The blue-cheeked butterflyfish, Chaetodon semilarvatus, is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of butterflyfish

Butterflyfish

The butterflyfish are a group of conspicuous tropical marine fish of the family Chaetodontidae; the bannerfish and coralfish are also included in this group. Found mostly on the reefs of the Atlantic, Indian and Pacific Oceans, there are approximately 120 species in 10 genera...

 (family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Chaetodontidae). It is found in the Red Sea

Red Sea

The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez...

 and the Gulf of Aden

Gulf of Aden

The Gulf of Aden is located in the Arabian Sea between Yemen, on the south coast of the Arabian Peninsula, and Somalia in the Horn of Africa. In the northwest, it connects with the Red Sea through the Bab-el-Mandeb strait, which is about 20 miles wide....

, at depths of between 3 and 20 m.

Its length is up to 23 cm (9.1 in). It is mostly yellow, with thin slate

Slate

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

 blue

Blue

Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...

 vertical lines on the sides and a slate blue cheek patch in lieu of the usual black eyestripe of most Chaetodon

Chaetodon

Chaetodon is a tropical fish genus in the family Chaetodontidae. Like their relatives, they are known as "butterflyfish". This genus is by far the largest among the Chaetodontidae, with about 90 living species included here, though most might warrant recognition as distinct genera.Several subgenera...

.

The blue-cheeked butterflyfish belongs to the large subgenus

Subgenus

In biology, a subgenus is a taxonomic rank directly below genus.In zoology, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the generic name and the specific epithet: e.g. the Tiger Cowry of the Indo-Pacific, Cypraea tigris Linnaeus, which...

 Rabdophorus which might warrant recognition as a distinct genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

. In this group, it seems closest to a group containing the Blackback butterflyfish

Blackback butterflyfish

The Black-backed Butterflyfish or blackback butterflyfish Chaetodon melannotus, is a species of butterflyfish . It ranges from the Red Sea and East Africa to Samoa, to southern Japan and throughout Micronesia....

 (C. melannotus), the spot-naped butterflyfish (C. oxycephalus), or the peculiar black-wedged butterflyfish

Blackwedged butterflyfish

The Black-wedged Butterflyfish, Chaetodon falcula, is a species of butterflyfish . IT is found in the Indian Ocean, from eastern Africa south to 27°S and east to Indonesia.It grows to a maximum of 20 cm long...

 (C. falcula) and Pacific double-saddle butterflyfish

Pacific double-saddle butterflyfish

The Pacific Double-saddle Butterflyfish or False Furcula Butterflyfish , is a species of butterflyfish . It flourishes in coral-rich environments in the Indo-Pacific region. Their range extends from the Cocos-Keeling Islands to the Tuamotu Islands, and north to Japan...

 or "false falcula", (C. ulietensis). Though the present species does not share their white body with black on the back and caudal peduncle and even lacks the typical eyestripe of Chaetodon, it has the same tell-tale blue vertical lines as these species.