The Dotted Butterflyfish, Chaetodon semeion, 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 Indian Ocean

Indian Ocean

The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

 and Pacific Ocean

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

 from the Maldives

Maldives

The Maldives , , officially Republic of Maldives , also referred to as the Maldive Islands, is an island nation in the Indian Ocean formed by a double chain of twenty-six atolls oriented north-south off India's Lakshadweep islands, between Minicoy Island and...

 to the Tuamotu Islands, north to the Ryukyu Islands

Ryukyu Islands

The , also known as the , is a chain of islands in the western Pacific, on the eastern limit of the East China Sea and to the southwest of the island of Kyushu in Japan. From about 1829 until the mid 20th century, they were alternately called Luchu, Loochoo, or Lewchew, akin to the Mandarin...

 and south to the Great Barrier Reef

Great Barrier Reef

The Great Barrier Reef is the world'slargest reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometres over an area of approximately...

.

It grows to a maximum of 26 cm (10.2 in) long. The body is golden yellow. The base of the posterior portions of the dorsal

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 and anal fins is black, a prominent vertical black bar runs across the eye and there are diagonal rows of black dots on the sides. A filament originating from the soft portions of the dorsal fin rays trails posteriorly.

The Dotted 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 appears to represent a distinct lineage, with the Saddle Butterflyfish

Saddle butterflyfish

The Saddle Butterflyfish, Chaetodon ephippium, is a species of butterflyfish . It is found in the Indian and Pacific Oceans from Sri Lanka and the Cocos-Keeling Islands to the Hawaiian, Marquesan and Tuamoto islands, north to southern Japan, south to Rowley Shoals and New South Wales in...

 (C. ephippium) perhaps the only somewhat closely related species. Next closest seems to be a group including the Black-backed Butterflyfish (C. melannotus), Spot-tailed Butterflyfish (C. ocellicaudus) and Yellow-dotted Butterflyfish

Yellow-dotted butterflyfish

The Yellow-dotted Butterflyfish, Chaetodon selene, is a poorly known fish species which has been recorded in the Indo-Pacific region from Malaysia, Indonesia, New Guinea, Taiwan and Japan....

 (C. selene), but these are already so distant that their ancestors must have diverged from the Saddle Butterflyfish's soon after the Rabdophorus lineage started to diversify.

C. semeion is found at depths between 2 and 30 m. An uncommon species occurring in coral rich areas of clear water lagoon

Lagoon

A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...

 and semi-protected seaward reef

Reef

In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....

s, it is usually found in pairs or small groups and more wary than most species of 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...

.