Lives on the underside of sea stars and takes on their colour, changing over about a week if moved to a different host.
A small palaemonid shrimp, most individuals between about 8 and 14 mm long, living obligately on sea stars. The rostrum is short and broad and curves downward, carrying ten to fourteen dorsal teeth, and the fingers of the chelae are subspatulate with denticulate cutting edges. Colour is the least reliable character and the most striking one, because it follows the host: blue on Linckia laevigata, bright red with a pale dorsal streak on crown-of-thorns, and transparent, maroon, white, yellow or pale green on Culcita. In one Mo’orea population roughly three-quarters of animals were transparent and only an eighth were fully coloured. The likely confusion is with the emperor shrimp Zenopontonia rex, which is two to three times the size and carries purple antennal plates, chelae and walking legs; it lives mainly on sea cucumbers and nudibranchs but occasionally turns up on sea stars, which is where the two meet. Note the name: this animal appears in almost all dive literature as Periclimenes soror, and WoRMS now places it in Zenopontonia.
The association is obligate rather than casual — shrimp isolated from a host die whether or not they are fed. Recorded hosts span the cushion stars and their relatives, Culcita, Choriaster, Protoreaster, Pentaceraster and Halityle, along with Acanthaster, Linckia and Echinaster. The practical point for finding it is position: the shrimp sits on the oral surface, underneath, and is usually not visible from above. Counts run from none to ten on a single star, with around two being typical, and the occupants of a given host turn over within days. Colour is acquired rather than fixed, an animal moved to a differently coloured host taking roughly one to two weeks to match it, and larvae locate hosts by detecting asterosaponins in the water. Whether it takes only mucus or feeds on host tissue is currently unsettled, with recent isotope work arguing against the traditional reading of a purely commensal relationship.
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.