''Heterandria formosa'' (known as the least killifish, dwarf topminnow, mosqu or midget livebearer) is a species of livebearing fish within the family

Poeciliidae The Poeciliidae are a family of freshwater fishes of the order Cyprinodontiformes, the tooth-carps, and include well-known live-bearing aquarium fish, such as the guppy, molly, platy, and swordtail. The original distribution of the family was t ...

. This is the same family that includes familiar

aquarium An aquarium (plural: ''aquariums'' or ''aquaria'') is a vivarium of any size having at least one transparent side in which aquatic plants or animals are kept and displayed. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aq ...

fishes such as

guppies The guppy (), also known as millionfish and rainbow fish, is one of the world's most widely distributed tropical fish and one of the most popular List of freshwater aquarium fish species, freshwater aquarium fish species. It is a member of the ...

and mollies. ''Heterandria formosa'' is not as commonly kept in aquaria as these species. Despite the common name "least killifish", it belongs to the family

and not to one of the

killifish A killifish is any of various oviparous (egg-laying) cyprinodontiform fish (including families Aplocheilidae, Cyprinodontidae, Fundulidae, Profundulidae and Valenciidae). All together, there are 1,270 species of killifish, the biggest family ...

families. ''H. formosa'' is one of the smallest fish species; the 1991 ''Baensch Aquarium Atlas'' listed it as the 7th smallest fish in the world, and it remains the smallest fish species found in North America.

Range and habitat

''Heterandria formosa'' is the only member of its genus found in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. Its geographical range spans the southeastern United States, from

South Carolina )''Animis opibusque parati'' ( for, , Latin, Prepared in mind and resources, links=no) , anthem = " Carolina";" South Carolina On My Mind" , Former = Province of South Carolina , seat = Columbia , LargestCity = Charleston , LargestMetro = ...

south to

Georgia Georgia most commonly refers to: * Georgia (country), a country in the Caucasus region of Eurasia * Georgia (U.S. state), a state in the Southeast United States Georgia may also refer to: Places Historical states and entities * Related to the ...

and

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

, and westward across the Florida Gulf Coast to

Louisiana Louisiana , group=pronunciation (French: ''La Louisiane'') is a state in the Deep South and South Central regions of the United States. It is the 20th-smallest by area and the 25th most populous of the 50 U.S. states. Louisiana is borde ...

. In recent years, this species has been collected in eastern Texas. It is recorded to occur along the western portion of the Sabine River basin, according to North American Native Fishes (NANFA). 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 that is

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...

to North America. ''Heterandria formosa'' live primarily in vegetated, slow moving or standing

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

habitats, but this species also occurs in

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

waters.

Description

''Heterandria formosa'' is one of the smallest fish and smallest

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

s known to science. Males grow to about 2 centimeters (0.8 inches), while females grow a little larger, to about 3 centimeters (1.2 inches). 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 A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...

and females also have a dark spot on their

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

. Like most poeciliids, male anal fins are modified into a

gonopodium Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

(intromittent organ) that is used to deliver sperm and impregnate females during mating.

Diet

''Heterandria formosa'' primarily eats aquatic

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s such as

worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wor ...

s and

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

s. They also eat plant matter.

Breeding

Like most poeciliids, ''H. formosa'' is a

livebearer Livebearers are aquarium fish that retain the eggs inside the body and give birth to live, free-swimming young. Among aquarium fish, livebearers are nearly all members of the family Poeciliidae and include guppies, mollies, platies and swordtails ...

. The male uses his modified anal fin, or

, to deliver sperm to the female. The fertilized eggs grow within the female until they hatch, and the free-swimming young are released into the water. ''Heterandria formosa'' has an uncommon breeding strategy even among livebearers: rather than all of 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.

Inbreeding depression

The effect of

inbreeding Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and o ...

on reproductive behavior was studied in ''H. formosa'' in at least one published work. 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 Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding (the breeding of related individuals). Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. In ...

were offspring viability and maturation time of both males and females.

References

External links

"Heterandria formosa - Der Zwergkärpfling", German language
{{Taxonbar, from=Q244221 Poeciliidae Endemic fauna of the United States Freshwater fish of the United States Freshwater fish of the Southeastern United States Fish described in 1859 Taxa named by Charles Frédéric Girard