Threadfin Jack

The threadfin jack is a moderately large species, growing to a known maximum length of 60 cm. The species is similar in appearance to a number of jacks in the genera Carangoides and Alectis in its adult form, having a compressed, oblong body, with the dorsal and ventral profiles approximately equal in concavity. The head profile is quite angular, being most steep immediately above the mouth, and being moderately steep to the nape, becoming more horizontal posteriorly. The juveniles have a more oval to diamond shape, looking much like juveniles of the genus Alectis. The dorsal fin is in two parts, the first being greatly diminished and consisting of eight spines entirely embedded in the skin, with the second dorsal fin composed of one spine and 18 or 19 soft rays. The anal fin is composed of two anteriorly detached spines followed by one spine and 16 or 17 soft rays. Both juveniles and adults have highly elongated second dorsal and anal fin lobes, extended out into long filaments; they are most pronounced in juveniles. The lateral line has a moderate, regular arch anteriorly, which is roughly equal in length to the straight posterior section. The straight section has no to 15 scales followed by 40 to 52 small scutes. The breast is devoid of scales ventrally to behind the pelvic fin origin and diagonally up to the pectoral fin base. …

The threadfin jack is distributed throughout the tropical regions of the eastern Pacific Ocean, inhabiting the western coast of the Americas. The northern limit to the species range is southern Baja California, with its range extending south to Mexico and Central America to a southern limit of Ecuador, with the species also recorded from the Galapagos Islands. It is one of only two species of the genus Carangoides found on the western coastline of the Americas, with the other species being the widely distributed island trevally, Carangoides orthogrammus. The species appears to undergo a major transition in lifestyle after its juvenile phase, with young individuals leading a pelagic lifestyle, able to be transported to offshore islands such as the Galapagos by currents. Older individuals are more benthic in nature, inhabiting the bottom of the water column in mostly coastal waters on reefs. Smaller fish have been recorded from estuaries, mangrove-lined creeks, shallow bays, and lagoons, although larger fish live in much deeper waters up to 50 m (160 ft) in depth.