DNA labelling and analyses of confocal z-series reveal that the brain of Macrobiotus cf. harmsworthi contains ∼200 nuclei that are arranged in a bilaterally symmetric pattern (Figure 2D and Figure S1). Since some of these nuclei are likely to be from non-neuronal cell types, such as glia cells, the tardigrade brain might contain fewer than 200 neurons.
Mayer et al. "Selective neuronal staining in tardigrades and onychophorans provides insights into the evolution of segmental ganglia in panarthropods": https://doi.org/10.1186/1471-2148-13-230
Also note that tartigrades apparently have a constant number of cells throughout their life, so instead of increasing the cell count, they just increase cell size. The cells can have increased complexity anyways. All it really means that the nervous system has about 200 nuclei. Also don't forget that even single celled organisms are capable of the classical behaviours like hunting for food, running away from predators, forming colonies (e.g. bacteria), etc.
https://doi.org/10.1371/journal.pone.0059090
Some more reading:Martin et al. "The nervous and visual systems of onychophorans and tardigrades: learning about arthropod evolution from their closest relatives": https://bib-pubdb1.desy.de/record/401705/files/Martin_et_al....
Mayer et al. "Selective neuronal staining in tardigrades and onychophorans provides insights into the evolution of segmental ganglia in panarthropods": https://doi.org/10.1186/1471-2148-13-230
Also note that tartigrades apparently have a constant number of cells throughout their life, so instead of increasing the cell count, they just increase cell size. The cells can have increased complexity anyways. All it really means that the nervous system has about 200 nuclei. Also don't forget that even single celled organisms are capable of the classical behaviours like hunting for food, running away from predators, forming colonies (e.g. bacteria), etc.