Since we're all theorizing and getting philosopical, here's my 2 pennies worth:
It looks mostly like a large green sunnie to me but seems to have too many bluegill characteristics to dismiss them entirely. I would bet that a long time ago someone was thinking about making a little fishing hole out of the watering hole and dumped a bucket (maybe a dozen or two) of bluegills in there.
I've read that any time you have a small population of one type of sunfish with an abundance of other sunfish you are likely to end up with some hybridization, like it or not. So those bluegill would have cross bred with the green sunnies. Then over generations the bluegill traits would dwindle but the helpful traits would remain.
Also the darker than normal color would be from natural selection as well, since you said that this pond has lots of plant life. The small ponds I've worked on that have lots of plant life seem to end up with darker colored fish over time because of the advantages of being dark in all those dark plants and shady places. It seems to often be that way regardless of water clarity, even though you would think otherwise.
That's my guess until we get better pictures. (Altough there's always the question about why none of my theoritical bluegill were able to stay pure?)
Paul
