Some reef fish get by on colour. Some get by on attitude. Surgeonfish have both, plus a hidden weapon.
Surgeonfish, tangs and unicornfish all belong to the family Acanthuridae. On our reef, that family includes the pencil surgeonfish, bluespine unicornfish, spotted sawtail and convict surgeonfish. As an aside, I have also recorded a brown tang and a Pacific sailfin tang here, although those are not fish I see every day.
The family name refers to the sharp spine or spines near the base of the tail, on the narrow part known as the caudal peduncle. At first glance, these can look like tiny white, blue or dark marks. Look more closely, though, and you can see why surgeonfish have that name.
In the pencil surgeonfish, Acanthurus dussumieri, there is a single scalpel on each side (see the white scalpel at the base of its tail in the photo below). In the bluespine unicornfish, Naso unicornis, there are two on each side, showing as those lovely blue spots near the tail (see the image below). They are not there for decoration. If the fish needs to defend itself, a sharp sideways flick of the tail can make those small blades very effective.
Pencil surgeonfish, Acanthurus dussumieri, Norfolk Island
I have been trying to get decent photos of these scalpels for some time. Pencil surgeonfish are quite dark fish and can be hard to photograph well, especially in early morning light. Pencil surgeonfish can also look surprisingly different from one swim to the next. Sometimes they appear quite pale and silvery-brown; at other times they look much darker, with the fine pale lines standing out more clearly. I have not found a species-specific explanation for this in the literature, but many fishes can change colour rapidly through pigment cells in the skin, often in response to stress, light, background, social behaviour or general arousal. So when I see a pencil surgeonfish shift from pale to dark, I am probably seeing a mix of real colour change, body angle and the usual underwater tricks of light. The numbers of these fish wax and wane, and right now we have the most I think I have ever seen: a school of anything between eight and fourteen, and a few others dotted around the place, including quite a few juveniles, which tend to be solo.
The bluespine unicornfish’s scalpels
The bluespine unicornfish are a different sort of character altogether. They are show ponies. They pose, turn, peek over coral, and sometimes charge forward in what feels very much like a game of ‘call my bluff’. The adults have that slightly haughty, assessing look, as though they have already formed an opinion of you. The bluespine unicornfish can also shift colour, although in my experience not as dramatically as the pencil surgeonfish. Sometimes the change is subtle – a deepening of the body colour, a shift in tone, or a more obvious contrast around the head and flanks. In unicornfish, rapid colour changes have been linked with social and reproductive displays, although light, background, stress and general fish mood can also affect how they appear underwater.
They are called unicornfish because of the horn-like bump between the eyes, known as a rostral protuberance. The young ones do not have it yet, which makes the little bubbas especially endearing. I have watched their numbers inside the lagoon change over the past few years as well. At one stage there were several adults. Later, that seemed to drop to one. Then came the baby boom in 2024 (a drought year), when I counted 24 tiny bluespine unicornfish scattered along the reef in a single swim. Some of those little fish stayed for a while, but currently I only know of one, which is on the cusp of adulthood.
The age of these fish also changes how I look at them. Bluespine unicornfish are not short-lived little reef extras. Studies in Hawaii have aged them at more than 50 years old, using their ear bones, known as otoliths. So when a large adult unicornfish turns sideways and gives you that assessing look, it may genuinely be an old fish, not just a big one.
The fact that these fish are algae grazers is an important part of this story.
Many surgeonfish and unicornfish browse on algae, and on our reef that is a very useful job. Algae belongs on a reef. It is part of the system. But when it grows too thickly, or starts spreading over the spaces where corals might otherwise grow, it can shift the balance. It can cover hard surfaces, crowd living corals, trap sediment, and reduce the clean settlement space needed by coral larvae and other reef life.
The photographs above demonstrate just some of the colour changes in bluespine unicornfish and pencil surgeonfish
Anyone who swims regularly in Emily and Slaughter Bays knows we have plenty of algae. Sometimes we have it by the bucketload. That is one of the reasons I always look for these fish as I swim. The same goes for the other members of this family when they appear. Convict surgeonfish, spotted sawtails, tangs and unicornfish are not all doing exactly the same thing in exactly the same way. Different species graze differently, feed in different places, and target different kinds of algal growth. Taken together, though, they are part of the reef’s grazing clean-up crew.
A very haughty bluespine unicornfish, Norfolk Island
They are not a magic fix. Herbivorous fish cannot undo poor water quality, nutrient inputs, sediment, freshwater stress or repeated disturbance. No fish can graze its way out of all that. But take away too many grazers from a small reef already carrying a heavy algal load, and the algae gets one less check on its spread.
That is why I get excited when I see young bluespine unicornfish in the lagoon. And, in fact, I saw my first tiny baby bluespine just a few days ago. The little ones do not have their horns yet, but they already have the look. Some are now old enough for their tiny scalpels to be visible as small blue marks near the tail. They are beautiful, ridiculous, haughty little things, and they are already part of the grazing crew.
So yes, today’s photos show the scalpels, which make them fascinating in their own right. But the fish carrying them are doing something more useful than looking pretty for the camera. On our algae-rich little reef, every grazer counts.
Convict surgeonfish, Acanthurus triostegus, Norfolk Island
A new baby bluespine unicornfish. We had a baby boom in 2024, which was a drought year on Norfolk Island
A juvenile bluespine unicornfish. Its horn has not yet formed. Norfolk Island