Flashes hard metallic blue at one angle and disappears at the next.
A copepod a few millimetres long, flattened, oval and pointed at both ends, with a pair of large forward-facing eyes set close together. Males carry stacked layers of guanine crystal plates in the skin, and the colour that comes off them is structural rather than pigmented: at one angle the animal is a hard flash of blue, gold, violet or green, and a few degrees away it is gone, because the colour is produced by interference in the crystal stack and switches off with the viewing angle. Females are larger, lack the crystal plates, and are transparent with conspicuous dark eyes. Nothing else in the water column appears and disappears in this way.
Females live inside salps and doliolids, entering the barrel and feeding on the host’s tissue, while males swim free, which is where the flashing is seen and where it is thought to serve in finding females. The tunicates drifting past on a blackwater dive are worth as much attention as the open water between them, since an occupied barrel can hold more than one passenger.
The genus is widespread in warm surface waters worldwide and contains a number of species that are not separable from a photograph, so the record is normally left at Sapphirina.
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.
