News / Feature Articles / What Exactly Is A Pygmy Angelfish? (09/18/18)

What Exactly Is A Pygmy Angelfish?

What Exactly Is A Pygmy Angelfish?
The marine angelfishes of the family Pomacanthidae are among the most iconic of reef fishes, and, for many, an aquarium isnt quite complete without a representative from this charismatic group. For those with larger, fish-only systems, there are many beautiful options to choose from among the larger species (e.g. Pomacanthus, Holacanthus, Apolemichthys, Chaetodontoplus, Pygoplites). For those looking for a mostly reef-safe option in a mid-sized system, there are the swallowtail angelfishes in the genus Genicanthus, which, rather uniquely, feed upon zooplankton. But the remainder of this familygenerally referred to as the pygmy angelfishesdoesnt quite fit into either of these distinctions.

Pygmy angelfishes are, as the name implies, generally quite small. The majority of these reach somewhere around 3-4 inches when fully grown, but there are a few exceptions, such as the Keyhole Angelfish (Centropyge tibicen), which grows to an impressive 7-8 inches. Traditionally, these smaller pomacanthids found themselves lumped into a single genus, Centropyge, but it was always questionable whether the three-dozen or so species that fit this description truly represented a single evolutionary lineage. As recent genetic study has argued, they dont.

The True Pygmy Angelfishes
The type species of the genus Centropyge is the aforementioned Keyhole Angelfish, described in 1831 by the famed 19th century anatomist George Cuvier. At the time, the angelfishes were all placed in the genus Holacanthus, but a study of this group in 1860 by the German naturalist Johann Jakob Kaup broke them up, establishing the name Centropyge for those angelfishes which possessed 4 anal fin spines. Oddly, though, no angelfish actually possesses 4 anal fin spines. When Cuvier described his Holacanthus tibicen, he mistakenly recorded it as having a fourth spine, and so this genus is fundamentally based on a lie.

But the name does correlate to a natural grouping of some small angelfishes. Those most closely related to C. tibicen, which we can rightly regard as being the true Centropyge, can usually be recognized for having a reduced number of gill rakers and, in most,

Centropyge tibicen

pointed tips to the dorsal and anal fins. The primary exception to this are a trio of closely related species with more rounded fins, which includes the Lemonpeel Angelfish (C. flavissima), the Half-black Angelfish (C. vrolikii), and the Red Stripe Angelfish (C. eiblii). These three have seen an immense amount of study in recent years, owing to the rampant hybridization that takes place amongst them, the most famous examples being the famed Cocopeel and Tigerpyge Angelfishes from parts of the Eastern Indian Ocean.

The earliest species to be described for this group dates to the work of Marcus Bloch in 1787, when he described the Bicolor Angelfish (C. bicolor) from Indonesia. This is also a fairly large species, measuring in at a robust 6 inches. The same can be said for the Indian Oceans Multispined Angelfish (C. multispinis). This is often regarded as the least visually appealing of the various pygmy angelfishes, as is the related Yellowfish Angelfish (C. flavipectoralis), a species rarely seen in captivity and one which is only found in the Western Indian Ocean.

Then there is the Yellow Angelfish (C. heraldi), a species easily confused with the similarly colored Lemonpeel. This fish has a tendency to develop a black edge to its dorsal fin, and at times these variants have been recognized as their own unique species, though theres no strong evidence supporting this. And rounding out this group we find a couple species that are highly reclusive on the reef, the Golden Angelfish (C. aurantia) and the Midnight Angelfish (C. nox), the latter of which is mimicked by the Midnight Dottyback (Manonichthys paranox).

Xiphypops, the Spiny Pygmy Angelfishes
This group is easily diagnosed be the presence of three well-developed spines on the rear margin of the preopercle, giving these fishes an especially spiny appearance when examined closely. To see these, look at the small space between the eye and mouth, and youll see these backwardly directed spines. The scientific name alludes to this, coming from the Greek for sword and face. Also, compared to the true Centropyge we just looked at, these fishes have a consistently higher number of gill rakers, which is a trait shared with the next group well be looking at.

Additionally, the mouth in Xiphypops is subterminal (i.e. directed downwards), while Centropyge has its mouth more terminally positioned (i.e. opening at the anteriormost tip of the body). And, lastly, we see a common patterning to the rear margin of the dorsal and anal fins, with a series of black and blue stripes being present. Given all the distinctive qualities

Centropyge argi

of these fishes, they have at times been regarded as a genus separate from Centropyge or, alternatively, as their own subgenus. Genetic data shows that they are in fact quite distantly related to the true Centropyge, and are actually more closely related to both Genicanthus and Apolemichthys than they are to their other pygmy cousins.

All but one of the species classified here falls into a single circumtropical species complex, and its worth noting that these are the only examples of pygmy angelfishes to occur in the Atlantic. Their occurrence here is apparently quite recent, and, despite consistent differences in the color patterning of the three Atlantic species, genetic study has thus far failed to find meaningful differences between them. This is even true to some extent with those from the Indo-Pacific, and there have even been some researchers who have gone so far as to suggest that these are all really one highly variable species.

