A small smooth-shelled crab, reddish brown to purple, that lives among the spines of long-spined urchins - most often Diadema. It clings close to the test with long thin legs and shifts around the host as the spines move, which is why it is far easier to find with a torch than by daylight.
The relationship is one-sided. Females are considerably larger than males and can take up residence inside the urchin's rectum, feeding on its tissues and on what passes through; smaller males live outside among the spines. The urchin gains nothing and loses a little, which puts this closer to parasitism than to the tidy partnerships elsewhere on the reef. Females carry eggs under the abdomen until they hatch. Handling the host is the only real risk here, and it belongs to the urchin rather than the crab.
- small smooth crab clinging among urchin spines
- reddish brown to purple, matching the host
- long thin legs held close to the body
- width under 1.5 cm
