This is Pyura chilensis, a tunicate of the Pyuridae family. The fact that it looks like a rock with guts is not even the strangest thing about it. It's completely immobile, and eats by sucking in water and filtering out micro-organisms. If s clear blood secretes a rare element called vanadium. Also, all individuals are born male and become hermaphrodic at puberty. They reproduce by tossing clouds of sperm and eggs into the surrounding water and hoping they bump into each other.