How to Identify Three-spined Stickleback

To identify Three-spined Stickleback (Gasterosteus aculeatus), look at these key features.

Physical Characteristics

Body shape

elongated

Coloration

Highly variable. Typically olive-green to brownish on the dorsal side, fading to silvery-white on the ventral side. During breeding season, males develop vibrant red or orange coloration on their throat and belly, and their eyes turn iridescent blue. Females may become slightly paler and plumper.

Scale type

none

Mouth position

terminal

Distinctive features

Three prominent, isolated dorsal spines (can range from 2 to 4, but typically 3)., A series of bony plates (scutes) along the flanks, the number and extent of which vary significantly between populations (fully plated, partially plated, or low plated morphs)., A slender caudal peduncle.

Dorsal fin

Consists of 3 (rarely 2 or 4) strong, isolated spines followed by a soft-rayed dorsal fin with 8-14 rays.

Tail fin

Slightly forked or truncate.

Pectoral fin

Well-developed, fan-shaped, with 9-11 rays.

Distinctive fins

Pelvic fins are modified into a strong spine and one soft ray, often with a membrane connecting them. Anal fin has 1 spine and 8-10 soft rays.

Identification Tips

1. 1

   Dorsal Spines

   Count the isolated spines on the back. Gasterosteus aculeatus typically has three, distinguishing it from other stickleback species like the Ten-spined Stickleback (Pungitius pungitius) which has 9-12 spines, or the Four-spined Stickleback (Apeltes quadracus) which has 3-5 spines but a different body shape and habitat.

2. 2

   Lateral Plates

   Examine the bony plates along the flanks. The number and extent of these plates are highly variable and can be used to identify different morphs (e.g., 'trachurus' - fully plated, 'leiurus' - partially plated, 'gymnurus' - low plated). This variation is a key characteristic for evolutionary studies.

3. 3

   Breeding Coloration

   During the breeding season, males develop a striking red or orange throat and belly, and iridescent blue eyes. This coloration is a strong indicator of a mature male G. aculeatus.

4. 4

   Size

   Adults typically range from 5-10 cm (2-4 inches) in total length, though some marine forms can reach up to 12 cm.

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