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Secrets of the Achilles Tang

The Achilles tang or surgeonfish (Acanthurus achilles) can be difficult for many fishkeepers.

By Terry Siegel

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Q. I am looking for information on the Achilles tang. I need to know its behavior, feeding requirements, reaction with reef/inverts, past experiences with this species and longevity in captivity. I have not been able to find much info on this tang.

A. The Achilles surgeonfish or tang (Acanthurus achilles) is a member of the family Acanthuridae, order Perciformes (perch-like fish). In the wild, surgeonfish ingest primarily plant food. In other words, with two exceptions — A. thompsoni and A. mata — they are herbivores and have often been referred to as the “cows of the sea.” Surgeonfish are widely distributed, and, because they are easily netted and beautifully colored, they are readily available to marine aquarists.

The common name, surgeonfish, which is characteristic of the family Acanthuridae, refers to the scalpel-like spine at the base of the tail fin (caudal peduncle). When the fish is threatened or is threatening others, all its spines, including its tail-based scalpel, are extended. By whipping its caudal peduncle it can inflict serious wounds.

Interestingly, the areas where the spines are most prominent are often highly colored. It is suspected by many biologists that intense coloration evolved as a warning to would-be enemies. In my experience, Acanthurus achilles is one of the most aggressive of the surgeonfish, and not surprisingly, the area surrounding its scalpel is colored bright orange or red.

In the wild, young surgeonfish are highly territorial, but when they mature they form large schools that graze the reefs clean of algae. Territorial fish, such as damselfish, which defend a specific area, are unable to defend their turf against a school of surgeonfish, but are able to defend against a solitary fish.

The need and ability to defend a territory has a distinct survival advantage — food and shelter are finite. In the case of surgeonfish, aggression against their own kind (interspecific aggression) has to be blunted by numbers so this genus can successfully roam over large areas of the reef while feeding. Understanding territorial aggression is essential if the aquarist is to keep one in the small confines of a reef aquarium with any other fish that even looks like a potential rival.

During the course of my 40-plus years as a marine aquarist I purchased three Achilles surgeonfish. One, which I kept in a 220-gallon reef aquarium, lived only about a month. It was starved to death by an established powder blue surgeonfish (Acanthurus leucosternon). I tried every trick I knew of to blunt the territorial interspecific aggression — placing a mirror on the side of the aquarium, rearranging what hiding places I could in a reef aquarium, putting the new saltwater fish in at night and so on — but nothing worked.

The powderblue’s attacks were most vigorous at feeding time. It kept the Achilles pinned to a small upper corner of the aquarium, where it eventually gave up trying to feed.

Many years ago I was able to keep a large Achilles in a fish-only aquarium for about five years, but it took regular additions of a copper/formalin anti-parasite solution to keep the fish in that aquarium healthy. I should also note that the aquarium only became relatively parasite free when I no longer added any fish.

At the time, many aquarists were surprised by my success with this Achilles, but I ascribed the success to a relatively large aquarium, a stable, parasite-free environment and heavy feedings of flake food and romaine lettuce. It was also the only surgeonfish in the aquarium.

The Achilles I now have in my 10-foot aquarium has been my guest for over five years. For four years it was in my old 220-gallon reef aquarium. This is the same aquarium that contained only fish, but was converted to a reef aquarium over a dozen years ago. When the Achilles was added, there was already an established yellow tang (Zebrasoma flavescens) and a large Red Sea sailfin tang (Zebrasoma desjardinei) in the aquarium.

Although the Achilles quickly became the most aggressive fish in the aquarium, it essentially ignored and was ignored by the other established surgeonfish. There are a number of factors of note here. First, the most aggressive of the surgeonfish (the Achilles) was added last and it was larger than the yellow (different body shape) tang and smaller than the Red Sea sailfin (different body shape and mellow for a surgeonfish). As noted marine fish expert Scott W. Michael has pointed out, surgeonfish are most aggressive (intraspecific aggression) toward members of their own species than toward other surgeonfish with a similar body shape.

