LysoTracker Green TMRE NucBlue

The brain of a water bear weighs nothing at all

One animal, 152 micrometres long, scanned by X-ray and stained while alive. Four of the numbers below had never been measured by anyone.

A live tardigrade stained with three fluorescent dyes: green lysosomes, magenta mitochondria, blue nuclei

Live H. exemplaris, three dyes at once. Harry, Hibshman, Damatac et al. 2024, STAR Protocols 5:103232, Figure 2 — CC BY-NC-ND. This is where the colours on this page come from: they are sampled from the brightest decile of each channel, not chosen by eye.

1,389µm³ · brain
1% of the animal

1,386µm³ · four ganglia
added together

288×the unc-86 switch
across 38 hours

2,728µm³ · whole CNS
a cube 14 µm a side

I · the chain

Four ganglia nobody had measured

Behind the brain, four swellings sit under the gut, one for each pair of legs. The paper that published this scan states plainly that it did not measure them.

So we did, by cutting the nerve cord wherever its cross-section passes 10 µm²: 385, 430, 379 and 194 µm³, front to back. The fourth is half the size of the others, and stays half the size however the cut is made.

They add to 1,386. The brain is 1,389. We have no explanation for that and we are not going to invent one.

The nervous system inside the translucent animal: brain in blue, four ganglia in purple

The head, close: brain, first ganglion and stylets

II · the switch

And a gene that turns on

Sixty-two embryos, each killed and sequenced at a known moment across the four days from laying to hatching. The genes that lay out the nervous system fire between 41 and 53 hours — twelve hours out of four days — and then go quiet.

Inside that window one gene comes on that is absent from almost every earlier embryo: 288-fold, from nothing. It is this animal's version of unc-86, the gene that decides which cells become which neurons in a nematode.

No POU gene has ever been reported in any tardigrade.

III · the honesty

What this is not

One adult animal, fixed and dried, and bulk expression from whole embryos — it cannot tell you which cells anything is in. The ganglion volumes depend on a threshold, and we report the range they survive. An orthologue is not a function: what unc-86 does in a nematode is a reason to look, not a result.