The most familiar to aquarists are the Atlantic taxa. The Cherubfish (C. argi) is a widespread Caribbean species, which gives way to the Fireball or Flameback Angelfish (C. aurantonotus) on the reefs south of the Amazon. Its also recently been reported from parts of Western Africa, and, sitting right in the middle of these locations, we find the legendary Resplendent Angelfish (C. resplendens) at the isolated islands of St. Helena, Ascension and Tristan da Cunha. Wild specimens of this fish are no longer collected, but, on rare occasion, captively-bred specimens (hybrids with C. argi) can be seen in the aquarium trade. Wild hybrids of C. resplendens and C. aurantonotus were even recently photographed at St. Helena, illustrating the connectivity that all of these fishes have across this ocean basin.

Crossing over to the Indian Ocean, we find a confusingly similar fish, the Flameback Angelfish (C. acanthops). This one differs from its Brazilian doppelganger in having a yellow (versus blue) caudal fin, and youll find this fish throughout the Western Indian Ocean, with possible hybrids reported as far as the Maldives. Here, it gives way to a fish that is variably referred to as either C. flavicauda or C. fisheri. Most recent references favor the latter name, which is a species originally described from Hawaii. The Hawaiian population is distinctive, usually having a mostly orange body (though exceptions exist), but, from a morphological standpoint, it is no different from other populations in the Indo-Pacific.

Though it is most often treated as a synonym, the name C. flavicauda will probably be recognized as valid when this group is further studied. This species, commonly known as the Whitetail Pygmy Angelfish, was described from the South China Sea, and it almost always has a blue body and a white caudal fin. Those from the Eastern Indian Ocean and the Maldives match this general appearance, but their distant biogeography suggests they might also be distinct.

And, lastly, there is C. nigriocellus, the Black-spot Angelfish, possibly the single most reclusive member of this whole family. This fish is very rarely seen on the reefs and is even rarer in aquariums. Researchers have most often collected it indiscriminately by using narcotizing agents squirted into reef crevices, which flushes out whatever species are living deep within the rocks. This is of course a very shy species in captivity, and very few have ever been offered.

The Other Spiny Angelfishes
The apparent sister group to Xiphypops are the species referred to by Angelfish researcher Richard Pyle as the multicolor complex and the bispinosa complex. As with the preceding group, these fishes have a bit more of an armament to their preopercle, though this feature is usually not quite so prominent when compared to Xiphypops. The higher number of gill rakers, the subterminal mouth, and the black and blue fin margins (present only in the bispinosa complex) further support their close relatedness. At some point, these groups are likely to be split off from the genus Centropyge, and we may eventually see them lumped together as a broadly defined Xiphypops (or, alternatively, as two separate genera).

The Coral Beauty Angelfish (C. bispinosa) is the most characteristic of these, and also the most widely distributed. The species occurs from the eastern shores of Africa to Japan, Micronesia, and Polynesia,

Centropyge bispinosus MAC

being absent only from Hawaii, where it is instead replaced by the similar Potters Angelfish (C. potteri). Theres an interesting parallel here between the distribution of these species and the situation with C. fisheri/C. flavicauda discussed above. Theres even evidence for a distinctive (and unstudied) population in the Western Indian Ocean for C. bispinosa, which can be recognized for having a more colorful and striped caudal fin, referred to in the aquarium trade as the Tigertail Angelfish.

Its here that we find the Flame Angelfish (C. loriculus), a species that can lay strong claim to being the most vibrantly colored reef fish, full stop. This species, restricted primarily to the Central Pacific, is highly variable in its coloration, with specimens from Hawaii and the Marquesas potentially representing cryptic species (though genetic study has argued against this actually being the case).

Very closely related is the Rusty Angelfish (C. ferrugata) from Japan and the Northern Philippines and Shepards Angelfish (C. shepardi) from the Mariana Arc and the Northern Philippines. The Philippines sits at an interesting biogeographic crossroads, where these species meet and commingle, producing some confusing hybrids. One example from the Southern Philippines, thought to be a mix of loriculus X ferrugata, is nearly identical to C. shepardi and has been exported to aquarists as the False Shepards Angelfish.

The multicolor complex includes an interesting mix of species, generally found in deeper reef habitats and, for many of these, from isolated portions of the Indo-Pacific. The most widespread is the aptly named Multicolor Angelfish (C. multicolor), which youll see all across the Central Pacific. At the remote Johnston Atoll, it gets replaced by the nearly identical Nahackys Angelfish (C. nahackyi). This is one of the few fishes reported to be endemic to this small island, though occasional strays have been found to the north in Hawaii. Aquarium specimens are almost unheard of.

Another isolated relative occurs at Easter Island, which sits at the easternmost edge of Polynesia. This is the Blackear Angelfish (C. hotumatua), named after the mythological first settler of this island. This is another species that is mostly absent from the aquarium trade, though you will see one at the Waikiki Aquarium.

