Unicorns of the Fish World
The surgeonfishes of the Naso genus are interesting fish with a “unicornlike” head growth.
Text and photos Scott W. Michael
We have all heard of the unicorn. You know — that mythical beast that looks like a horse with a lionlike mane, a goatlike beard, cloven hooves and a long horn protruding from its head. But have you heard of a unicornfish? While not mythical, they are somewhat mystical, in that they differ significantly from the rest of the family to which they belong. Unicornfishes are members of the family Acanthuridae (known collectively as surgeonfishes), but they are disparate from most other members of this group in both appearance and lifestyle, much the same way as the legendary unicorn differs from its supposed equine relatives.
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The palefin unicornfish (Naso brevirostris) is one of many members of this group with fantastic head ornamentation; this trait has resulted in the common name often applied to the genus.
The males of a few species sport long streamers on the tail. This is the elegant unicornfish (Naso elegans), which is often mistaken for the orangespine unicornfish (N. lituratus).
While they can make fascinating aquarium inhabitants, unicornfishes have special care requirements that will need to be addressed if they are to be kept long-term.
The unicornfishes comprise the genus Naso, which is distinct enough from other surgeonfishes to be placed in its own subfamily: Nasinae. Like other acanthurids, Naso species have spines (actually modified scales) at the base of the tail, but these structures are different in the unicornfishes. In the Naso species, the caudal peduncle usually has one or two bony plates (most species have two), and each plate has a bladelike keel. In some species, these peduncular keels have sharp, forward-projecting points that can do damage to competitors, predators and even the aquarist. In other species, the keel lacks the point but instead is simply bladelike. In most other surgeonfish groups (except the sawtails of the genus Prionurus), the sharp peduncular plate is folded into a groove where it rests until needed.
Eight of the 18 known Naso species have some sort of bony protuberance on the forehead. In four species, this consists of a horn (hence the common name of unicornfishes), while in the other four it is rounded and bumplike. This forehead ornamentation is not present in young fish but develops as the fish grows. It usually begins to develop when the fish is from 4 to 8 inches long, depending on the species. In some species, this structure is found in both sexes (e.g., whitemargin unicornfish, N. annulatus; bignose unicornfish, N. vlamingii). In others, only the male has the cranial embellishment (e.g., humpback unicornfish, N. brachycentron), or it is much larger in males than in females (e.g., bulbnose unicornfish, N. tonganus; bluespine unicornfish, N. unicornis).
Other sexual differences have been noted in various Naso species. The adults of five species have filaments that extend from the top and bottom lobes of the tail. In some of these species, only the males possess these filaments (whitemargin unicornfish; orangespine unicornfish, N. lituratus), while in other Naso species, the females also have filaments, but they are not as long as those found in the males (e.g., bluespine unicornfish). In most of the unicornfishes, the peduncular keels are better developed in males, and the males are larger in overall size than the females.
The members of this subfamily have another unique physical characteristic. They have scales on the body that are more similar to the placoid squamation (platelike scales) found on sharks than the flat, cycloid scales in other reef fish. In both sharks and unicornfishes, these scales reduce turbulence along the body so that they can move through the water with greater ease and speed. Because unicornfishes are regularly found far from the shelter of the reef, they need to be able to move rapidly to elude piscivorous species, such as sharks and jacks. Many of these open-water unicornfishes also have bodies that are built for speed. They are more elongate and laterally compressed, and they have truncated or emarginated tail fins (tail fin shapes that are indicative of a sprinter).
Many of the unicornfishes are drably attired. Many species are brown, gray or bluish-gray overall. This helps those species that live up in the water column to disappear against the blue open water where they live. Some of these more muted species are adorned with spots, mottling or fin highlights.
Unicornfishes can engage in abrupt color changes. Some species change color to facilitate the activities of cleaner fish. For example, the sleek unicornfish (N. hexacanthus) will change from a dark to light hue when they are being inspected by cleaner wrasses. In this way, the parasites may show up better to the parasite-picking cleaners. Some male unicornfishes also engage in sudden color changes during courtship, and many species exhibit a mottled coloration when refuging at night.
