The Least Killifish (Heterandria formosa): A great little fish that isn’t a killifish!

Heterandria formosa is a nice little fish native to the Southeast United States. The common name for this petite fish is the Least Killifish, but this is somewhat of a misnomer. The Least Killifish is…well…not a killifish. It is in fact a livebearer in the family poecilidae, the family that mollies and swordtails are members of. Why this fish is called the Least Killifish when it is in fact not one, I don’t know. Heterandria formosa is an attractive fish with nice olive coloration and a red dorsal fin. Some Carolina populations are known to throw amelanistic morphs known as sometimes sold as Heterandria formosa “Gold”.

Pregnant Female Heterandria formosa. This is a very easy livebearer to keep and breed.

Heterandria formosa is very diminutive and is the smallest known livebearing species in the world. A large female would be slightly over an inch while large males are around 3/4″ in length. This makes this a great fish for nano aquariums. I keep mine in a 5 gallon tank with Red Cherry Shrimp and Tylomelania snails, both of which they get along with perfectly. While the Heterandria formosa may pick off some baby Red Cherry Shrimp, I still get new shrimplets all the time.

Heterandria formosa is a very easy fish to breed. Males can be differentiated from females by the presence of a gonopodium, a skinny and elongate modified anal fin used for reproduction. Males also stay smaller than females on average.

Heterandria formosa male. This species is easy to sex as males have a distinct modified anal fin, a gonopodium.

Heterandria formosa has an interesting breeding strategy in that it is superfetation. This is where the female is nurturing more than one fetus at a time, but the fetuses are at different stages of development. This results in females giving birth to 1-2 fry every few days. The gestation period for this fish is about 4 weeks.  The fry are extremely small, only a bout 3-4 millimeters in length. They can eat baby brine shrimp right away and grow quickly. The parents generally will not eat their fry, but it is still advised to keep plants in the tank as refuge for the young. I personally like to use Java Moss as the adults won’t swim into it but the fry will.

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One Response to The Least Killifish (Heterandria formosa): A great little fish that isn’t a killifish!

  1. Hy says:

    Called Least Killi which was Dutch meaning smallest fish in pond.

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