A silver mesopelagic fish shaped like a blade, with light organs along the belly.
A small deep-bodied fish of two to six centimetres, silver and so strongly compressed that it reads as a thin blade edge-on and as a broad disc from the side. A row of tubular light organs runs along the belly, pointing downward, and the eyes are large; in several genera they are directed upward. The body is fragile, and animals met on a blackwater dive are often already damaged.
The group lives in the mesopelagic layer through the day, several hundred metres down, and rises towards the surface at night with the rest of the migrating community. The ventral light organs are used for counterillumination: output is matched to the faint light coming down from above, so that the silhouette is cancelled for anything looking up from below.
Records on shallow blackwater dives are occasional rather than reliable, and a photograph will not usually settle the genus. The water worked is the top 25 m over deep ground.
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.
