I wish I had taken a video of this weird little guy. I saw him snorkeling (incidentally, the same time I saw the octopus), and later on the voyage I saw a crab shell. Pretty guy! This was all at Porto de Galinhas, Pernambuco.
Those lighter dots sort of in the middle are eyes...Or, based on the shell below, maybe they're not. And for sure those little wrap arounds in the front are the claws.