Yeah, the lion didn't do that. A lion will suck something in and spit it back out, but you said the mouth wasn't big enough to do that, and also if it spit the crab back out it would still be alive.
The fact that the crab has disappeared is a good sign, the exoskeleton is likley to disintegrate and get swept away. If it was actually dead, i'd expect it to be sitting in the same place.
Oh yeah, was the exoskeleton/crab standing up? How was it laying?