How to Identify Three-spined Stickleback (partially plated morph)

To identify Three-spined Stickleback (partially plated morph) (Gasterosteus gymnurus), look at these key features.

Physical Characteristics

Body shape

torpedo

Coloration

Variable, typically olive-green to brownish on the dorsal side, fading to silvery-white on the ventral side. Breeding males develop vibrant red or orange coloration on the throat and belly, and iridescent blue eyes.

Scale type

none

Mouth position

terminal

Distinctive features

Three prominent, isolated dorsal spines (the first two are stout and erectile, the third is smaller and often connected to the soft dorsal fin), A single, strong spine in front of the anal fin, Absence or significant reduction of lateral bony plates, particularly on the posterior flanks (distinguishing feature of 'gymnurus' morph), Slender caudal peduncle

Dorsal fin

Three isolated spines followed by a soft-rayed dorsal fin (typically 10-14 soft rays).

Tail fin

Slightly forked or truncate caudal fin.

Pectoral fin

Well-developed, fan-shaped pectoral fins (typically 10 rays).

Distinctive fins

Pelvic fins are modified into a strong spine and one soft ray, often with a membrane connecting them.

Identification Tips

1. 1

   Lateral Plates

   The most crucial identifying feature for G. gymnurus is the absence or significant reduction of lateral bony plates, especially on the posterior half of the body. Run a finger gently along the flank; a smooth feel indicates the 'gymnurus' morph, whereas a rough feel indicates a fully plated or partially plated (but not 'gymnurus') morph.

2. 2

   Dorsal Spines

   Count the three distinct, isolated dorsal spines. These are characteristic of all Gasterosteus species.

3. 3

   Body Shape and Size

   Typically small, reaching up to 10 cm (4 inches) in length. The body is elongated and laterally compressed.

4. 4

   Breeding Coloration

   During breeding, males develop a striking red throat and belly, and bright blue eyes, which are unmistakable.

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