A reddish brown mantis shrimp to about 12 cm (5 in) with a tail plate unlike anything else in the group: a domed disc covered in stout radiating spines, so close to a small sea urchin that the resemblance is the whole point. The animal backs into its burrow and seals the entrance with it, leaving a predator facing what looks like a spiny urchin wedged in a hole.
Using part of the body as a door has a name - phragmosis - and it turns up independently in ants, frogs and lizards, but among crustaceans this species is the striking example. It occupies burrows in coral rock and rubble from about 1 m to 25 m (3 to 80 ft), hunting near the entrance after dark and retreating tail-first at any disturbance. That habit is exactly why it is so rarely seen: a diver passing in daylight sees a spiny plug, not an animal, and reads it as one more urchin among thousands. Anyone who finds one out in the open at night has had a genuinely lucky dive.
- tail plate covered in radiating spines like a sea urchin
- reddish brown body
- plugs the burrow entrance with its spiny tail
- length up to 12 cm
