Emperor angelfish are large reef fish, with compressed bodies and a multitude of yellow and blue horizontal stripes along their sides. Their mouths are light blue or white and their eyes and gill covers are coloured dark blue with pale blue borders. Their gill covers each have a prominent spike. Juveniles are dark blue with blue and white rings.
One of the reasons for this difference is that adults are very territorial and fight with one another. Because the juveniles have a different colour and pattern they are allowed into adult territory. It takes 24 to 30 months for an emperor angelfish to obtain its adult colouring.
The emperor angelfish hunts small invertebrates, or forages on sponges in coral reefs in the Indian and Pacific Oceans.