The central nervous system of Hypsibius exemplaris, measured from data that was already public

Tardigraph · exploration 07 · every figure on this page is on this page

Four ganglion volumes, never previously measured, from a 2019 scan; a twelve-hour patterning window and a 288-fold gene switch, from a 2016 transcriptome. No new experiments. Nothing below is behind a click.

1 · Volumes

The brain occupies 1,389 µm³, one per cent of the animal. Behind it the nerve cord runs as a single connected strand of 1,607 µm³ whose cross-section swells four times, once per pair of legs.

The source study segmented the brain and reported its volume. It states that it did not measure the ganglia. Items 2–5 in the table are new.

Cutting that strand wherever the cross-section passes 10 µm² separates swellings from cables without ambiguity: the connectives run at 2.5–6 µm², the swellings peak at 47–55.

Swept from 8 to 25 µm²: volumes move <10%, and the relative order (1.00, 1.12, 0.98, 0.50) does not move at all.

Table 1 · central nervous system, one adult, 0.27 µm/voxel
Part	µm³	rel. I	% of CNS	source
Brain	1,389	—	50.9	Gross 2019
Ganglion I	385	1.00	14.1	this work
Ganglion II	430	1.12	15.8	this work
Ganglion III	379	0.98	13.9	this work
Ganglion IV	194	0.50	7.1	this work
Connectives	214	—	7.8	this work
Ganglia I–IV	1,386	—	50.8	cf. brain: Δ 3
CNS total	2,728	—	100	cube 14 µm/side

The four ganglia come to 1,386 against the brain's 1,389. Reported without interpretation.

2 · Timing

Sixty-two embryos, each sequenced once at a known hour across the ~101 hours from laying to hatching. Three traces, each scaled to its own maximum, all shown below at the same width so their shapes may be compared directly rather than described.

Patterning peaks 41–53 h; synaptic climbs from 45 h to hatching; the unc-86 orthologue is near zero until 38 h.

The patterning set fails a monotone-trend test at every group size (p = 0.38) and passes a pulse test decisively (mean pairwise r = +0.521, p = 0.0002). Both results are true; they answer different questions.

We published the null first. A pulse does not trend. The failure was the statistic's, and it is kept in the record beside the correction.

3 · The switch

One gene rises 288-fold across the 38-hour boundary: 0.17 CPM in the 19 embryos before, 51.1 in the 43 after, 31 of its 33 detections falling late (hypergeometric P = 6.7 × 10⁻⁶). On a concatenated POU-domain + homeodomain tree against 18 references it is sister to unc-86 at 93% ultrafast bootstrap.

Three tardigrade genomes carry the same 5–6 POU genes, one per class; POU1 and POU5 absent, as expected for vertebrate innovations. PubMed returns zero records for POU genes in any tardigrade.

The nervous system: brain and four ganglia inside the translucent body — **Fig. 1.** Lateral, body at 7% opacity. Brain (blue) and ganglia I–IV (violet) with paired connectives. Geometry: Gross et al. 2019, CC BY 4.0.

The animal cut across with the X-ray slice on the cut face — **Fig. 2.** Transverse cut with the original scan shown on the face rather than a surface; bright rings are storage cells. The published segmentation records the gut as a **filled** volume — the lumen is in the scan, not the labels.

4 · Limits

One adult, fixed and critical-point dried. Bulk expression from whole embryos: no statement here can be assigned to a cell type. Ganglion volumes depend on a threshold whose tolerance is given above. Orthology is not function.