How to Identify Japanese Swallowtail Angelfish

To identify Japanese Swallowtail Angelfish (Genicanthus semifasciatus), look at these key features.

Physical Characteristics

Body shape

compressed

Coloration

Males exhibit a pale blue to whitish body with prominent dark, vertical to oblique bands on the upper half of the body, extending onto the dorsal fin. The caudal fin is deeply forked with elongated filaments, often edged in black. Females are generally paler, often whitish to light blue, lacking the distinct dark bands of the male, and may have a yellowish wash on the dorsal fin and caudal fin. Both sexes have a small mouth and relatively large eyes.

Scale type

ctenoid

Mouth position

terminal

Distinctive features

Sexual dimorphism in coloration and finnage, Deeply forked, swallowtail-like caudal fin, Males with distinct dark bands on upper body, Small mouth adapted for planktivorous feeding

Dorsal fin

Single dorsal fin with 15 spines and 15-16 soft rays. In males, the anterior soft rays may be slightly elongated.

Tail fin

Deeply forked, swallowtail-like caudal fin. Males typically have elongated filaments on the upper and lower lobes.

Pectoral fin

Pectoral fins are relatively large and fan-shaped, used for propulsion and maneuvering.

Distinctive fins

Pelvic fins are small and pointed. Anal fin has 3 spines and 16-17 soft rays.

Identification Tips

1. 1

   Sexual Dimorphism

   Males are easily identified by their pale body with several prominent dark, oblique bands on the upper half, extending into the dorsal fin, and elongated caudal fin filaments. Females are generally plain whitish to light blue, lacking these bands, and have less elongated caudal fin filaments.

2. 2

   Caudal Fin Shape

   The deeply forked, swallowtail-like caudal fin is a key characteristic of the Genicanthus genus, and particularly pronounced in G. semifasciatus, especially in males.

3. 3

   Habitat Preference

   Look for them in deeper reef environments, typically below 15 meters, often swimming in open water above the reef structure to feed on plankton.

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