Jacks
In the western Atlantic, the lookdown is found from Canada and Maine south to Uruguay, which includes Bermuda and the Gulf of Mexico. Although lookdown are common in tropical, Atlantic waters, they are rarely seen in the Greater Antilles. Lookdown are found in both marine and brackish waters at depths of 1 to 53 meters. They usually live in shallow water near the coast, along hard or sandy bottoms. Juveniles may be seen in estuaries or near sandy beaches Lookdown usually swim together in schools, but they often break away into smaller groups or even in pairs.
The golden trevally (Gnathanodon speciosus) is a tropical marine fish in the jack family (Carangidae). Native to the eastern Pacific Ocean from southern California to Ecuador, it is associated with reefs and usually found within the upper ten m (30 ft) of the surface. Juveniles are usually bright yellow with vertical black bars, which alternate between broad and narrow. In adults this coloration fades to a silvery color with the bars faint or absent.
Golden trevally are schooling fish which often follow other large fish, such as sharks, as well as SCUBA divers, for protection from predators. They feed on small fishes and on benthic crustaceans and other invertebrates. Juvenile individuals may be found living in the tentacles of jellyfish. Juvenile golden trevally may be found in the aquarium trade due to their bright coloration. Adults may be used as food fish.
Although the lookdown is similar to the Atlantic moonfish, it can be distinguished by its dorsal and anal fins, the second ray on each fin being many times longer than the surrounding rays. This gives the dorsal and anal fins a noticeably scythe-like shape. Also, the dorsal fin of the lookdown has 9 spines and 23 rays, while the anal fin has only 3 spines and 18 rays. The lookdown, like the Atlantic moonfish, had a deep, rhombus-shaped, laterally-compressed body. The head has the mouth set low and the eyes high. The overall profile of a lookdown's head is concave. The caudal fin of the lookdown is forked, as in the Pompanos, while the pectoral fin is similar to a scythe and reaches the middle of the second of the lookdown's dorsal fin.
Lookdown are usually described as being silvery on both sides with darker tints on top. Young lookdown have several faint, vertical bars that fade as the fish grows. The longest lookdown known was 48.3 cm long, the heaviest known lookdown weighed 2.1 kg.