There are at least a couple of dozen species of jacks prowling through Gulf waters. Most jacks are flattened, silvery fishes, with strongly forked tails and tiny, almost invisible scales. Almost all have two unattached sharp, stubby spines in front of the anal fin, but in large adults, these may be hidden beneath the skin.
Many species have a hardened row of the tail-fin.
Each of these groups, with their several species, has distinctive habits and characters mentioned in the species treatments. The young juveniles of many jacks hang close to floating sargassum weed and other protective cover, drifting along until big and bold enough to venture out, form schools, and begin the hunt. There is not much about this fish to let you know its family affiliation. It's not flat and silvery, it doesn't have the ridge of scutes. And whereas in most jacks there are two stubby little spines just in front of the anal fin, in the rainbow runner there is only one, while the second has become incorporated into the anal fin proper.