Family : Pomacanthidae
Text © Giuseppe Mazza
English translation by Mario Beltramini
Genicanthus melanospilos (Bleeker 1857), commonly known as Swallowtail angelfish or Spotbreast angelfish, belongs to the class of the Actinopterygii, the ray-finned fishes, to the order Perciformes and to the family of Pomacanthidae, counting 8 genera and about 90 species, often present in the aquaria with their multicoloured liveries.
Very compressed laterally fishes and with a characteristic spine at the base of the operculum, present also in the genus Genicanthus that counts about a ten species and comes from the old Greek “γένυς” (genys) = cheek and “ἄκανθα” (akantha) = spine, with reference, rightly, to this peculiarity.
The specific name melanospilos comes, always in ancient Greek, from “μέλαν” (mélan) = black and “σπιλος” (spilos) = spot, to remind the showy black spot that, apart from the lateral bands, the males have ventrally just before the pelvic fins.
The Swallowtail angelfish has a vast diffusion in the western Pacific Ocean, crossing into the neighbouring areas of the Indian Ocean. Indicatively, starting from Indonesia, it reaches, northwards, China and Japan, southwards, after Australia, New Caledonia. Eastwards, after Papua New Guinea, we find it in Micronesia, the Solomon Islands, Vanuatu and in the Fiji Islands. Recently, it has been found also in Tonga waters.
It is a sedentary species, present between 20 and 45 m of depth, strictly linked to the madreporic formations. It loves the external steep slopes of the reefs, rich of protrusions and of grottoes where to hide, but it can be found also on the sandy seabeds, close to coralligenous remains, and usually, looking for food in sites where the currents abound of zooplankton.
The males can reach the length of 18 cm. The females are smaller, but can, growing, become males. In fact, as for instance occurs for Genicanthus lamarck and Genicanthus personatus, this is a sequential protogynous hermaphrodite species. In fact, upon birth, the young have both sexes, but the first to manifest is the female one. Then, with age, the females may transform into males and can have a small harem. Usually, they are small groups of 3-7 individuals. When the leader dies, the biggest female turns male and takes its place.
The mouth is small with rows of tiny brush-like teeth. The long dorsal fin has 15 spiny rays and 15-17 soft that extend far beyond the caudal peduncle, like the anal fin, symmetrical at the end, that has 3 spiny rays and 17-18 unarmed. The pectoral caudal is semilunate with very long lobes forming elegant filaments also in the females apart the golden colour of the eyes and the great bluish defensive spine at the base of the operculum. The livery displays a strong sexual dimorphism.
The males are immediately recognized due to the zebra pattern usually formed by 15 parallel vertical lines on a white-bluish background, paler on the body and dark on the back and towards the head. On the caudal peduncle stands an orange hatch line like the elegant speckling present on the dorsal fin, the anal and the caudal that all end with a thin blue little border. Finally, there is a black spot in the chest, visible only if the fish is observed from below, that has granted the name to the species.
In females the upper part of the body tends to yellow with scales more and more bordered, towards the belly, with the bluish grey that covers the remaining parts of the body. The “swallow tail” of the caudal fin is more emphasized by the bluish strips on the sides, edged with blue.
Liveries that can be found also, roughly, also including the black spot of the males, in a similar species, the Genicanthus caudovittatus, centered however in the Indian Ocean.
The Swallowtail angelfish nourishes of zooplankton that often catches, without moving too much, in the water column over the den. When a female is ready for spawning, the fecundation takes place swimming, and the eggs are entrusted to the currents.
The resilience of the species is good (2021), with a possible doubling of the populations in 1,4-4,4 years and, considering the vast diffusion, the vulnerability index is modest, presently marking 33 on a scale of 100.