Chaetodon mertensii, the atoll butterflyfish, yellowback butterflyfish or Merten's butterflyfish, is a species of marine ray finned fish, a butterflyfish belonging to the family Chaetodontidae. It is found in the western Pacific Ocean.

The Eritrean butterflyfish or crown butterflyfish is a species of marine ray-finned fish, a butterflyfish belonging to the family Chaetodontidae. It is essentially just known from the Red Sea and the Gulf of Aden, but has been reported from East Africa. It grows up to 14 cm (5.5 in) in length. It is white with black chevrons, except for a red zone stretching from the aft dorsal fin across the caudal peduncle to the end of the anal fin. A red bar runs vertically through the caudal fin. There are black eyestripes and a black "crown" with white border.