The Mertensii Butterflyfish (Chaetodon mertensii), also known as the Atoll Butterflyfish, Yellowback Butterflyfish, or Merten’s Butterflyfish, is a species of marine ray-finned fish belonging to the family Chaetodontidae. It is found in the western Pacific Ocean.

Distribution and Habitat

The Mertensii Butterflyfish is found in the Pacific Ocean from the Ryukyu Islands to the Philippines and extending to Lord Howe Island, Rapa Iti, and the Tuamotus. It is found in deep lagoons and seaward reefs at depths of 10 to 120 meters.

Physical Description

The Mertensii Butterflyfish grows to a maximum of 12.5 cm (5 in) long. The body color is white with 5 to 7 chevron-shaped dark grey bands on the sides. The posterior portion of the trunk, the adjacent dorsal and anal fins, and the posterior portion of the caudal fin are orange or yellow. A vertical black bar runs across the eye and over the nape without a break.

Diet and Feeding

The Mertensii Butterflyfish feeds on algae and small benthic invertebrates. In captivity, it accepts a varied diet including mysis shrimp, brine shrimp, spirulina, and marine algae.

Behavior and Reproduction

This species is typically found alone or in pairs. It is an oviparous species that forms pairs for breeding.

Aquarium Care

The Mertensii Butterflyfish requires a tank of at least 45 gallons with stable water conditions and abundant live rock. It is peaceful and can be kept with a variety of tankmates. It may nip at corals, so it is not universally considered reef-safe.

Conservation Status

Classified as Least Concern by the IUCN Red List.