Those of you who have been following this column know I retired to Cape Cod, in Massachusetts, and moved the inhabitants from my three reef aquariums into a newly set-up 10-foot reef aquarium, which currently houses seven surgeonfish that live together in relative harmony. How I got these fish to live together in peace is the result of a combination of luck and planning. Ideally, I would have moved all of the fish together, but only after I had already moved and reestablished the corals from the three old reef aquariums. However, this was not practical.

Instead, I moved the corals and fish from the smallest reef aquarium (135-gallon) first. This aquarium housed only one surgeonfish, a very aggressive sohal tang (Acanthurus sohal). This saltwater fish, along with the other occupants from the 135, had the new aquarium to themselves for several weeks.

On the next trip I moved the corals and fish from the other two aquariums, which meant adding the six other surgeonfish to the new aquarium. Obviously, the physical characteristics of the new aquarium changed with the addition of corals from the other two reef aquariums. Once everything had been placed into the new aquarium I turned the lights out and went to sleep wondering, not without a little trepidation, what I would find the next morning.

In fact, found an enraged sohal trying desperately to drive the six other surgeonfish from what it claimed as its territory — the entire 10-foot reef aquarium. It would violently chase the Achilles into the rocks only to have the other five pop out. This spectacle went on for about two weeks. Finally, the sohal simply gave up trying to chase the other surgeonfish out of its territory. Somehow, it recognized that it was a hopeless cause.

However, and here's where luck came into play. Because the other surgeonfish spent so much time defending themselves from the sohal they had little time to pay attention to one another. Now, approximately 18 months later, the seven surgeonfish coexist together peacefully. Aggression rarely results in more than an occasional torn fin. The most peaceful of the group (the kole tang) was only slashed twice by the spines of one of the more aggressive surgeonfish.

For those of you who want to keep more than one surgeonfish in a reef aquarium there are certain things you need to remember about this group. The fish with the most similar body shapes display the most aggression toward one another. The Acanthurus achilles and the A. leucosternon barely tolerate one another. When they met in my reef they often swam in circles, head to tail, fins splayed, until one chased the other into a coral, where the one being chased whipped its caudal peduncle at the chaser. Fortunately, my aquarium is large enough so that they are not continuously in one another's faces.

The lesson to be learned here is not to attempt to keep an Achilles surgeonfish with a powderblue unless they are added at the same time to an aquarium at least 10-feet long. In my experience, the other two surgeonfish that are particularly aggressive toward one another are the yellow tang (Zebrasoma flavescens) and the purple tang (Zebrasoma xanthurus). Their coloration is quite different, but their body shapes are almost identical, and, although I haven’t tried this, I suspect it would be difficult to keep an Acanthurus sohal with an A. lineatus.

With active aggressive fish like those described here it is important that they receive enough calories. Those reef aquarists who just feed a small pinch of flake food and an occasional feeding of live brine shrimp because they are concerned with polluting their reef aquarium(s) by overfeeding will starve most herbivores, such as surgeonfish, quite quickly. I have found it essential to feed large-size flake foods heavily at least once a day. It is also important to feed dried seaweed (Nori or Julian Sprung’s “Sea Veggies”) by suspending sheets of it in the reef aquarium for the surgeonfish in particular to graze on.

An additional note about my Achilles: I was dismayed to find that it refused the flake food I offered when I originally introduced it into my reef aquarium. If it continued to refuse flake food I knew I would have trouble keeping it from starving. Dehydrated, flake food contains significant calories. I was feeding TetraMarin marine and Spirulina flakes at the time. It took the flakes into its mouth, but spit them out. I decided to try OSI flakes, which I noticed had a stronger odor. It eagerly accepted these. It may be that another surgeonfish will prefer the TetraMarin flakes. The point here is that it is important for the aquarist to experiment to find a vegetable-based fish foods that will be accepted by the fish and that is high in calories.

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