The genus Genicanthus boasts of ten medium to large angelfish species that have evolved away from the regular substrate perusing stereotype. All members of this genus are rather streamlined, being longer than they are tall. Their caudal fins are festooned with long filamentous extensions on both lobes, lending the colloquial moniker of “swallowtail angelfish” highly befitting. These adaptations have allowed Genicanthus to reign dominance high above the water column in rocky seaward slopes – a habitat devoid of most other angelfish genera.
Their evolution towards a pelagic lifestyle has brought upon radical change in their diet and metabolism. Unlike other angelfish genera, zooplankton and pelagic tunicates make up the bulk of their diet. Although gut contents reveal the presence of filamentous algae and other benthic microfauna, these make up the minority of their daily food intake. While sometimes found close to the substrate, rarely do Genicanthus seek shelter within the reef catacombs when threatened. They instead prefer fleeing for safety, and the swallowtail angelfishes do this with relative ease, owing to their nimble and agile speed.
All species of Genicanthus are sexually dichromatic. In most species, the sexes can appear so vastly different that, to the uninformed, may be mistaken for separate species altogether. The genus are also protogynous sequential hermaphrodites, in which all fish are born female, and males are derived from sex changed females. This process happens fairly quickly, and matured females under the influence of testosterone can quickly shed their colors to adopt that of the male’s. In as little as two weeks, a female can assume the role of a fully functional male, complete with color change. Genicanthus adopt a haremic, or more appropriately, a lek mating style. As such, they are usually never alone and the presence of one should signal the proximity of a nearby group.
All species are found in moderately deep waters at least, although they can easily extend their depth preference well into the mesophotic twilight zone. This, as well as the above ecology, can be applied to every Genicanthus species. In this article however, we’ll focus on one beautiful member – Genicanthus bellus. Genicanthus bellus has the largest distribution of all species in this genus. It is widespread across the Pacific Ocean, and can be found in Tahiti in the Society Islands (French Polynesia), Guam, Palau, Tonga, Cook Islands, the Marshall Islands, Philippines, southern Indonesia and the Coral Sea. It reaches its northernmost limit in Japan. The species also strays weakly into the Indian Ocean, and here it occupies the Cocos-Keeling and Christmas Island.
The specific epithet “bellus” is latin for beautiful, and indeed, the females of this species are rather ornately patterned with a series of complicated stripes. In the females, the ground coloration is bluish white to light grey, fading to a lighter shade ventrally. The body is adorned with a series of complex broad stripes, and their formation is reminiscent of a badly thought out tetris game.
The first black stripe presents itself vertically on the nape, masking the eye completely but never venturing south past the orbital limit. The second black stripe starts closely behind, making a ninety degree bend at the start of the dorsal fin before continuing along its entire length. The third black stripe starts at the flexure of the second stripe and makes an oblique transition downward, connecting and finishing through to the lower caudal fin lobe. The fourth and final stripe lines the upper caudal lobe and does not pass the caudal peduncle. All these stripes are edged copiously along their periphery in white or the faintest shade of blue. A thick cobalt swoosh is present behind the pectoral fin and below the third body stripe. This swoosh is variable in its extensiveness, and the messy, poorly demarcated borders are reminiscent of a brush stroke.
The anal fin is rather variable, and is edged either with a single or double stripe in orange. It is also not uncommon to find specimens lacking this orange trim. The dorsal fin is predominantly black, but very large and matured females may have it variably shaded in orange. The upper lip of the females are black with two silvery blue dots. From a head on perspective, this gives the fish a gapped tooth appearance. Juveniles are identical to the females.
Males of this species are contrastingly more simple. In the males, the ground color ranges from pale dirty grey to light beige. The entire body is unmarked sans the presence of two parallel stripes in bright yellow to kumquat. The first stripe starts at the nape, where it emerges from a medusa of random squiggling. Here, it travels along the base of the dorsal fin entirely along its length and terminates at the caudal peduncle. The second stripe forms an equatorial belt and runs parallel to the dorsal stripe starting from behind the operculum. Both stripes converge at the caudal peduncle. The pectoral fin base and dorsal fin are colored as with the body stripes. The caudal fin is hyaline and often fenestrated with orange, much like a stained glass window. Both caudal lobes including their filamentous extensions are bright cyan. The lips are of the same color.
Like the females, the male sex shows slight variations as well, especially in the caudal fin, dorsal fin and the peduncular convergence of the stripes. In some specimens, these regions can be so extensively fenestrated and filigreed that their connections are broken. Despite the vast extent of its geographical range, these variations do not seem to correspond closely to any potential crypts or specific insular forms. G. bellus is almost exclusively deepwater and is rarely found in depths shallower than 50m (160ft). This species makes an exception in Cenderawasih bay, where it dwells in depths as shallow as 20m (65ft). Here, it cavorts with other deepwater species such as Chaetodon burgessi, who curiously enough penetrate shallower regions of the reef. No one knows why, but they are never found at this depth anywhere outside of this location.
Genicanthus bellus is uncommon in the trade, and appears to be “seasonal”, for lack of a better word. At times numerous individuals may appear at various wholesalers, corresponding to mass collection and distribution of the species from Indonesia or the Philippines. Between these periods the availability of this species remains very sporadic and random. As with all deepwater Genicanthus, affliction of decompression related maladies are not uncommon. Infection from poor needling practices and swimming with their head down are some common problems associated with this species. Healthy specimens that are collected properly prove to be quite hardy and resilient once adjusted to aquarium life.
The enormous geographical distribution of G. bellus puts it in close sympatry with nearly half the total members of the genus. This species is often collected for the aquarium trade alongside G. watanabei, G. semifasciatus, G. melanospilos and G. lamarck. It comes as a surprise then, that this species has only been reported to hybridize with just one species. The hybrid itself is exceedingly rare and known only from a few specimens. Genicanthus bellus x G. lamarck have been documented from Indonesia, where both sexes have been collected for the aquarium trade.
The likelihood of this hybrid arising is not at all surprising, considering that G. bellus is closely allied to G. lamarck, for which it is paired cladistically with G. watanabei. Together the three form a monophyletic clade that features males with horizontal stripes and females with oblique markings on their caudal lobes. More information on the evolution of Genicanthus can be found in Joe Rowlett’s upcoming magazine article from reefs.com.