The black arowana has an elongated body and a tapered tail. Their maximum total length is typically considered to be 0.9 m (3.0 ft), but there are reports of individuals up to 1.2 m (3.9 ft). The juveniles are black with yellow markings down the length of the body, head and the tail. Once it reaches about 15 cm (0.5 ft), the markings disappear and the fish will develop a dark iridescent steel grey to blue coloration, hence its common name. Additionally, there are yellow and red outlining or the dorsal, caudal and tail fins. In contrast to the juveniles, adults are very similar to the silver arowana (O. bicirrhosum), but the two species can be separated by meristics.