How to Identify Whitemargin Unicornfish

To identify Whitemargin Unicornfish (Naso annulatus), look at these key features.

Physical Characteristics

Body shape

compressed

Coloration

Adults are generally greyish-brown to olive-brown, often with a paler belly. The most striking feature is the prominent white margin on the caudal fin, which is particularly noticeable on the posterior edge and often extends along the dorsal and ventral rays. A dark band may be present at the base of the caudal fin. Juveniles are typically lighter in color, often silvery-grey.

Scale type

ctenoid

Mouth position

terminal

Distinctive features

Long, bony, horn-like projection on the forehead, extending forward from between the eyes (more pronounced in older males)., Two prominent, keeled bony plates (scalpel-like spines) on each side of the caudal peduncle, which are sharp and can inflict wounds., White margin on the caudal fin, especially on the posterior edge., Relatively small mouth with a single row of small, spatulate teeth.

Dorsal fin

Single dorsal fin with VI spines and 28-31 soft rays. The soft-rayed portion is long and continuous with the spiny portion.

Tail fin

Lunate (crescent-shaped) caudal fin with a distinct white posterior margin. Filaments may develop on the tips of the caudal fin lobes in large males.

Pectoral fin

Pectoral fins are relatively large and rounded, with 17-19 rays.

Distinctive fins

Pelvic fins are small, with I spine and 3 soft rays. Anal fin has II spines and 27-29 soft rays.

Identification Tips

1. 1

   Horn-like projection

   Look for the prominent bony horn on the forehead, which is a key identifier for adult Naso species. In N. annulatus, it is long and pointed, extending well beyond the snout.

2. 2

   Caudal fin coloration

   The distinct white margin on the posterior edge of the caudal fin is a definitive characteristic, especially when observed underwater.

3. 3

   Body color

   General greyish-brown to olive-brown body color, often with a darker band at the caudal fin base.

4. 4

   Size

   This is one of the larger unicornfish species, reaching up to 100 cm (39 inches) in total length, though commonly observed at 50-70 cm.

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