Some of the most unique features of this genus are related to their biology. Like other surgeonfishes, these are active fish, but most roam over larger home ranges than the more site-attached members of the family. Unicornfishes also differ from most acanthurids when it comes to their culinary proclivities. Like other surgeonfishes, there are some unicorns that feed on algae. However, rather than eat microalgae like their relatives, the favorite food of Naso species is fleshy macroalgae. They seem to have a particular fondness for brown macroalgae species, such as Sargassum, Pocockiella and Dictyota. They will also eat fleshy green (e.g., members of the genera Dictyosphaeria, Microdictyon and Ulva) and red algae species (e.g., members of the genera Acanthophora, Champia, Laurencia and Hypnea). The teeth of these unicornfishes differ from those of other acanthurids and can also vary somewhat between species and age groups within a species. For example, the teeth of all size classes of the palefin unicornfish (N. brevirostris) are finely serrate, while the young bluespine unicornfish has serrate teeth (these serrations disappear as the fish grows). The dentition of the algae-eating Naso species is adapted for stripping off the leaflike fronds from the main thallus of leathery algae species (Sargassum) or biting chunks from softer algae types (Dictyota).
How do those unicornfish species that have a long horn on their heads eat algae from the substrate? It turns out that as this potential obstruction grows, many of these fishes begin to exhibit a change in diet (they feed more on zooplankton than algae). Other horn-bearing species, such as the bluespine unicornfish, continue to feed on algae as the “nose” grows, but they browse on larger macroalgae, such as Sargassum, that are easier to get their teeth around than shorter microalgae that grow close to the substrate. A few species that lack the potential forehead-feeding impediment (e.g., orangespine unicornfish) feed heavily on macroalgae throughout their lives.
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The sleek unicornfish (Naso hexanchus) fades in color when it is cleaned by juvenile hogfishes or cleaner wrasses. This change in color may help make the parasites more conspicuous to the cleaners.
Here is a close look at the caudal peduncle of the orangespine unicornfish. This is where the fish’s spines are located, though they can be difficult to see.
At night, many unicornfishes (Naso spp.) exhibit a blotchy or mottled color pattern. Unhealthy or stressed unicornfishes may exhibit similar color patterns during the day in captivity.
Some of the unicornfishes, such as this humpback unicornfish (N. brachycentron), get very large and require much more space than is available in the vast majority of home aquariums.
Yet not all of the unicornfishes eat algae. There are 12 species in the genus that are known to feed almost entirely on zooplankton as adults. These species tend to prefer larger, more gelatinous, semitransparent planktor types, including chaetognaths (arrow worms), larvaceans (free-swimming tunicates) and fish eggs. Some species also feed on copepods and crab megalops, while free-floating bits of filamentous algae may be ingested when available.
Another unusual item utilized by some zooplankton-feeding unicorn-fishes is poop. Yes, that’s right — fish poop. These unicornfishes will hang under schools of predatory fish (e.g., jacks, fusiliers and barracuda) and feed on feces that rain down from the piscine groups above. They are able to extract some of the nutrients that remain in the feces of predatory fish.
The zooplankton-feeding Naso species are not limited to water depths where algae is abundant (remember that algae is most common in relatively shallow, sunlit waters) and are thus found at much greater depths than their plant-eating kin. For example, the palefin unicornfish has been observed at depths of 397 feet, while the spotted unicornfish has been spotted as deep as 715 feet from submersibles. The zooplankton-feeding species also tend to gravitate toward current-prone drop-offs and exposed reef-front habitats where their prey abound. They will feed far from the reef, often high in the water column.
As mentioned, the unicornfishes are built for speed. This helps them elude predators when they roam away from the reef. But like many other fish species that feed away from shelter, these unicornfishes often form groups (there is safety in numbers). Some of these Naso species will even school with each other.
Unicornfishes are less likely to form large groups when young, as they tend to feed on algae near the safety of the substrate. Instead, juvenile Naso species occur singly, but they are sometimes found in small, loose groups, or they even mix with similar-sized heterospecifics (e.g., parrotfishes, other surgeonfishes). Even the adults of the zooplankton-feeding unicorns utilize the reef as a place to slumber (many refuge in reef crevices at night) and search out the services of cleaner fish to remove annoying parasites.
The fascinating unicornfishes are not for everyone. First of all, most species are not that chromatically blessed. In fact, the majority of Naso species are downright drab. There are three species that are sought-after because of their more attractive coloration. The elegant unicornfish (N. elegans), which is frequently imported from the Indian Ocean (sometimes from around Bali), is also sold in the aquarium trade as the “blond naso tang.” It gets its name from its yellow dorsal fin. This same fin is black in the closely related and equally sought-after orangespine unicornfish. The orangespine is a common import from the Pacific Ocean (many are from Hawaii), and it is also sold in the aquarium trade simply as the “naso tang.” Both of these fish are popular because of their orange and yellow highlights. The bignose unicornfish, also known as the vlamingii tang in the trade, is another more colorful species that is usually available as a juvenile.
