Brunei

Charles Nakamura wrote about the Brunei form when describing this photo.

"They aren't so well colored because they are in a bare tank and older. At the time of this photo, I think they were about 12 years old and just about 8". Much larger than the Marudi form. They were still very actively breeding. In the photo, some eggs were already laid and gathered."


Sarawak
Marudi

In 2018, Peter Petersen led by Sarawak guide Michal Lo, found Betta macrostoma downriver from Brunei.




The first account of artificial incubation of eggs by Sumer Tuwari can be found here:
https://www.reef2rainforest.com/2020/10/11/artificial-incubation-of-betta-macrostoma-eggs/