Chaetodon mertensii
A small black-and-white butterflyfish logged year-round on reefs from Palawan to Queensland, most reliably in the Philippines.
A small, deep-bodied butterflyfish about 8 cm long, mostly white with fine black chevron stripes angled across the flanks and a black band running through the eye. The tail and rear dorsal edge carry a soft yellow wash, and there is no obvious eyespot or false eye near the tail. It moves in tight, deliberate pecks along the reef rather than drifting like many of its relatives, which helps separate it from busier-patterned look-alikes at a glance.
Usually spotted alone or in a loose pair, picking at coral polyps and small invertebrates along the reef face rather than schooling in the open. It tends to stay close to structure, ducking into crevices or coral heads when a diver gets too close, and divers most often log it as an uncommon rather than a headline sighting on a dive. Expect it on the reef slope during daylight hours, working the same patch rather than roaming far.
Like most Chaetodon butterflyfish it feeds heavily on coral polyps and tends to pair off with the same partner, patrolling a shared patch of reef rather than ranging widely.
There is no detected seasonal pattern, and with a comfortable range of roughly 24 to 29 C it sits happily on tropical reefs across the Indo-Pacific all year, so any month you are diving the Philippines, Indonesia or Queensland is a fair shot.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most consistently logged in the Philippines, with strong numbers across Luzon, Palawan, Mindanao and the Visayas, and it turns up widely through Indonesian sites in Sulawesi and Nusa Tenggara. Australia's Queensland reefs hold a solid population too, and it is a reliable if less frequent find in Malaysia's Sabah, Fiji, Micronesia, New Caledonia and French Polynesia.
Mapping where Merten's Butterflyfish is logged…
It is most commonly logged in the Philippines, particularly Luzon, Palawan, Mindanao and the Visayas, with good numbers also in Indonesia's Sulawesi and Nusa Tenggara and on Australia's Queensland reefs.
No clear season shows up in sighting data, and it tolerates a wide 24 to 29 C range, so it is a year-round find on tropical Indo-Pacific reefs.
Look for a small white butterflyfish with fine black chevron stripes angled across the body, a black eye band and a yellowish tail, usually seen alone pecking at the reef.
They stay small, reaching only around 8 cm, making them one of the more modest-sized butterflyfish divers encounter on Indo-Pacific reefs.
No, they pose no threat to divers and are typically shy, retreating into reef structure rather than approaching.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
