How about auditory singals in murky water? My understanding is that male sunfish make a series of low pitched voaclized grunts while chasing competitors out of their territory when they are on the bed and while courting females. I have also read that sound production in sunfish is species specific meaning that vocalizations from BG and RES should be noticable different. One thing is for sure, the research into the area sound vocaliztion in sunfish is pretty sketchy, but it might explain why hybridizations doen't occur more often than it does.