fishes of the month
FISH OF THE MONTH - MAY 2003
common, drab, seldom-noticed reef fish that might be more interesting
than you thought
BROWN SURGEONFISH ·
Acanthurus nigrofuscus (Forsskål, 1775)
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
behavior and ecology
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 Bays north shore,
however, Brown Surgeons display little territoriality. Here they migrate
considerable distances to feed, and often graze peacefully in schoolssometimes
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.
Surgeonfish attempting to drive off a feeding school of Convict Tangs.
Hanauma Bay, O`ahu.
Brown Surgeonfish schooling to feed. Makaha, O`ahu.
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 its about to happenor else
has just happened.
March of 1993 newspapers around the world proclaimed that the biggest
bacteria known to scienceeach about the size of a hyphen in
a newspaper articlehave 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....