Divers often encounter jacks under water. They are fast-swimming predatory fish, which range from the bar-jacks and horse-eye jacks which school in silvery thousands to the trevallys, which often hunt in small groups.
     Largest of all the jacks is the mighty almaco (Seriola rivoliana), a deep-bodied amberjack with a dark olive-coloured diagonal stripe that stretches from mouth across the eye to around the dorsal fin.
     The almaco jack is found in the Mediterranean, the Atlantic and the Eastern Pacific south of California, and prefers deep, open water away from reefs. Often weighing in at more than 50kg and approaching 2m in length, these are magnificent animals, much prized by deep-sea sport fishermen.
     In common with other jacks, almaco jacks like to remove parasites from their skin by rubbing themselves on the rough exterior of passing sharks.
     Divers at offshore sites in places such as the Galapagos and Cocos Island often enjoy the strange experience of an almaco jack mistaking them for sharks!