FMF- Frogs will cannibalize equal size frogs if the opportunity presents itself.
The opening on their "knee" is the "exhaust" from their concealed gills. Part of their camouflage are internal gills which reduces movement that would give them away when they're waiting in ambush. It also serves as propulsion so they can appear to drift and not have to use their fins when approaching prey. These guys are like a balloon with a mouth and that stomach can expand to several times it's own size to accomodate huge meals so nothing in the tank is safe with these.
LFS marine guy, thank you for the input. I have the frogs in a tank of their own.