I thought root hairs only happened in aero?

Maybe I read wrong - but I thought root hairs were reserved only for those growing in aero.... not in other forms of hydro.

Not sure what the darker areas were. Look white as rice in person.

Once they hit water you will have a dramatic change in root structure.... they will look like hair noodles.
In soil roots use the tiny hairs to move towards water .... in DWC roots don't need to look for water so they loose the little hairs.
That is why you can't plant a well established water root plant back into soil.

Hope this helps
Happy growing !!