This is ONE OF 2 responses to VC 23 Roger 2 ("What conclusions?")...
1) Not all fish can do this. So, fish aren't even similar in this regard!
2) There are salt water variations to this theme. In the some marine fish, the largest female fish becomes the male. Once a male, always a male, until it dies. Then the next largest female fish becomes a male. I don't know if anyone has looked at their brains for any changes.
3) Salmon also experience brain cell growth at the time they are triggered to migrate back to the home waters. They don't, however, change sex. The brain cells are apparently for "smelling" the way home.
4) Male canaries develop new brain cells every season. The growth corresponds to their singing. No cell growth, no singing.
5) Mammals seem to only undergo brain cell growth at a very early stage of development. "We" don't seem to be able to grow brain cells after that time. "Swelling" of any of our brain cells will result in our death or dysfunction. We only have so much room in our boney craniums.
6) Trying to make observations across species lines can lead to trouble. Most of the early primate behavior studies, which were justified on the premise that we would gain an understanding of human behavior, ultimately were shown to have NO bearing or relationship to people! In retrospect, they were just cruel experiments that didn't even help the animals themselves.
7) Muscle cells, cartilage cells, and bone cells are also "similar" across species lines, but you can't fly with your arms like a bat or a bird can!