I agree with Adrian.. I think you guys are judging the crabs a bit prematurely.
Much like you and me, those crabs are real good judges of a creature's health. People can usually tell if someone is sick by their watery eyes, sniffing, coughing, red nose, etc etc...
So can crabs. And if the starfish wasnt doing so hot, they probably took advantage of it. But in my experience, emeralds are opportunists, not really murderers. If they attack something like that, usually whatever they attacked was well on its way to dying anyway.
alianated