Brown Surgeonfish

Acanthurus nigrofuscus, commonly known as the brown surgeonfish, blackspot surgeonfish, brown tang, dusky surgeon, lavender tang, or spot-cheeked surgeonfish, is a species of marine ray-finned fish within the family Acanthuridae. This family is notable for comprising surgeonfishes, unicornfishes, and tangs. Acanthurus nigrofuscus is prevalent and abundant across an expansive range in the Indo-Pacific region.

This species was initially described as Chaetodon nigrofuscus in 1775 by the Finnish explorer, orientalist, and naturalist Peter Forsskål, with Jeddah recorded as the type locality. The genus Acanthurus is one of two genera within the tribe Acanthurini, which is part of the subfamily Acanthurinae, itself one of the subfamilies within Acanthuridae.

The specific epithet, nigrofuscus, derives from the Latin terms "nigro," meaning "black," and "fuscus," meaning "dark," reflecting the fish’s dusky or brownish-black coloration.

Acanthurus nigrofuscus is characterized by having nine spines and between 24 and 27 soft rays supporting its dorsal fin, while its anal fin is supported by three spines and 22 to 24 soft rays. The dorsal head profile is slightly humped, and the overall body coloration ranges from brown to purplish or bluish-brown, with distinctive small orange spots on the head and breast. Black spots are present at the base of both the dorsal and anal fins, which are edged with a pale blue margin. The caudal peduncle spine is embedded in a patch of black pigment. The species may also exhibit sinuous horizontal lines of small blue dots. It can reach a maximum total length of 21 cm (8.3 in).

Geographically, Acanthurus nigrofuscus is widely distributed across the Indo-Pacific, from the eastern coast of Africa, including the 🇮🇱 Israeli 🌊 Red Sea coast and extending south to the Aliwal Shoal in 🇿🇦 South Africa. Its range extends across the 🌊 Indian Ocean, reaching as far as the Pitcairn Islands, Hawaii (🇺🇸 United States), southern 🇯🇵 Japan, and New South Wales. This species is commonly found in small schools inhabiting sheltered coral and rocky reefs, often around isolated rocky outcrops at depths up to 25 m (82 ft).

The species primarily feeds on benthic algae. Although typically seen in smaller groups, it is capable of forming large schools in oceanic environments and congregates in significant numbers during spawning. Despite being relatively small, and often dominated by larger species, their large schools have the capacity to displace other grazing fish.

Acanthurus nigrofuscus is host to the unusually large symbiotic bacterium Epulopiscium fishelsoni, found in its intestine. This bacterium can grow as large as 600 by 80 μm, approximately the size of a printed hyphen, and plays a vital role in controlling the pH of the host’s gut, thereby enhancing the host's digestive and nutrient absorption capabilities.