The second unicorn concern is their size. These are not diminutive creatures. They range from 9 to 39 inches in length, with most species exceeding a length of 20 inches. In fact, all three species just mentioned exceed 18 inches. Not only are they large, but they are also very active. Many of these species swim over large portions of the reef or even in open-ocean habitat. This means that in order for your Naso to remain healthy, it will require plenty of swimming room. A cute little bignose unicorn may be “happy” in a 75-gallon tank, but it will rapidly outgrow such quarters and will eventually need a tank of a couple hundred gallons.
Of course, it will not do your unicornfish any good if the space is filled with decor, so make sure they have unencumbered swimming room. Also, be aware that these fishes are known leapers, and therefore their aquarium homes will need to be covered.
At this point, if you are still willing and able to house a unicornfish, you will need to know more about keeping them properly nourished. If you are going to keep an algae-eating unicorn (e.g., N. elegans, N. lituratus), you will need to feed it plenty of plant material. One of the best foods for these unicornfishes are the sheets of dried brown algae available at most aquarium stores. Use a lettuce clip to attach them to the side of the tank so that the Naso can rip off mouthfuls of the plant matter. Add at least one of these algal sheets to the aquarium every day (more if the fish is larger or if it shares its tank with other herbivores).
In most cases, unicornfishes will also ingest Spirulina-laced frozen preparations — some may even eat flake food created for herbivores. Feed them these foods at least once a day. In the case of the zooplankton-feeding unicorns (e.g., N. vlamingii), frozen mysids and frozen preparations for herbivores are usually heartily accepted. Feed these unicornfishes at least once and preferably several times a day. Juveniles will require more frequent feeding (two or three times a day), as they have greater metabolic needs than the adults.
Fortunately for the potential unicorn owner, the members of this genus are not overly aggressive, especially when compared to the “bad boys” in the family, such as the powder blue (Acanthurus leucosternon), the lined (A. lineatus) and sohal surgeonfish (A. sohal). But they are likely to quarrel with members of their own kind and may squabble with congeners. If you want to keep more than one Naso species in a larger aquarium, the aquarium should be larger, say 180 gallons or more. Also, add smaller specimens to the tank simultaneously. You can keep them with unrelated surgeonfishes. They are usually able to defend themselves with their large peduncular spines. However, always add them to the tank before more aggressive acanthurids.
One of the great things about the unicornfishes is that they rarely bother unrelated species. Thus, you can house them with butterflyfishes, angelfishes, wrasses, rabbitfishes, triggers and puffers without them causing problems with their neighbors. When it comes to invertebrates, the Naso species are usually benign. Like any herbivore, an occasional specimen may go off the rails and start nibbling on large-polyped stony corals or some of the soft coral polyps (e.g., Xenia) that do not store toxins that dissuade fish predators. This is especially true if they are not being fed enough.
As mentioned earlier, unicornfishes have stout, sharp caudal peduncle spines, and unlike other surgeonfishes, they are fixed in an erect position. As a result, these fish are easily entangled in fishnets. These spines can also be used to slash a finger or hand during the process of removing one of these fish from an aquarium. These spines can produce deep, painful gashes, so handle these fish with great care.
If stressed, they will adopt a mottled coloration (the same one they exhibit at night). If the fish retains this color, there is a problem. It may be harassed by tankmates, or water conditions may be suboptimal, or it may have a severe parasite problem. Unicornfishes will usually adapt to aquarium life more rapidly if housed in a tank in a low-traffic area. But once they acclimate, they will cruise all about the aquarium and even beg passing people for food.
The ailments most often encountered by unicornkeepers are the typical ectoparasites, such as Cryptocaryon and Amyloodinium. Being “dry-skinned” fish (i.e., they produce relatively little body slime), they seem even more susceptible to parasites than other bony reef fish. They can be treated with the common antiparasitics or by lowering the salinity of the aquarium for a while. If going with the salinity treatment, drop the specific gravity in your quarantine or hospital aquarium to 1.012 for at least two weeks. You can decrease the specific gravity relatively quickly (over a period of four to six hours), but you will need to raise it back up more slowly (over two or three days).
The unicornfishes are also frequently vexed by head and lateral line erosion. This may be caused, in some cases, by the use of carbon as a filter medium or the lack of vitamins (namely vitamin C) in their diet.
While most of the Naso species do not sport stunning colors, these are interesting creatures that are worthy of consideration. If you find you are tempted by the unicorns, and have the tank space to keep them happy and healthy, be aware that acquiring one is a long-term commitment. The unicornfishes have been reported to live at least 13 years in the aquarium, and it has been estimated that many of the larger Naso species may live for 30 to 45 years of age in the wild. Happy fish watching!