In my experience, bluegill don't develop the basidorsal spot until they're about 2" long. Some other sunnies have a little dark pigment there at a small size, that doesn't ever develop into a distinct spot.
For most part what I seen as well fro all the subspecies plus they can "turn" the spot on and off at will. We have been using the spot to distinguish bluegill hybrids and it works well providing you are willing to watch the fish for extended periods of time.
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