Mertensii Butterflyfish (Chaetodon mertensii)

Mertensii Butterflyfish (Chaetodon mertensii) known to tropical fish keeping enthusiasts as Chaetodon mertensii, the Atoll Butterflyfish, Yellowback Butterflyfish, Orangetail Butterflyfish, Crowned Pearlyscale Butterflyfish, Merten’s coralfish. and Merten’s Butterflyfish is found in the western Pacific Ocean, East Indian Ocean, West Indian Ocean, Australia, The Red Sea, and Indonesia.

The Mertensii Butterflyfish was first discovered in the Kermadec Islands Marine Reserve north of New Zealand in 2015 and has a range that extends from the Ryukyu Islands to the Philippines, all the way to Lord Howe Island, Rapa Iti, and the Tuamotus.

Mertensii Butterflyfish are encountered alone, in pairs, and loose groups in the deep lagoons and seaward reefs of their range in depths from 30 to over 390 feet where they feed on algae, corals, tubeworms, and small benthic invertebrates.

Mertensii Butterflyfish (Chaetodon mertensii)

Mertensii Butterflyfish have a silvery white body color, with 5 to 7 dark gray chevron shaped bands on along the sides. A black vertical bar, outlined in white, runs interrupted through the eye and over the nape at the head.   A broad yellow to orange band extends from the top of the dorsal fin to the anal fin at the posterior of the body.   The tail is yellow or orange with the caudal peduncle and outer edge of the tail colored white.

Except for the yellower hind parts and the continuous head band; Chaetodon mertensii look almost identical to the Seychelles Butterflyfish (Chaetodon madagaskariensis).

Similar species include the Pearlscale Butterflyfish (Chaetodon xanthurus) and closely related Eritrean (Chaetodon paucifasciatus).

Although  Mertensii Butterflyfish can be housed in a mature reef tank with caution; they are better housed in a well established FOWLR aquarium of at least 100 gallon capacity with a fine crushed coral or sand substrate, large amounts of mature live rock arranged into caves, crevices, and overhangs for them to graze on and hide among, and plenty of free swimming space.   They thrive in mature aquariums where the fish can forage on mature live rock growths.

This species requires a good filtration system, efficient protein skimming, a wave maker for water movement, and possibly a chiller to help maintain their cooler water requirements.

Like Chaetodon xanthurus, Mertensii Butterflyfish feed on several species of hard and soft corals, tubeworms, bristle worms, small invertebrates and are not considered reef safe.

Many tropical fish keeping enthusiasts with very large reef tanks have been successfully keeping them with some of the more noxious, unpalatable soft corals,  despite them occasionally nipping or nibbling on the polyps of SPS corals like Zoanthus, some LPS coral species, Tridacna clams, Aiptasia anemones, tubeworms, bristleworms, and smaller benthic invertebrates.

Mertensii Butterflyfish tankmates should be of the same size and peaceful in nature to minimize aggressive behavior.

Mertensii Butterflyfish may become territorial towards similar patterned or butterfly species of the same color, particularly when kept in smaller tanks.   When simultaneously introduced into large mature aquariums as juveniles,  they will normally coexist peacefully with other inhabitants however, its usually not a good idea to mix butterflyfish species of the same genus.

Mertensii Butterflyfish will often refuse to eat or venture out of the protection of their live rock crannies when kept with more belligerent species in FOWLR tanks.   In peaceful surroundings, they acclimate well and will almost always immediately begin to eat.

Compatible species include Banggai Cardinalfish (Pterapogon kauderni), Firefish Goby (Nemateleotris magnifica), Royal Gramma (Gramma loreto), Clownfish (Amphiprioninae), and Cleaner Shrimp (Lysmata amboinensis).

In the wild, Mertensii Butterflyfish form distinct pairs and scatter their eggs in open water.   They have not been successfully bred in an aquarium environment.

In their natural habitat, Mertensii Butterflyfish feed on algae, soft corals, SPS and LPS corals polyps, gorgonians, tubeworms, and small benthic invertebrates.

In an aquarium environment, the Mertensii Butterflyfish diet should consist of a variety of live, frozen, or freeze dried foods that include prepared foods rich in algae, high quality marine flakes, omnivore pellets, spirulina, Zooplankton, chopped krill, clams, mussels, Mysis shrimp, and vitamin enriched brine shrimp.   Feed small portions several times daily.

Mertensii Butterflyfish (Chaetodon mertensii) are available to tropical fish keeping enthusiasts from a variety of online suppliers and specialty fish shops at prices that vary according to size, area of collection, and correct identification of the species.

The three most often misidentified species are listed below:  

Chaetodon mertensii and Chaetodon madagaskariensis, believed by many to be the same species; both have a larger orange area than Chaetodon xanturus.

Eritrean butterflyfish (Chaetodon paucifascatious) which are identified by the reddish area, which in the other two is orange.

The yellow to orange area of Pearlscale Butterflyfish (Chaetodon xanthurus) that is smaller than, and more moon shaped than the orange area of Mertensii Butterflyfish (Chaetodon mertensii).

Approximate purchase size for Mertensii Butterflyfish:  Small: 1-1/2″ to 2-1/4″; Medium: 2-1/4″ to 3-1/2″; Large: 3-1/2″ to 4-1/2″

Currently prices for small Fiji specimens start at around $69.99 for small specimens, to $110.00 or more for large specimens.

Mertensii Butterflyfish (Chaetodon mertensii)

Minimum Tank Size: 100 gallons
Aquarium Type: FOLR
Care Level: Moderate
Temperament: Peaceful
Aquarium Hardiness: Moderately Hardy
Water Conditions: 74-80° F, dKH 8 – 12°, pH 8.1 – 8.4, sg 1.020-1.025
Max size: 5.2″
Color Form: White, Black, Yellow
Diet: Omnivore
Compatibility: Reef with Caution
Origin: Indo Pacific
Family: Chaetodontidae
Lifespan: Over 7 years
Aquarist Experience Level: Intermediate