How to Identify Giant Threadfin

To identify Giant Threadfin (Polydactylus indicus), look at these key features.

Physical Characteristics

Body shape

elongated

Coloration

Dorsal surface is typically silvery-grey to bluish-grey, fading to silvery-white on the flanks and belly. Fins are often dusky or yellowish, with the pectoral fins sometimes having a darker margin. Juveniles may exhibit faint vertical bars.

Scale type

ctenoid

Mouth position

subterminal

Distinctive features

Seven free, filamentous pectoral fin rays, significantly longer than the main pectoral fin, extending well beyond the origin of the anal fin., Prominent, pointed snout that overhangs the mouth., Large, deeply forked caudal fin., Two distinct dorsal fins.

Dorsal fin

Two dorsal fins; the first with 8 spines, the second with 1 spine and 13-15 soft rays. The second dorsal fin is typically higher than the first.

Tail fin

Deeply forked caudal fin with pointed lobes.

Pectoral fin

Pectoral fins are located low on the body, with 15-16 rays in the main fin and 7 long, free, filamentous rays below it.

Distinctive fins

The seven free pectoral fin rays are the most distinguishing feature, used for sensing prey in the substrate.

Identification Tips

1. 1

   Free Pectoral Fin Rays

   Count the number of free, filamentous rays below the main pectoral fin. Polydactylus indicus has 7, which are notably long, often extending past the anal fin origin. This is a key differentiator from other threadfin species.

2. 2

   Snout Shape

   Observe the snout, which is prominent and distinctly overhangs the mouth. This gives the fish a somewhat 'pig-like' appearance.

3. 3

   Size

   Adults can reach impressive sizes, up to 1.5 meters (5 feet) in total length and over 50 kg (110 lbs), making them one of the largest threadfin species. Smaller individuals might be confused with other Polydactylus species.

4. 4

   Coloration

   Note the silvery-grey to bluish-grey dorsal coloration fading to white ventrally. The fins are often dusky, and the free rays may be whitish.

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