Western Mosquitofish
  • Side view adult.
  • Side view adult.
  • Side view adult.
  • Top view adult.
  • Bottom view adult.
  • Head-on view adult.
  • Close-up flank.
  • Close-up head.
  • Close-up mouth and snout.
  • Close-up tail.
  • Close-up dorsal fin.
  • Close-up anal fin.
  • Close-up pectoral fin.
  • Close-up pelvic fin.
  • Close-up head: The mouth is superior; there is a bar/blotch below the eye and a groove between upper snout and lip.
  • Close-up head: The mouth is superior and there is a continuous groove between upper snout and lip.
  • Notice the solid color; there are 29-32 lateral scales.
  • There is a thick dark lateral stripe, the dorsal origin is behind the anal origin, the pelvic fins are ahead of the dorsal origin.
  • Side view of a breeding male: The first anal rays are long and fused to form a gonopodium.
  • Side view of a breeding male: The first anal rays are long and fused to form a gonopodium.
  • Side view of a pregnant female: Notice that the anal fin rays are not elongated or fused, the belly is distended and there is a dark spot by the anus.
DescriptionSimilar Species
Livebearers (Poeciliidae)
Mouth and Snout: Superior with small teeth in jaws. No barbels. Body Pattern: Tan to olive solid cover or with a few fine dark spots, especially dorsally. Dark bar or blotch under eye. Belly cream or white. Paired fins generally clar but dorsal and tail fins usually with fine black spots. Lateral line absent; 29-32 cycloid lateral scales. Body Shape and Size: Body fusiform, somewhat laterally compressed in males; oval to round in cross-section. Typically 25-40 mm (1-1.5 in) TL; maximum (females) about 71 mm (2.8 in).
Rounded tail. Single dorsal with no spines and 6-8 (usually 7) rays. Pelvic fins abdominal. Adipose fin absent. Anal fin with 9 rays, fused to form a gonopodium in males.
Distinguishing Characteristics
None. See also similar species information.
Breeding Adults
Males have the anal fin rays fused to form a long thin gonopodium, and they do not reach as large a size as females. Pregnant females often have a belly distended with developing young and a dark blotch around the urogenital pore (photos).
Similar to adults.
None known from Wisconsin.
Compare Similar Species
Lateral Scales
Killifish, Banded39-43
Mosquitofish, Western29-32
Mudminnow, Central34-37
Topminnow, Blackstripe31-36
Topminnow, Starhead30-34
Dorsal Fin Origin
Killifish, BandedAhead of anal origin
Mosquitofish, WesternBehind anal origin
Mudminnow, CentralAhead of anal origin
Topminnow, BlackstripeBehind anal origin
Topminnow, StarheadBehind anal origin
Color and Pigment Pattern
Killifish, BandedSilvery with irregular dark vertical bars (often faint) on flanks; no dark bar or blotch below eye
Mosquitofish, WesternSolid tan or olive, sometimes with fine spots, especially dorsally; dark bar or blotch below eye
Mudminnow, CentralDark olive green to brown black, mottled with up to 14 distinct to vague dark brown vertical bars; belly yellow to white; dark bar or blotch below eye
Topminnow, BlackstripeOlive with thick dark lateral stripe, sometimes with finer incomplete vertical bars giving stripe a zig-zag appearance; no dark bar or blotch below eye
Topminnow, StarheadOlive or silvery with either fine dark vertical bars or lateral stripes (consisting of rows of fine spots); dark bar or blotch below eye
Similarity Index
Killifish, BandedVery similar
Mosquitofish, WesternN/A
Mudminnow, CentralModerately similar
Topminnow, BlackstripeVery similar
Topminnow, StarheadVery similar
Mosquitofish, WesternNot established in WI, but stocked in the past and known from Illinois north of Chicago