Coleman Shrimp

Photo: 104623964081378888743 · CC BY-NC

Coleman shrimp are obligate commensals of the fire urchin Asthenosoma ijimai, and pairs — always one large female and a smaller male — are found exclusively on the white-tipped venomous tube feet of this urchin species, on which they have cleared a small bald patch by removing spines to create a safe living platform. Their translucent bodies are patterned with white, red, and purple markings that match the colour of the fire urchin's surface precisely, a camouflage that is effective against visual predators approaching from above but less useful to a diver looking down from above the urchin's curved surface. The presence of Coleman shrimp is entirely dependent on the availability of fire urchins, making them an indirect indicator of reef health and the absence of urchin harvesting, which is common on unprotected East African reefs.