News / Species Spotlight / Thompsons Surgeonfish (Acanthurus thompsoni) (07/14/17)

Thompsons Surgeonfish (Acanthurus thompsoni)

Species Spotlight - Thompsons Surgeonfish (Acanthurus thompsoni)
Close your eyes and picture a surgeonfish any surgeonfish. You are probably imagining it swimming along the bottom, gently rasping away at some bit of algae growing upon the rocks. This is the stereotypical behavior of acanthurids that we as aquarists are most familiar with, but it is important to recognize the exceptions to this rule. Many of us purchase surgeonfishes to help clean any nuisance algae which might grow in our tanks, but there are a few species which arent up for the job like Acanthurus thompsoni.

Thompsons Surgeonfish fills a somewhat unique ecological niche within the speciose genus Acanthurus. This is the largest major lineage within the surgeonfish familyfar larger than the Naso unicornfishes or the Zebrasoma tangsand it has somewhere around 50 species if we include the Ctenochaetus bristletooths in the discussion. These species form a handful of subgroups, as has been shown in recent studies of the groups evolutionary history, with each tending to specialize on a different feeding ecology.

For instance, the familiar Convict Tang (A. triostegus) and Powder Blue Tang (A. leucosternon) are the quintessential schooling, algae-feeding species. Then theres the Clown Tang (A. lineatus), which is a more solitary and territorial herbivore. And you might be surprised to learn that most species in Acanthurus are actually feeding almost entirely on detritus. The aforementioned bristletooths do this by combing bits of organic matter trapped within algal mats using their specialized teeth, while many others, such as the Orange-shoulder Tang (A. olivaceus), simply mop up mouthfuls of sandy sediments. And then theres Acanthurus thompsoni

In studies looking at genetic data, it has been shown that Thompsons Surgeonfish is the sister group to all others in this diverse lineage, which befits its odd approach to being an Acanthurus. If youve ever seen this fish in the wild or in photographs, you might have noticed that it swims high off the bottom, far from where any algae might be growing. So what is it doing up there in the water columnfeeding on zooplankton.

Yes, A. thompsoni is the anthias of the surgeonfish family, actively scanning the reef for any edible morsel which might swim by. This could include things like small crustaceans, fish larvae, pelagic polychaete worms, or, perhaps most abundantly, gelatinous zooplankton, especially salps. If youre not familiar with salps, these are a bit like a free-swimming variety of sea squirt (AKA tunicates), and their bodies are of relatively low nutritional quality. In fact, their nutritional profile is probably not all that different from an herbivore, which might explain how the earliest acanthurid ancestors could have switched from a diet of salps to a diet of algae.

Thompsons Surgeonfish isnt the only member of the genus to have developed this unorthodox diet. The poorly known Pin-striped Surgeonfish (Acanthurus nubilus) has, as well as another small group comprised of the Elongate and White-fin Surgeonfishes (A. mata & A. albipectoralis). But A. thompsoni is probably the most interesting of the bunch on account of its unique position in the evolutionary history of the group, and, as it tends to be found quite commonly in vast shoals across the Indo-Pacific, it is without question the most dominant zooplanktivore in this genus.

To keep Thompsons Surgeonfish in captivity, a suitably large aquarium will be required to accommodate the nearly foot-long size of adults. The caudal spine is barely developed, making this one of the more peaceful members of the family to add to an aquarium, and it can potentially be mixed with others of its kind, as well as others in its genus (though only in appropriately large tanks). Most any food will be accepted, including both dry and frozen, as well as bits of nori. Its hard to say whether it is better to prioritize herbivorous or carnivorous foods for this unusual surgeonfish, but probably any diverse mix will suffice.