It sounds like a genetic thing; either a genetic tendency toward a misshapen pupil/iris wall (Coloboma) or genetic dark pigments in the iris that cause a similar appearance to Coloboma. If all else seems well, as you describe, I personally would not worry about it. 
I think you would have seen other symptoms and possibly losses if you were dealing with Marek's. Breathe easy! You have some pretty chickens, by the way!
I think you would have seen other symptoms and possibly losses if you were dealing with Marek's. Breathe easy! You have some pretty chickens, by the way! 
