Not a shrimp at all, and it stands on hydroids swaying like a stick insect.
An amphipod rather than a shrimp, to about 3 cm, with a body reduced almost to a thread, two pairs of grasping claws at the front and hook-like rear legs that grip the substrate. Translucent white, brown or green, and so thin that a colony is usually noticed as movement rather than as animals. Males are much the larger and carry a long spine on the head; females brood the young in a ventral pouch and the juveniles ride on the mother.
Grips hydroids, algae, rope, netting and discarded fishing line with the rear legs and holds the body upright and free, swaying to catch particles from the current with the feeding appendages, then strikes at passing plankton with the claws. Colonies of dozens on a single hydroid are normal, and a rope hanging off a jetty is often solid with them. Recorded from the shallows to about 25 m. It is at the limit of what a standard macro lens resolves, and it is one of the subjects that a wet dioptre is actually for.
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.