I don't think anybody has seen the sharks or the groupers hunt lionfish for themselves yet, but what I have seen firsthand down there is groupers "pointing" at lionfish like a bird dog. They hang around a coral head, and when a diver approaches, they basically angle themselves to face directly down under the coral ledge. If you go look under the ledge, there is a lionfish. I saw it several times in the last two years on Little Cayman. We also had a whole boat of divers watch a nurse shark and a couple of grouper wrestle over something that was hiding under a coral head in about 15' of water right behind the boat at Mixing Bowl. We never got a look at what they were after, but there was something hiding under a small coral head and the nurse shark and several groupers were trying every which way to wedge their heads under the edges to get at whatever it was, and had no worry whatsoever about the divers hanging around to watch. We watched them work for about 5 minutes, but they were still at it for a while after we left to get on the boat.