What is the purpose of this structure?  Communication between top minnows or entertainment for predators?  Something else?

LOL. Funny that this is the first time I have seen this asked, I have no idea, and honestly surprised that we haven't discussed it.

I would like to simulate what I bird sees using a fancy camera and filters, just for fun.  I think there is a major surprise hiding.

I was wondering the same thing... at first, I thought that they might be parasites, LOL.

Probably a well developed third or parietal eye that detects movement directly above.

They are very obvious on my monitor lizards, and they are also known in some fish species, though I haven't seen Fundulus listed.

Would be a good project no matter what it turns out to be.

I'm with Matt - the bright spot in the "starhead" group of Fundulus is probably related to the pineal "eye" organ.  But other species such as F. rathbuni have a bright spot just in front of the dorsal fin, not on the head.  Whazzup with that?

Another hypothesis:  Maybe the head spot (or back spot) is simply to announce:  "Don't hate me -- I'm NOT a Gambusia!"

I am betting money it is a signalling structure of some sort.  Either it helps ID conspecifics, confuses predators, or attracts flying eats to water surface.

Either way it would be extremely interesting. It certainly could be a target. One that alerts the fish via peripheral vision in time to move from the target, also keeping the(say kingfisher) fixated on the target until that instant burst of power when the topminnow does what it does. Bam! disappears deep, or not, but does so blindingly quick.

I hope someone finds the ending to this story. If anyone considers this as a project or any other project, please remember our grant program. It is one of our most important endeavors.

This problem looks kind of tough to crack.  Testing on birds would be difficult at best.  A tame bird would likely required.  I would like to sit and watch the fish for hours on end to make certain that something easier is not operating before pressing bird option.

Does anyone have a pet Green Heron I could borrow?

Early warning if it is third eye. Or false target if the bird is so zoned in on the gold spot that it stays course even though the fish's peripheral vision has alerted it to overhead movement. Either way I believe it is an early warning to the fish. I can't imagine that it some how confuses birds as the fish starts moving. Heron, kingfisher, have both chosen targets before they move. They don't adjust much once they go for it. I bet it sees movement, or holds a birds attention long enough and strong enough that the fish can move away in an instinctive manner. Instinct says that bird attacks this point on my body, once I see it, I evade it in this manner. Gives the fish a reference point, that it doesn't know exists, but it's instinct recognizes that point. Makes sense to me, but i may be explaining it poorly.

I wonder if it is camouflage, looks like the glint of sunlight off the water.  Maybe makes the overhead predator think that the water is moving when it is not? 

Fish got bling.

(Would be a good Far Side cartoon. Maybe it already was.)

Gold spot might serve as a warning or somehow overloads brain trying to process what the fish is.  I think the spot is much more visible to other animals than it is to us.  Our visual system is dumbed down in some ways to take in more information in others.

Zebrafish are known to have a parietal eye. Surely there are others.

Found this

Looks mosquito fishish

https://www.reddit.c..._aka_a_3rd_eye/