''Rhinecanthus lunula'', commonly known as the halfmoon picassofish, is a balistid triggerfish species first described by John E. Randall and Roger C. Steene in 1983. It belongs to the

Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...

triggerfish genus ''

Rhinecanthus ''Rhinecanthus'' is a triggerfish genus from the Indo-Pacific. They are found at reefs, and all except ''R. abyssus'' are restricted to relatively shallow depths. They are among the smallest members of the family, with no species surpassing in le ...

''.

Physical description

While the

standard length Fish measurement is the measuring of individual fish and various parts of their anatomies. These data are used in many areas of ichthyology, including taxonomy and fisheries biology. Overall length * Standard length (SL) is the length of a fish m ...

of ''Rhinecanthus lunula'' holotype is , the standard length of other type specimens ranges from . ''R. lunula'' can be differentiated from other ''Rhinecanthus'' species by its soft ray count and distinct coloration. It usually has 26 soft dorsal rays (though sometimes only 25 are observed), between 22 and 24 anal soft rays, and 14 pectoral rays. The longest dorsal soft ray is 11.7 percent of the standard length. A broad, black bar exists on the caudal peduncle. Anterior to the caudal peduncle is a curved black line which gives the species its name “lunula,” the Latin word for crescent. An orange-yellow band starts from the base of the upper lip and extends across the head to the pectoral base. In addition, the fish has a long snout, a concave dorsal profile, and an elongate body. The greatest body depth is between 2.2 and 2.6 percent of its standard length.Randall, J.E. and R.C. Steene (1983) ''Rhinecanthus lunula'' a new species of triggerfish from the South Pacific. Freshwater and Marine Aquarium (6)7: 45-51. Randall and Steene noted many differences between juvenile ''R. lunula'' and adults. For example, the

caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

of a juvenile is rounded in a juvenile before becoming slightly double emarginate as it develops into an adult. Juveniles are also missing some of coloration that stands out in adults: there is no blackish area on the ventral side, black bars below the eyes are not well-developed, and the black crescent near the caudal peduncle (which gives the species its name) is also missing. However, there are also two yellow stripes present on juveniles that are not seen on adults. Additionally, the interorbital space is slightly concave in juveniles but becomes slightly convex in adults.

Distribution

A rare species, ''R. lunula'' has a small geographic distribution. When first described as a species, ''R. lunula'' was only known to exist from the

Pitcairn Islands The Pitcairn Islands (; Pitkern: '), officially the Pitcairn, Henderson, Ducie and Oeno Islands, is a group of four volcanic islands in the southern Pacific Ocean that form the sole British Overseas Territory in the Pacific Ocean. The four isl ...

to Queensland, Australia. The holotype of the species was collected by John E. Randall in a barrier reef near the

Society Islands The Society Islands (french: Îles de la Société, officially ''Archipel de la Société;'' ty, Tōtaiete mā) are an archipelago located in the South Pacific Ocean. Politically, they are part of French Polynesia, an overseas country of the F ...

Tahiti Tahiti (; Tahitian ; ; previously also known as Otaheite) is the largest island of the Windward group of the Society Islands in French Polynesia. It is located in the central part of the Pacific Ocean and the nearest major landmass is Austr ...

, and juvenile specimens were collected near Queensland, Australia. While most reported sightings of the species have occurred within the same geographic range noted by Randall and Steene, a single sighting of ''R. lunula'' has been recorded by scuba divers in

Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Mediter ...

. However, the fish seen in Egypt may have been misidentified, as other members of the ''Rhinecanthus'', ''R. aculeatus'''' and R. rectangulus'', are known to exist in Africa.'''' ''Rhinecanthus lunula'' are generally found in the outer areas of

coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Co ...

s 10 meters or deeper. Though the maximum depth of their habitat is unknown, triggerfishes of the Balistidae are known to exist up to 50 meters deep. Habitat may differ by age for ''R. lunula'', as it is known that ''

Rhinecanthus aculeatus The lagoon triggerfish (''Rhinecanthus aculeatus''), also known as the blackbar triggerfish, the Picasso triggerfish, or the Picassofish, is a triggerfish, up to 30 cm in length, found on reefs in the Indo-Pacific region. The Hawaiian name ...

'' juveniles tend to live near beaches while adults tend to live on barrier reefs. Distribution of a given Balistid species among an ecosystem may also be influenced by their diet and the distribution of food sources.

Reproduction

A main characteristic of the triggerfish family Balistidae, to which ''Rhinecanthus'' ''lunula'' belongs, is a haremic mating system in which a male has multiple female mates.

References

{{Taxonbar, from=Q2415789 Fish described in 1983 Balistidae