I once kept a six inch large mouth bass as a pet. I caught a huge (5") crayfish and put him in the tank to clean up. The bass grabbed the crayfish by the head. The crayfish clamped on to both sides of the bass' jaw with his pincers. They swam around together like that for about 3 days before they reached that "tense state of mutual tolerance".
Yeah, I'm not exactly proud of the decision making process (or lack thereof) that led to that situation, but it was certainly educational.
I had a black PVC tee (3"?) in there for shelter that the convict really liked. They were both about the same size. The crayfish decided it wanted in there, so it marched in with claws raised and open, and the convict backed out.
The convict then proceeded to swim around to the side, grab one of the crayfish's legs, and either pull it off or flip it over and get a few nips in, and then back off. This happened several times, and after a few torn fins and a couple missing legs, the convict regained control of the inside of the tee, and the crayfish took to living under it. Every now and then the apple snail would get too close to the tee, and the convict would then rush out and headbutt it until it was an acceptable distance away.