Heterandria formosa

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Heterandria formosa
Least killifish female Heterandria formosa.jpg
Adult female
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Cyprinodontiformes
Family: Poeciliidae
Genus: Heterandria
Species: H. formosa
Binomial name
Heterandria formosa
Girard, 1859 [1]

Heterandria formosa (known as the least killifish, mosqu or midget livebearer)[2] is a species of livebearing fish within the family Poeciliidae. This is the same family that includes familiar aquarium fishes such as guppies and mollies. H. formosa is not as commonly kept in aquaria as these species. H. formosa is one of the smallest fish in the world (7th smallest as of 1991),[3] and is the smallest fish found in North America.[4] Despite the common name "least killifish", it belongs to the family Poeciliidae and not to one of the killifish families.

Range and habitat[edit]

Heterandria formosa is the only member of its genus to be found in the United States.[5] Its range covers the southeastern United States, from South Carolina south to Georgia and Florida, and through the Florida Gulf Coast to Louisiana.[5][6] In recent years, this species has been seen in eastern Texas. It is recorded to be in the west side of the Sabine river, according to North American Native Fishes.[7] It has also been collected as far west as Humble, TX in small sand pit ponds after the 2017 floods associated with hurricane Harvey. It is one of the few aquarium fishes to come from North America.

H. formosa lives primarily in vegetated, slow moving or standing freshwater but also occurs in brackish waters.[6]


Heterandria formosa is one of the smallest fish and smallest vertebrates known to science.[5] Males grow to about 2 centimeters (0.8 inches), while females grow a little larger, to about 3 centimeters (1.2 inches).[5][8]

The fish is generally an olive color, with a dark horizontal stripe through the center of the body. There is also a dark spot on the dorsal fin and females also have a dark spot on their anal fin. Like most poeciliids, males' anal fins are modified into a gonopodium that is used for impregnating females during mating.


Heterandria formosa primarily eats invertebrates such as worms and crustaceans.[6] They also eat plant matter.[6]


Like most poeciliids, H. formosa is a livebearer. The male uses his modified anal fin, or gonopodium, to deliver sperm to the female. The fertilized eggs grow within the female until they hatch, and the young are released free swimming. H. formosa has an uncommon breeding strategy even among livebearers. Rather than all the young being released at once, as many as 40 fry are released over a 10- to 14-day period, but occasionally over a longer period.[3][5][8]

Inbreeding depression[edit]

The effect of inbreeding on reproductive behavior was studied in H. Formosa.[9] One generation of full-sib mating was found to decrease reproductive performance and likely reproductive success of male progeny. Other traits that displayed inbreeding depression were offspring viability and maturation time of both males and females.


  1. ^ Nicolas Bailly (2010). Bailly N, ed. "Heterandria formosa Girard, 1859". FishBase. World Register of Marine Species. Retrieved May 29, 2012. 
  2. ^ "Common names of Heterandria formosa". FishBase. Retrieved May 29, 2012. 
  3. ^ a b Baensch, H. (1991). Baensch Aquarium Atlas. pp. 592–593. ISBN 3-88244-050-3. 
  4. ^ Jason C. Chaney & David L. Bechler (2006). "The occurrence and distribution of Heterandria formosa (Teleostei, Poeciliidae) in Lowndes County, Georgia" (PDF). Georgia Journal of Science. 64 (2): 67–75. Archived from the original (PDF) on 2013-10-14. 
  5. ^ a b c d e Dawes, J. (1995). Livebearing Fishes. pp. 186–187. ISBN 0-7137-2592-3. 
  6. ^ a b c d "Fishbase Heterandria formosa". Retrieved 2008-08-10. 
  7. ^ Combest, Lisa. "North American Native fishes". North American Native Fishes Forum. North American Native Fishes. Retrieved 5 March 2018. 
  8. ^ a b Dawes, J. (2001). Complete Encyclopedia of the Freshwater Aquarium. p. 276. ISBN 1-55297-544-4. 
  9. ^ Ala-Honkola O, Uddström A, Pauli BD, Lindström K (2009). "Strong inbreeding depression in male mating behaviour in a poeciliid fish". J. Evol. Biol. 22 (7): 1396–406. doi:10.1111/j.1420-9101.2009.01765.x. PMID 19486236. 

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