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Taxon profile

species

Chocolate Surgeonfish
Acanthurus pyroferus Kittlitz, 1834

kingdom Animalia - animals »  phylum Chordata - chordates »  class Actinopterygii - ray-finned fishes »  order Perciformes - perch-likes »  family Acanthuridae - surgeonfishes and unicornfishes »  genus Acanthurus

Scientific synonyms

Acanthurus armiger Valenciennes, 1834
Acanthurus celebicus Bleeker, 1852
Acanthurus fuscus Steindachner, 1861
Acanthurus leucocheilos (non Herre, 1927)
Hepatus celebicus (Bleeker, 1852)
Hepatus pyriferus (Kittlitz, 1834)
Hepatus pyroferus (Kittlitz, 1834)
Rhombotides celebicus (Bleeker, 1852)

Other names

= Chocolate Tang
= Mimic Surgeonfish
= Mimic Tang
= Orange-gilled Surgeonfish
= Yellowspot Surgeon

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Description

Body moderately deep, its depth 1.85 to 2.1 times in standard length. Snout distinctly protruding, its dorsal profile concave; snout length 4.4 to 4.7 times in standard length. Mouth small; teeth spatulate, close-set, with denticulate edges; 14 upper and 16 lower teeth in jaws of subadults, to as many as 16 upper and 21 lower teeth in adults. Total gill rakers on first gill arch 23 to 26. A continuous unnotched dorsal fin with VIII spines and 27 to 29 soft rays; dorsal and anal fins moderately high, the longest dorsal-fin ray 3.9 to 4.8 times in standard length; anal fin with III spines and 25 to 28 soft rays; pectoral-fin rays 16. Caudal fin rounded in juveniles, progressively more lunate with age in adults, the caudal concavity of adults 4 to 5 times in standard length. A lancet-like spine on each side of caudal peduncle which folds into a deep horizontal socket. Stomach round and gizzard-like.

Color: Brown, shading to brownish red on chest; a large vertically elongate orange-red spot on body above pectoral-fin base; a broad black band from upper end of gill opening along edge of operculum to isthmus; a narrow white band encircling chin and extending above corner of mouth; median fins dark brown, the caudal fin with a broad yellow posterior border; juvenilies exhibit 4 different colour phases, one all yellow, one bright yellow with blue markings on head (a precise mimic of the small angelfish Centropyge flavissimus), one light greenish grey shading posteriorly to black (an exact mimic of C. vroliki), and one yellow except posterior half of body which is deep blue (a mimic of C. bicolor).

Size

Maximum total length up to 29 cm; commonly to 18 cm.

Distribution

Western Pacific from southern Honshu to New South Wales, east throughout Oceania, except Hawaii, Pitcairn Group, and Easter Island. It has not been reported from Rapa (French Polynesia). The juvenile phase that mimics Centropyge flavissimus occurs in eastern Oceania, such as the Society Islands (where the mimicry was first discovered), the one mimicking C. vroliki is found more to the west such as Palau and the Philippines, and the one mimicking C. bicolor occurs in New South Wales. Acanthurus pyroferus is replaced in the Indian Ocean by the closely related A. tristis Randall (the juvenile of which is a mimic of C. eibli). A. pyroferus and A. tristis have been observed together in eastern Bali.

Ecology

Occurs on coral reefs; feeds on filamentous benthic algae on reef and sandy surfaces. Usually solitary at depths from 2 to at least 60 m. The juveniles are remarkable for mimicking different small species of angelfishes (family Pomacanthidae) of the genus Centropyge, not only in colour but in having a strongly rounded caudal fin. The basis for the mimicry appears to be the tendency for the species of Centropyge to stay very close to shelter; predaceous fishes learn that they are difficult to capture. The surgeonfish juvenile is then able to feed in the open on the reef with less risk from predators.

One of three species of the genus Acanthurus with 8 dorsal spines. The second and third dorsal spines fused to form one thick spine, hence the count is reduced from usual 9 to 8. Randall (1960) noticed that specimens from the Society Islands have higher average count of soft rays of the dorsal and anal fins, whereas specimens from the Philipines the lowest count, and the Marshall Islands population intermediate. Additional counts confirm that the three populations differ which may perhaps result in some taxonomic adjustments in the future.

Myers (1999) noted that that juvenile A. pyroferus at Palau mimic C. vrolikii which is a common species in that area, while C. flavissimus is rare. At Kwajalein Atoll in the Marshall Islands, it is far more common than C. vroliki but this atoll (and at least also Majuro, as Randall observed) is unique in having many hybrids of C. flavissimus and C. vroliki. Juvenile A. pyroferus at Kwajalein mimic both species but the hybrid is mimicked more often that either of the two species, as noted by Scott Johnson.

Taxonomy

One variant of the juvenile form that mimics C. vroliki has small yellow spots on the grey anterior part of the body. Some authors have misidentified it as Acanthurus chronixis, a species know only from one adult specimen from Kapingamarangi Atoll in the Caroline Islands.

Interesting facts

The species name "pyroferus" means "fiery". It is evidently derived from Greek "pyros" meaning “fire.” It is not clear why Kittlitz decided to use this name. One possible reason is the bright orange bar behind the gill opening.

Sources

Carpenter, K.E.; Niem, V.H. (eds). FAO species identification guide for fishery purposes. The living marine resources of the Western Central Pacific. Volume 6. Bony fishes part 4 (Labridae to Latimeriidae), estuarine crocodiles, sea turtles, sea snakes and marine mammals. Rome, FAO. 2001. pp. 3381-4218.

Randall, John E. Surgeonfishes of the World. Mutual Publishing and Bishop Museum Press. 2001.
Author: Petr Čech

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