adjective

(of the mouth) wide open, especially with surprise or wonder.

Downes listened, mouth agape with incredulity

amazed,filled with astonishment,filled with amazement,astounded,staggered,surprised,startled,stunned,thunderstruck,aghast,taken aback,confounded,dumbfounded,stupefied,dazed,nonplussed,dumbstruck,open-mouthed,agape,lost for words,wide-eyed,awed,filled with awe,filled with wonder,awestruck,wonderstruck