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Certainly one of the most beautiful marine angelfishes, but one with a mixed record of longevity, the emperor, Pomacanthus imperator is well-deserving of it's majestic name.
Most individuals are lost due to poor capture technique, transport, acclimation and lack of nutrition. In this article I will detail the most common symptomology for avoiding a doomed specimen, and offer my insights into the species appropriate care.
Classification: Taxonomy, Relation With Other Groups
Emperors are grouped with the marine angelfishes, family Pomacanthidae. This family was long considered as a sub-family of the closely related Butterflyfishes, Chaetodontidae. No longer. They can be easily distinguished by the presence in angelfishes of a strong opercular spine which butterfly's lack and the lack of a well-developed pelvic axillary process which they have. Butterflyfishes also pass through a unique larval stage termed a trolichthys, peculiar to their family.
Both are in the suborder Percoidea and order Perciformes with several thousand other advanced bony fishes, many familiar to the pisciphile. Cichlids, scats, basses, sunfishes, cardinals, remoras, jacks, snappers, croakers, archers, monos, goatfishes, Spadefishes, Leaffishes, surf perches, Hawkfishes, among many many others. This is the largest suborder in the largest living order of fishes. Please see Nelson's treatment for the group's higher taxonomy.
As with many Pomacanthids, juveniles are very different in appearance. Prior to 1933, young emperors were considered a separate species, P. nicobariensis. There is a wide variation in range of natural color and pattern in small to large specimens.
Selection: General to Specific
Juveniles are similarly marked as Koran (P. semicirculatus) and several other large related angels, with series of concentric dark blue, black and white rings. Considered individuals should be well colored and marked with no apparent traumas.
However the primary selection character/criteria I would describe as "brightness"; that is an active/searching interest in it's immediate environment. Prospective specimens should be curious, reactive to your presence, not so much dashing about their enclosure, nor totally "spaced-out" but aware, following motion, light and anything else near them.
For most hobbyists I strongly encourage the trial of individuals only starting between 10-14 cm. size. These seem best suited for beginning life in captive conditions. Larger one's have more difficulty adapting to aquariums and smaller one's don't accept prepared foods as well. There are, of course, exceptions, but this is my rule of thumb guideline.
Roger Steene, good buddy of Gerald Allen, offers his opinion that P. imperator "is a sturdy aquarium species". This is so for specimens hailing from Australia, the Indian Ocean and Red Sea; but not for most of the Indo-Pacific. Only individuals that appear full-bodied, alert as described above, and heartily accepting several foodstuffs should be considered. Be especially wary of overly-brightly-colored disoriented specimens from the Philippines and Indonesia. Many of these I've found to be cyanided and doomed. I consider the species not as easy to maintain as the Koran P. semicirculatus, but certainly hardier than those in the genera Euxiphipops and Pygoplites
Similarly avoid rapid breathers; more than eighty gill movements a minute's too many.
One last note re selection: Take care in capturing and transporting members due to their gill cover (opercular) spines. These are often easily fouled, torn by nets, leading to subsequent infection. I advise net-less directing the intended specimen into a suitably double or otherwise puncture-insulated bag or at worse lifting ultimately by hand. Be Careful! The family name is derived from the Greek roots "pom" = cover and "acanthus" = thorn, for good reason.
Imperators are found exclusively associated with coral reefs in the wild, in shallow to one hundred feet depths.
Good water quality must be optimized and constant. use a good quality salt mix, at a 1.022-1.025 specific gravity and maintain pH in the higher range, 8.2-8.4. A protein skimmer is a must.
Angels are sensitive to "new tank syndromes". Place them in systems that have been seasoned for a few months.
Very important to the group of angelfishes is vigorous circulation and removal of metabolites.
Even in captivity emperors seek out/prefer the comfort of shelter from boulders, caves and coral niches. They will do well only where offered the possibility of retreat.
Size of aquaria? The bigger the better. I would not even start an emperor in a system of less than four hundred liters. Ultimately, you will need one twice plus this size.
Emperor angels are considered amongst the larger members of their family, able to reach some forty centimeters. Like most species in their and related genera, they do best kept singly and not with other angel species of a similar size.
Place in a non-occupied or re-disturbed system in subdued lighting and leave some indirect light on for a day. Make favored foods available a few times a day at first.
May be readily eaten by typical predaceous types. Beware of Billy Grouper, Eli the Eel, Larry the Langusto and Porky-boy the Puffer...
Lifelong pairing/mating seems to be the rule. The family are egg-scatterers, clued from tide, temperature and light stimuli. Some authors cite sex-change capacity and harem/lek activity. I doubt it.
Feeding/Foods/Nutrition: Types, Frequency, Amount, Wastes
Similar to members of the genus Holacanthus, emperors and other Pomacanthus are known consumers of a large amount of sponge (phylum Porifera) material, then algae, other animal material and vascular plants in the wild. Will you go broke buying live sponges or specialty frozen foods made of the same? Nah. In captivity they can/usually will adapt/adopt to prepared frozen, fresh and dry foods.
Disease: Infectious, Parasitic
All specimens are suspect. Most have been impaired/traumatized by handling and weakened by handling to the extent that invariably-present protozoan infections (and possibly internal and external parasite fauna) may gain an upper hand (fin?). All newcomers should be preventively dipped and otherwise quarantined for a good two weeks. Trust me.
If yours appears to be sick, look first and foremost to water quality, next to feeding/nutrition, and last chemical therapeutic treatment. Be careful about interrupting biological conversion with copper compounds, though they are the treatment of choice for the two common scourges Cryptocaryoniasis and Amyloodiniumiasis (Oodiniumiasis). For general or non-specific malaise I would recommend a massive water change, vitamin application and a few grams of fruit sugar (fructose).
I like and agree with Hans Baensch, the man behind Tetra's advice as regards this and other angels; try other easier marines first and learn the value of culturing the algae genus Caulerpa or other suitable bio-assay species as viable indicators of water quality, before plunking down the big bongo bucks for something like an emperor angel.
Allen, Gerald R. 1978. Butterfly and Angelfishes of the World. Vol. 2. Hans Baensch, W. Germany.
Campbell, Douglas. 1981. Marines: Their Care and Keeping, Pomacanthus. FAMA 9/81.
Hemdal, Jay. 1989. Marine Angelfish, Color and Style. Aquarium Fish Magazine. August 1989.
Nelson, Joseph S. 1976. Fishes of the World. Wiley Interscience, N.Y.
Spies, Gunter. 1988. The Emperor of the Reef: Pomacanthus imperator. T.F.H. November 1988.
Steene, Roger C. 1977. Butterfly and Angelfishes of the World. Vol. 1. Hans Baensch, W. Germany.