature but sluggishly, you’d conclude that everyone has the same machinery and cold slows it down, which is what cold does to almost everything.  A hard yes-or-no says something else is going on: a different component, a different lipid composition in the membrane, a different adhesive that stays workable when the ordinary one won’t, something.  Present-versus-absent is usually a more tractable question than faster-versus-slower, because you can go looking for the missing part.

Nobody has.  The comparison that would answer it - line up the adhesion and motor machinery of ice-gliding and non-ice-gliding species and see what differs - isn’t in the paper and as far as I can tell isn’t anywhere.  It’s sitting there.

There’s a term in spectrophotometry, path length, which is just the distance light travels through your sample on the way to the detector.  It’s a number you set once by choosing a cuvette and then never think about again, because it’s a property of the glass rather than of anything alive.  I like that these organisms have taken the phrase back.  For them the path length is not a constant, and it isn’t a property of the box either.  It’s a decision, made continuously, by something with no legs, in the dark, under the ice.