Jacks, also called scads, trevallys, and crevalles, are marine bony fishes living in open waters. Amberjacks, runners, and pompanos also belong to the same family Carangidae, order Perciformes. Jacks are swift, predatory fishes, found widely in warm and tropical seas. The younger fish tend to travel in vast schools, but the older ones may be solitary.
Many jacks are valued for commercial and sport fishing, and some species are successfully exhibited in public aquariums. The Florida pompano (Trachinotus carolinus), which grows to 1 ft (30.5 cm) in length, is considered a seafood delicacy. The crevalle jack (Caranx ruber) may reach the size of 2 ft (61 cm) and weigh over 20 lb (9 kg). It is the most common jack of the West Indian waters, and is often seen near coral reefs. In the summer, large schools of this species cruise by the Bahamas, where it is known as the "passing jack." The more than 200 species in the family Carangidae vary greatly in form, from long and streamlined to deep-bodied and very thin from side to side. Generally they share the following features in common: two dorsal fins (the first may be greatly reduced in size); anal and second dorsal fins usually high in the front; slim, often sickle-shaped pectoral fins; a strong, fork- or crescent-shaped tail with a slim base; small scales. Many species in this family are quite small, but some can reach very large sizes. For example, the amberjack (Seriola dumerili) can grow to 6 ft (1.8 m) in length and can weigh as much as 150 lb (70 kg). Some jacks have a series of scutes (comb-like scales) along the caudal peduncle (the fleshy part of the tail), which reinforce it for fast swimming.
Most carangids are silvery in color, but some exhibit lovely colors or markings. The rainbow runner (Elagatis bipinnulatus) of the tropical Atlantic and Indo-Pacific has beautiful blue bands on the sides. It is a hard-fighting sports fish, and supposedly very tasty. Color changes may occur in some species as the fish mature. The Indo-Pacific blue-banded golden jack (Gnathanodon speciousus) is solid yellow when young. The African pompano and other threadfins of the genus Alectis have streaming fins which trail behind them, resembling the long tentacles of jellyfish. When adult fish reach the size of about 3 ft (91 cm), the fins appear shorter. Palometas (Trachinotus goodei) are silvery jacks that tend to form schools in shallow water, and often approach wading people. In public aquarium exhibits they form attractive schools.
The permit (Trachinotus falcatus) lives in shallow Atlantic waters near reefs and sandy flats. It may grow to the size of 3.5 ft (1.1 m). When young, it lives in sheltered waters and feeds on small crustaceans; later, it includes molluscs and sea urchins in the diet. The greater amberjack is the most common species of the genus Seriola in the tropical and subtropical waters of western Atlantic. It has a lengthwise brassy stripe on the side of the body at the level of the eye. The back above the stripe is olive to blue, and the body below the stripe is silvery white. There is also a diagonal dark band running from the snout, through the eye, to the nape. The greater amberjack and several other jack species may at times carry a toxic substance in their flesh that causes ciguatera poisoning when these fishes are eaten by humans. This toxin comes from algae that the fish ingests either directly or in the smaller fish it consumes.