Description
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 are 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 8 spines entirely embedded in the skin, with the second dorsal fin composed of 1 spine and 18 or 19 soft rays. The anal fin is composed of 2 anteriorly detached spines followed by 1 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, and are most pronounced in juveniles.
The lateral line has a moderate, regular arch anteriorly, which is approximately equal in length to the straight posterior section. The straight section has 0 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. Both jaws contain uniform bands of small, weak teeth becoming wider posteriorly and irregular conical outer teeth in adults. There are 21 to 23 gill rakers and 24 vertebrae.
In life, the threadfin jack is a silvery blue above, becoming silvery white on the underside, with golden to yellow reflections. The first dorsal and pelvic fin is grey, while the second dorsal, anal, pectoral and caudal fins are hyaline or grey with a yellow tinge. Juveniles have clear dark vertical bands, fading with age. Much like the shadow trevally, there are small black spots on the bases of soft dorsal rays and the body immediately below them that increase in size with age, and there is a small black blotch on the upper operculum.
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