Another entry in the "there's some weird stuff down there" category. This is not a nice friendly guy like cleaner shrimp: mantis shrimps are predators with the nickname "thumb splitter." They hide in holes like this, with only their eyes showing, and have one arm that is essentially a hammer that they use to stun prey that comes close enough. The nickname comes from fishermen who got hammered in the obvious place when the shrimp was trapped in the net they were pulling in. The hammer hits with enough force to cause some serious pain, which is pretty impressive given that the total length of the shrimp is about the same size as your average thumb.