Charisma plus! The bluespine unicornfish

Out on A Swim

‘Out on a swim’ is a coral reef blog that tells the stories of the characters who live under the waves and what has caught my eye when ‘out on a swim’ in the lagoons of Norfolk Island. It is also a record of the difficulties Norfolk Island’s reef faces, like many others around the world, as a result of the poor water quality that has been allowed to flow onto it.

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One of the many characterful fish in the Emily and Slaughter Bay environs are the bluespine unicornfish, more properly known as Naso unicornis. They belong to the family Acanthuridae, which includes surgeonfishes, tangs and unicornfishes. These fish all have a distinctive characteristic, but more on that later.

They are distributed widely throughout the Indo-Pacific region, probably because their pelagic eggs float freely on the currents in the water column.

These guys love to pose for the camera, showing off their best side, and then shifting so you can get a shot of the other. They also like to peek at you over the coral. I love them to bits and get a real thrill from quietly watching their antics.

Here’s a few vital stats about these charismatic guys.

The caudal peduncles are razor sharp daggers

They are called unicornfish because of the horn-like appendage between their eyes known as a rostral protuberance. No one is really sure what these ‘horns’ are for, but it certainly isn’t for fighting.

They are not generally aggressive, but if they do need to assert themselves they have two pairs of razor-sharp scalpels, or peduncles, near their tails – this is the characteristic they share with surgenofishes and tangs that I mentioned above, and the reason why surgeonfish have that name. They look just like two dots when you first see them, but look closely and you will notice that these modified scales are like tiny daggers. They have not been shown to be venomous in unicornfish.

When they are angry they will darken in colour. When they are mating, the males will flash intense colours. And when they are feeding their colour will subtly change to match their environment.

Importantly – and particularly as juveniles – they are herbivores. As in, they eat plants. And that is a Good Thing. As they get older they will eat plankton found in the water column as well. Leave them be and they will quietly munch the algae that can cover, and even smother, corals, allowing coral larvae to settle and create new coral colonies. So don’t fish for them!

Their skin is particularly tough. This is because they have modified scales that allow for greater speed by reducing the water turbulence as they swim. The Hawaiians used the skins of these fish to make their drums.

The males are generally slightly larger, have larger caudal peduncles, and longer caudal filaments on their tails. The bluespine unicornfish can grow up to about 70 cm in size over the course of its lifespan.

And, amazingly they can live for as long as 55 years. That look they are giving you is one of a wise old fish just sizing you up!

Addendum:

We have at least three beautiful adult unicornfish resident in our lagoons that I am aware of. There could be more. And just recently I discovered a new addition to the fam. Meet the latest member of the unicorn club! He (or maybe she) has that same haughtly but curious way of looking at you. Just beautiful!

A baby (juvenile) bluespine unicornfish, photographed on 11 May 2021.

Reference: Unicorns: More Than a Myth in Reef Aquariums; The Genus Naso

October 28, 2026

When storms rearrange the reef

Sometimes the sea rearranges a reef in a matter of hours! In the third article of my six-part photographic series, I revisit corals broken, overturned or swept away by powerful swells and storm surges. Some disappeared altogether, while others found surprising ways to survive. Five photographic sequences reveal just how dramatically Norfolk Island's reef can change.

October 26, 2026

When the reef fights back – coral growth and recovery

Not every coral story ends badly! Among six years of underwater photographs are some wonderful examples of growth, survival and recovery. From corals that have grown spectacularly to others that have survived bleaching, breakage or even being turned upside down, this second article in my photographic series celebrates some of the reef's more encouraging stories.

October 24, 2026

The neighbours – six years of watching the same reef

When I began photographing Norfolk Island's reef in 2020, I simply recorded whatever caught my eye. Six years later, those photographs have become a record of individual corals and their changing fortunes – growth, competition, survival and loss. This is the first of six articles looking back at what I've seen, and sometimes what I wish I hadn't.

October 10, 2026

Pulling the rugosity from out of the reef

What do ‘rugosity’, structural complexity and channeling your inner fish have in common? Quite a lot, as it turns out. This week I’m looking at how all the lumps, bumps, branches, holes and ledges on a reef create habitat – with a brief detour into one of my long-standing tricks for navigating tertiary study: write down the jargon until it starts to make sense.

September 27, 2026

How does a sea slug smell?

It started with a close-up of a little Elysia and two rhinophores that looked like tiny rolled-up scrolls. Once I noticed them, I started going back through years of sea-slug photos and realised just how wildly different these sensory organs can be – ridged, rolled, curled, tubular and, in sea hares, distinctly rabbit-eared. So this week’s rabbit hole is all about how sea slugs ‘smell’ underwater, and why their noses come in so many different shapes.

September 19, 2026

The Elite Fleet sails in

Nearly six years ago, my very first blog post was about the strange blue and violet creatures that live at the ocean’s surface – a group I christened Norfolk Island’s ‘Elite Fleet’. Since then I’ve seen plenty of bluebottles and the occasional violet sea snail, but never a by-the-wind sailor. Until this morning. Velella velella is not a true jellyfish, but a colonial hydrozoan with a transparent sail that lets the wind help carry it across the ocean. So what exactly is this extraordinary little creature, how does it ‘sail’, and what connects it to the rest of the fleet?

September 18, 2026

In a nutshell – where I stand on the proposed Norfolk Island Marine Park

I support the proposed expansion of Norfolk Marine Park and its renaming as Norfolk Island Marine Park. There is much to welcome – strong offshore protection, no mining, and recognition of the importance of sustainable local fishing. But I also have some reservations. Why does Cascade need to be removed from the Marine Park at all? How will more than 430,000 km² actually be monitored and enforced? How do we make sure Norfolk people have a genuine role in management? And, above all, how do we ensure that the enormous new park does not draw attention away from the small, stressed reef immediately beside us?

September 13, 2026

No seagrass, but plenty of turtles

Norfolk Island does not have seagrass, although one of our most conspicuous green algae is often mistaken for it. Feather and grape Caulerpa can both form striking patches across the lagoon. I take a closer look at what Caulerpa actually is, why it is so easily confused with seagrass, and where our green turtles fit into the story.

September 6, 2026

Watching El Niño from Norfolk Island

El Niño is strengthening across the Pacific, but that does not tell us exactly what the coming months will bring to Norfolk Island. After a long wet spell, I am watching our rainfall and water supplies more closely again – and, underwater, keeping an eye on sea temperature, algae, coral recruits and the fish that turn up on the reef.

September 2, 2026

What the world’s reefs are telling Norfolk Island

Two major new coral reef reports carry a clear message for Norfolk Island: reefs can recover when pressures ease, but the time available for recovery is shrinking. Norfolk Islanders were raising concerns about water quality, runoff and the health of Emily and Slaughter Bays nearly forty years ago. Today we have much better science, a changing climate and a far clearer understanding of how closely the catchment and reef are connected. This article looks at what that means for water quality, long-term monitoring, dredging and how we manage Kingston from land to sea.