Silver with a dark blue and black bar running along the back and into the lower tail lobe. It follows rays and turtles to take what they disturb.
To about 70 cm, typically under 40. Elongate, moderately deep and compressed with a slender tail base, silvery and light blue above.
One mark identifies it and nothing else in the region has it: a black stripe running from below the start of the second dorsal fin along the back and down into the lower lobe of the tail, with an electric-blue stripe immediately below it. Juveniles carry up to six dark bars and a darker lower tail lobe.
Nought to thirty-five metres in clear water, along reef edges and over soft and seagrass bottoms, and it crosses open sand to move between reefs. It is solitary or in large schools, and it shoals with barracuda, stingrays and sharks in lagoons. It follows bottom feeders — stingrays, and in one documented case the puddingwife wrasse — to take what they turn up. It hunts by sight and is inactive at night, and it is easily approached.
Two things to expect. The whole body turns blackish when it is feeding near the bottom, so a black fish here may still be a bar jack. And it spawns in aggregations of hundreds, with pairs breaking away.
Ciguatera risk is intermediate overall, but most cases in St Thomas trace to this species. Two published scute counts for it disagree outright, so this entry does not give one.
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.