How to Identify Indo-Pacific Flounder

To identify Indo-Pacific Flounder (Pseudorhombus arsius), look at these key features.

Physical Characteristics

Body shape

flat

Coloration

Highly variable, typically mottled brown, grey, or sandy with darker spots and blotches, allowing for excellent camouflage against the substrate. The blind side is whitish. Often has a prominent dark spot on the lateral line, sometimes with smaller spots.

Scale type

ctenoid

Mouth position

terminal

Distinctive features

Left-eyed (eyes on the left side of the head), Large mouth with prominent teeth, Lateral line strongly arched above the pectoral fin, Often has a distinct dark spot on the lateral line, sometimes with smaller accessory spots, Pectoral fin on the ocular side is longer than that on the blind side

Dorsal fin

Originates anterior to the upper eye, with 71-82 rays. Rays are unbranched.

Tail fin

Rounded to slightly double emarginate. Caudal fin rays are unbranched.

Pectoral fin

Well-developed; pectoral fin on the ocular side (left) is longer than that on the blind side (right).

Distinctive fins

Pelvic fins are symmetrical in position, with the base of the ocular-side pelvic fin slightly anterior to that of the blind-side pelvic fin. Both pelvic fins have 6 rays.

Identification Tips

1. 1

   Eye Position

   Always check that both eyes are on the left side of the head. This distinguishes it from right-eyed flounders (Pleuronectidae).

2. 2

   Lateral Line

   Observe the strong arch in the lateral line above the pectoral fin. This is a key characteristic of the genus Pseudorhombus.

3. 3

   Mouth and Teeth

   Note the relatively large mouth and prominent, sharp teeth, indicative of its predatory nature.

4. 4

   Coloration and Spots

   Look for the characteristic dark spot (or spots) on the lateral line, though coloration can vary greatly with substrate.

5. 5

   Pectoral Fin Length

   The pectoral fin on the eyed (left) side is noticeably longer than the one on the blind (right) side.

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