How to Identify Western Mosquitofish

To identify Western Mosquitofish (Gambusia affinis), look at these key features.

Physical Characteristics

Body shape

elongated

Coloration

Generally dull, olive-green to brownish-gray on the back, fading to silvery-white on the belly. Some individuals may have faint dark spots or a lateral stripe. Females often have a dark gravid spot near the vent when pregnant.

Scale type

cycloid

Mouth position

superior

Distinctive features

Small size, typically under 7 cm (2.8 inches), Upward-pointing mouth, Rounded caudal fin, Males possess a prominent, elongated gonopodium (modified anal fin for internal fertilization), Females are generally larger and more robust than males

Dorsal fin

Single, short-based dorsal fin located far back on the body, typically with 6-9 rays.

Tail fin

Rounded caudal fin.

Pectoral fin

Paired pectoral fins, relatively small.

Distinctive fins

Anal fin of males is modified into a gonopodium, a long, pointed intromittent organ used for internal fertilization. Females have a fan-shaped anal fin.

Identification Tips

1. 1

   Size and Shape

   Small, slender body, typically 3-5 cm (1.2-2 inches) for males and 4-7 cm (1.6-2.8 inches) for females. The body is somewhat compressed laterally.

2. 2

   Mouth

   Distinctive upturned mouth, indicating its surface-feeding habits.

3. 3

   Coloration

   Generally drab, olive-gray to light brown, with a lighter belly. Some individuals may have a dark spot below the eye or a faint lateral stripe. Females often display a dark 'gravid spot' near the vent when pregnant.

4. 4

   Male Gonopodium

   The most definitive feature for males is the long, pointed, and highly mobile gonopodium, which is a modified anal fin. Females have a normal, fan-shaped anal fin.

5. 5

   Dorsal Fin Position

   The single dorsal fin is set far back on the body, almost directly above the anal fin.

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