Crabs

Apparently coconut crabs are capable of disassembling a corpse and eating it. Being around these bastards helpless and weak on a desert island is definitely not a situation you'd want to be in. It sounds like the perfect idea for a horrific survival story. Imagine being immobilized under the rubble of your crashed plane being slowly encircled by giant crabs prodding you with their sharp claws. At least if you become an afternoon snack to the crustaceans you'll have an interesting story to tell in the afterlife.