"Not every rock on the cliff is a rock, and not every crab stays in the water. God forbid you should be sitting on a Carcinus trying to relax when it decides to wake up."
A creature most often seen with it's claws hooked into the side of a cliff, the Carcinus tends to find a Roc nest, feasting on their eggs. The Carcinus can also be seen climbing the cliff to the land above, curling into a ball, and waiting for a victim to arrive. When a victim does arrive, the Carcinus will uncurl, and attack. Their bite's are slightly venemous, and can paralyze their targets, which they then push off the cliff into the sea, allowing the water to tenderize the meat.
If you see a boulder you want to sit on, poke it first. If it doesn't uncurl into a crab thing and try to eat you, it's safe. If it does... Well, good luck.
Don't take naps too close to the cliff. Problem solved.
There's a reason there's always a few fires lit in Gryphon's Keep. These things rely on keeping their skin moist, like whales or dolphins, fire dry's them out. Therefore, they've learned to avoid fire.