Striped lengthwise to match a spine, and it lives its whole life running along them.
A slender shrimp to about 2 cm, dark purple-black with two or three clean white or pale yellow stripes running the length of the body from rostrum to tail. Held against a spine the stripes continue the spine’s own line, and the animal reads as a thicker section of it rather than as a shrimp. The legs are long and the body is flattened side to side, so it lies close.
An obligate associate of long-spined urchins, chiefly Diadema and Astropyga, and it does not leave. It runs along the spines and around the test, keeping the spines between itself and whatever is approaching, and pairs are usual on one urchin. Its value to the host, if any, is unclear; what it gets is a defended space that nothing will reach into. Recorded from the shallows to about 25 m. Photographing it means working through a thicket of spines that break off in a lens port as readily as in a hand, so the frame is taken from the side at a distance rather than from above.
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.