Rides sea cucumbers and nudibranchs, which makes it one of the easier small subjects to frame.
Two to three centimetres, females larger. Orange-yellow to orange-red with purple antennal plates, chelipeds and walking legs, and white patches that expand by day into bands across the back. The body colour shifts with the host. The nearest confusion is the sea star shrimp, which is host-matched to sea stars and lacks the strong purple-and-orange contrast. On the name: WoRMS now calls it Zenopontonia rex, because Kemp’s older name has priority over Bruce’s imperator and because Periclimenes has been broken up; Periclimenes imperator is what every dive guide and image library still uses, so both names are worth knowing.
It rides its host — sea cucumbers, most often, sitting in the ventral groove or near the anus, and a long list of nudibranchs and sea slugs including the Spanish dancer, plus cowries and occasionally sea stars. It is symbiotic but not host-specific and not strictly obligate. The traditional account has it eating host mucus, faeces and trapped detritus, and that account is now known to be incomplete: it has been recorded catching and killing another shrimp on a nudibranch in Indonesia, and killing a post-larval fish on a sea star in Bali — one of very few observations of a caridean shrimp killing a vertebrate. It is at least a facultative predator. Shallow to about 45 m, throughout the tropical Indo-Pacific.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.