Another difference lies in their overall size. Although this is true only 80 percent of the time, it is a practical differentiation that could be helpful to fishkeepers.
Upper Jaw Bichirs are generally smaller than the Lower Jaw Bichirs. The latter are the giants, growing 24 inches or more, probably the biggest among the genus Polypterus.
Most Upper Jaw Bichirs generally grow about 12 to 14 inches, with P. ornatipinnis reaching 24 inches, P. teugelsi maxing out at 22 inches, and P. weeksii a little less. The longest Upper Jaw Bichir is the E. calabaricus, which is said to reach 36 inches in length but is really a very slender fish.
If you collect big fishes, go for Lower Jaw Bichirs.