Unicornfish Profile
Surgeonfish, members of the Acanthuridae family of reef fishes, are some of the prettiest fish you’ll find in shallow, reef habitats.
They’re pretty much all characterised by being laterally compressed (not having much space between their eye sand fins), oval or round-shaped and being extremely colourful.
One genus takes this a step further and contains multiple members with a bizarre and mysterious protrusion sticking out of their heads. These are the unicornfishes, and despite being very popular in reef aquariums, they’re still poorly understood.
Unicornfish Facts Overview
| Habitat: | Warm marine, reef environments |
| Location: | Indo-Pacific waters |
| Lifespan: | Around 15 years |
| Size: | Up to 1m (39 inches) long. |
| Weight: | Around 6kg (14 lb), up to 13 kg (28 lb) |
| Colour: | Varied, often silvery-grey, sometimes with yellow or blue markings |
| Diet: | Algae and zooplankton |
| Predators: | Jackfish, moray eels, probably sharks |
| Top Speed: | Unknown |
| No. of Species: | 20 |
| Conservation Status: | All listed are of Least Concern (IUCN) |
Unicorn fishes are named for their horns, but only a minority of the group sport one. Others have bulbous heads, and some look like normal tangs or surgeonfish. Nobody’s quite sure what they do with their horn, as self-defence appears to come from spines on the other end.
Some of these tail spines have venom, but it’s rarely used. This is a very cool and collected member of the reef, easily tamed and very comfortable around others, though it does have some filthy habits that come out when nobody is looking.
Interesting Unicornfish Facts
1. Nobody’s sure why they have horns
The obvious use of a spiky facial appendage would be for head-jousting or clashing it against the facial appendages of your enemy. But unicornfish don’t appear to use theirs in this way at all.
This horn appears to begin growing in both males and females from a young age and gets longer as they grow. It’s present in both males and females but can be slightly longer in males, suggesting some level of sexual selection, but the true nature of it remains a bit of a mystery.
While it’s not clear what the purpose of this bony protrusion is, it coincides with a developmental period in the fish’s growth that sees them switch from eating algae (seaweed) to zooplankton.
Many species don’t grow one at all. 1
2. Most aren’t horned
This entire genus of fish is related to the surgeonfish and tangs, and have the same laterally compressed and oval-shaped bodies as their family members. As mentioned, they’re named for the characteristic elongated ‘horn’.
But this isn’t a characteristic of the genus, only a handful of the 20 species of unicornfish has this long nose, and all the others got lumped with the name. 2
3. Some have tail knives
Self-defence would be an obvious hypothesis to test, but researchers haven’t seen any evidence that the horn is used for this, and besides, the unicornfishes that have been seen defending themselves do it from the back end.
Many, if not all, have bony plates on the tail fin, and at least some of these are armed with a venom that can inflict a sting. These spines can inflict deep wounds and are attached to venom glands.
Stings can be fatal to smaller fishes, but for large animals, it’s merely an unpleasant experience.
4. This is still not very well understood
The current level of knowledge around this topic is limited and the nature of their venomous spines is still up for debate. Some suggest that not all species are venomous, but others point to the fact that the venom glands are tiny, and may simply have been missed in dissections.
There’s also the variable of whether or not the spines are hollow, which they might need to be to inflict a sting, but others point to the fact that they are often grooved, like the primitive fangs of Colubrid snakes.
5. Yet, they’re pretty friendly
Fortunately, venomous or not, these pretty fish seem to be rather chilled. Their weird looks and vibrant colouration make them sought after in aquariums, and their active and inquisitive nature and low aggression make them nice to keep around.
They’re said to be quick to tame and will come up to their keepers to be hand-fed, and they’re quite hardy, so their large size remains the biggest challenge to successfully keeping them. 3
6. But they can handle themselves, too
While calm and good-natured, these fish are no pushovers. When fully grown, they can be kept among far more aggressive fish and will hold their own without causing trouble.
They prefer coral substrates with plenty of room to hide and graze and will be content with the algae that grow on the substrate, not preying on any crustaceans or other fish in there.
7. They can change colour
During mating season, male unicornfishes will mate with as many females as they can. When courting, he’ll search the female out, get her attention, and then try to persuade her by changing colour and wiggling about in a shimmery fashion.
Spawning occurs when the pair agree, and then engage in a choreographed trip to the surface where both eggs and sperm will be released.
8. They also eat poop
Even a fish as graceful and cool as the unicorn fish has some bad habits. For Naso vlamingii, the Bignose unicornfish, being an omnivorous feeder, it’s the habit of eating what comes out of larger, predatory fish. Apparently, the spoils of a hunt from the rear of a barracuda are pretty tasty leftovers.
Interestingly enough, the so-called “bignose” of the genus has no horn at all, and just carries around an upturned bulbous nose that likely allows it to get right up in there to feed. 4
Unicornfish Fact-File Summary
Scientific Classification
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Perciformes |
| Family: | Acanthuridae |
| Genus: | Naso |
Fact Sources & References
- (2018), “Spotted Unicorn”, Smithsonian National Museum of Natural History.
- “Naso lituratus”, USGS.
- “Naso Tang”, Aquarium Domain.
- “Orangespine Unicornfish (Naso Tang)”, Animal Spot.