The greenbone or butterfish, Odax pullus, a cale of the genus Odax, is found around New Zealand, in shallow reef areas where there is abundant brown seaweed. Its length is between 30 and 75 cm, and they can weigh up to 5 kg. The name greenbone comes from the colour of their bones.
The greenbone is a slender fish with a pointed snout. It changes its colouration as it grows, young fish being yellow-brown with a line of white dashes down each side from the snout to the caudal peduncle. Adult females are brown with a pale band down each side, and males are grey-brown with conspicuous blue patterning on the head, flanks, and fins giving the fish an overall blue-grey appearance. Males have greatly elongated dorsal and anal fins that reach as far back as the tip of the tail. Their teeth are fused in each jaw to form a parrot-like beak, and they are herbivorous, living on seaweed, preferring Ecklonia radiata.
Greenbones are protogynous hermaphrodites, beginning life as females, later changing to males. Males establish territories and spawning takes place during late winter. Larval greenbone settle out of the plankton into dense patches of bladder kelp in shallow wave-swept water, where they are initially protected from predators.
Greenbones are larger and more abundant in the south of New Zealand. They are excellent eating fish, giving rise to their second common name butterfish. They can be caught by anglers using the proper bait and hook but are generally netted commercially in some southern localities, they are also regularly speared by divers.