Just tossing out a bit of food for thought on this with no particular breeder in mind.

Hikui has been speculated as a virus, bacteria, skin cancer (akin to melanoma possibly?), etc...

We all know that there are certain skin types in virtually all organic species that are more prone to Melanoma type cancers and that tendency is commonly passed on in offspring. Others are highly resistant to it which also tends to be inherited. The proclivity for weak immune systems, certain congenital defects, and specific chronic illnesses also tends to run in families. Sometimes it is directly passed from one generation to the next while in other instances it generally skips a generation. Some are gender specific while others are not, adding a hormonal connection to the equation.

Why would we not presume that it would only be "natural" for a heavily inbred sub-species like Nishikigoi to produce similar tendencies? Anyone who has EVER done any home breeding or participated in early sembetsu knows the high rate of physical defects in a typical hatch caused by line breeding. We also know that Nishikigoi, while very sturdy fish, are far less hardy than their wild cousins in virtually every way.

I wonder if any of the Breeders have studied the difference in Shimmies and/or Hikui tendencies between their older pairings (8-10 year olds) and their younger ones (3-6 year olds). Older ladies would be observed over the long haul as to their own development of either Hikui or Shimmies while the Younger girls would be less likely to display those traits earlier in life (meaning they would not be culled from future breeding...yet).

Just a few random points to ponder.