The Joculator Angelfish (C. joculator) is an endemic of Christmas Island and the Cocos-Keeling Islands in the Eastern Indian Ocean. This is the same location that produces the Cocopeel Angelfish, which was itself controversially recognized as a distinct species in 2016. C. joculator is reported to be relatively common in this region, but only a small number are collected yearly, making it one of the most expensive of the regularly available pygmy angelfishes. But for an even pricier fish, there is Debelius Angelfish (C. debelius), known only from Mauritius and Reunion in the Western Indian Ocean. Oddly, there is no evidence for this group elsewhere in the Indian Ocean, despite there being plenty of available habitat in the Maldives, the Chagos Archipelago, and the Andaman Sea. This shows the striking influence that biogeography can have on the speciation of reef fishes.

The last of the multicolor complex is Japans gorgeous C. interrupta, often referred to (somewhat erroneously) as the Interruptus Angelfish. Due to the way species names have to grammatically align with the linguistic gender of the genus, there has been a tendency for the scientific names in this genus to be malformed. So youll see both C. bispinosus and C. bispinosa for the Coral Beauty, the latter of which is correct. The same applies to the Lemonpeel Angelfish, which is C. flavissima, NOT C. flavissimus. But, oddly, for the Flame Angelfish, it is C. loriculus which is valid, NOT C. loricula. Its very confusing.

Paracentropyge, The Taller-bodied Pygmy Angelfishes
Depending on who you consult, Paracentropyge is either a valid genus or a subgenus of Centropyge. Unfortunately, theres all sorts of confusion and inconsistency when it comes to the species involved here, with the Purple-mask or Venusta Angelfish (P. venusta) often being incorrectly lumped into Centropyge. This fish looks very different from the other Paracentropyge, having an almost abstract blue patterning across its back and nape. Youll find it occupying a similar range as the distantly related Rusty Angelfish in Japan and parts of the Philippines.

Where it overlaps with the widely distributed Multibar Angelfish (P. multifasciata) in the Pacific, the two produce one of the most beautiful of hybrid reef fishes, which aquarists have given the whimsical portmanteau Multivenusta. It features a wavy, irregular mixture of the patterns seen in both parent taxa, providing strong evidence of their close relatedness.

Paracentropyge venustus

Paracentropyge has a couple of distinctive morphological qualities to it. The species have a lower number of dorsal fin spines (either 13 or 14, compared to 15 in other pygmy angelfishes), and the shape of the body is taller, relatively speaking, especially when compared to the elongated members in Xiphypops.

The most prestigious of the Paracentropyge is without question the Peppermint Angelfish (P. boylei). The species is named after its discoverer, aquarium collector Chip Boyle, who first encountered it on his deep dives in the Cook Islands. It has subsequently been found in nearby French Polynesia and appears to be restricted to the South Pacific. In addition to its remote geography, this stunning red-striped fish is only found in the deepest of mesophotic habitats, making it exceptionally rare for specimens to find themselves collected. Until this one can be captively bred, expect to pay an arm and a leg (and possibly some other bodily parts too) to acquire this fish.

The Round-finned Pygmy Angelfishes
And weve reached the last group of pygmy angelfishes, comprised of just two species, the Narcosis Angelfish (C. narcosis) and Colins Angelfish (C. colini). In his revision of the angelfishes published in 2003, ichthyologist Richard Pyle was the first to report on the distinctiveness of these two when compared to others in their genus. Of note, they have a low number of gill rakers, a relatively tall body, a fairly steep and straight profile to their head (more convex is the true Centropyge), along with some distinctive color patterns and rounded dorsal and anal fins. This combination sets them apart from all of the other small angelfishes, but it wasnt until the genetics of these fishes were examined that their true evolutionary novelty was finally appreciated.

These two species are distant relatives of all the other fishes weve looked at thus far and seem to represent the earliest group to diverge from within the diverse

Centropyge narcosis

clade formed from Centropyge, Apolemichthys, and Genicanthus. And, interestingly, theres a third member of the group which has historically had a very confused placement in this family, the Bandit Angelfish (Apolemichthys arcuatus). At times, this fish was classified in its own genus, Desmoholacanthus, but it can be seen to possess some general commonalities with these two pygmy relatives. The general shape of juvenile Bandit Angelfishes conforms to the tall, steep profile of this group, and the dark coloration along its back is comparable to the patterning seen in C. colini. However, fully grown specimens reach an impressive 7-8 inches in length, making them among the largest of the pygmy angelfishes.

The outlier here is the Narcosis Angelfish, a pale yellow fish with a large black spot in the middle of its body. The unusual name stems from the nitrogen narcosisthe bendssuffered by the first researchers to collect it, Chip Boyle and Richard Pyle. While C. colini is a widespread species in the Central Pacific, C. narcosis only occurs in Polynesia, and, like with the Peppermint Angelfish, only at great depth. Its really rather remarkable that there are two such species in this region when, elsewhere in the Indo-Pacific, this deepwater niche is left unfilled.

So, to summarize what weve learned, Centropyge, as presently defined, includes a number of species that are quite distantly related, often by tens of millions of years, and the group will at some point be split into a number of smaller genera.