Fish of the Month: Brown Surgeonfish Acanthurus nigrofuscus

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FISH OF THE MONTH - MAY 2003

BROWN SURGEONFISH
Acanthurus nigrofuscus

another common, drab, seldom-noticed reef fish that might be more interesting than you thought

BROWN SURGEONFISH · ma`i`i`i
Acanthurus nigrofuscus (Forsskål, 1775)
(LAVENDER TANG)
This fish feeds in the shallows close to shore and is so common and unremarkable in color that few pay it any attention. Nonetheless, it is a fascinating animal and possibly the only Hawaiian reef fish to have made headlines around the world (see Giant Bacteria, below). It varies from dark brown to light grayish brown with a definite lavender tinge, especially on the fins. There are dull orange spots on the head and two dark spots at the base of the tail, one above the other. The tail fin is lunate (crescent-shaped). When displaying aggression its upper back and the entire dorsal fin lighten, sometimes becoming distinctly yellow. The species name nigrofuscus means "dark brown." The Hawaiian name refers to a variety of kalo (taro). To 8 in. Indo-Pacific. Photo: Kahe Point, O`ahu. 15 ft.

Brown Surgeonfish displaying aggression

Feeding behavior and ecology
The feeding behavior and ecology of the Brown Surgeonfish vary greatly according to local conditions. Along the gently shelving south shore of Kealakekua Bay, Hawai`i, for example, Brown Surgeons are territorial. Here individual fish are commonly seen defending their feeding areas against other herbivores, primarily Convict Tangs. Along the steep dropoff of the Bay’s north shore, however, Brown Surgeons display little territoriality. Here they migrate considerable distances to feed, and often graze peacefully in schools—sometimes with Convict Tangs! (On O`ahu, similar behavioral differences between the two species can be seen respectively at gently-sloping Hanauma Bay and the precipitous "Hale`iwa Trench" dropoff at Ali`i Beach Park.)
Dr George Barlow, who studied the intereactions of Brown Surgeonfish and Convict Tangs at Kealakekua in the 1970s, found that in areas where Browns were least numerous, Convicts were able to feed singly, even though attacked about once a minute. Where Browns were moderately common, however, Convicts could feed only by swamping the defenders in dense schools. Finally, where Browns were abundant, Convict Tangs could not feed at all and were absent.

Territorial Brown Surgeonfish attempting to drive off a feeding school of Convict Tangs. Hanauma Bay, O`ahu.

Non-territorial Brown Surgeonfish schooling to feed. Makaha, O`ahu.

Reproduction
Like many other surgeonfishes, Brown Surgeons engage in spectacular mass spawning events that are a lot of fun to watch if you happen to be in the right place at the right time. These seem to occur almost daily, from approximately February to early September. More study is needed to determine if the times are tide-related in Hawai`i. Dr. Linn Montgomery, who studied Brown Surgeons in the Red Sea, reports that these fish migrate in single file from various parts of the reef to the "traditional" spawning site, arriving from distances of up to half a mile. Here hundreds, sometimes thousands of fish mill in a dome-shaped aggregation that pulses and swells with activity. Every few minutes a dozen or so excited fish break away, dash upward, spawn, then return to the safety of the school below. Clouds of white gametes released at the apex of their rush resemble fireworks or bursts of flack. In Hawai`i, look for this spectacle in the late afternoon, although it sometimes occurs as early as 10 am. The easiest place to see it on O`ahu is right over the hot water outfall at Kahe Point Beach Park. Another site is along the top of the dropoff at Ali`i Beach Park (the “Hale`iwa Trench”). A constant stream of fish moving in single file along the reef flat is a good sign it’s about to happen—or else has just happened.

Giant bacteria
In March of 1993 newspapers around the world proclaimed that the biggest bacteria known to science—each about the size of a hyphen in a newspaper article—have been discovered in the gut of Brown Surgeonfish from the Red Sea. Named Epulopiscium fishelsoni, they apparently aid digestion. So large are these bacteria that they were assumed at first to be protozoans. Amazingly, micro-probes can be inserted into them, offering researchers a rare chance to study the inner life processes of a bacterial micro-organism. Dr. Linn Montgomery has found similar bacteria in Brown Surgeons from all over the Indo-Pacific, including Hawai`i. The bacterial forms, however, are not quite the same. Does that mean that Brown Surgeons in different geographical areas are dependent on different species of bacteria and might themselves possibly be different species? Which came first, anyway, the fish or the bacterium? As always, new knowledge only results in more questions....