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

Poeciliidae Poeciliidae are a Family (biology), family of freshwater ray-finned fishes of the order Cyprinodontiformes, the tooth-carps, and include well-known live-bearing aquarium fish, such as the guppy, Poecilia, molly, Platy (fish), platy, and Green swo ...

. This is the same family that includes familiar

aquarium An aquarium (: 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. fishkeeping, Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aquati ...

fishes such as guppies 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, Pantanodontidae, Cyprinodontidae, Fundulidae, Nothobranchiidae, Profundulidae, Aphaniidae and Valenciidae. All together, there ar ...

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 (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. Its geographical range spans the southeastern United States, from

South Carolina South Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders North Carolina to the north and northeast, the Atlantic Ocean to the southeast, and Georgia (U.S. state), Georg ...

south to

Georgia Georgia most commonly refers to: * Georgia (country), a country in the South Caucasus * Georgia (U.S. state), a state in the southeastern United States Georgia may also refer to: People and fictional characters * Georgia (name), a list of pe ...

and

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

, and westward across the Florida Gulf Coast to

Louisiana Louisiana ( ; ; ) is a state in the Deep South and South Central regions of the United States. It borders Texas to the west, Arkansas to the north, and Mississippi to the east. Of the 50 U.S. states, it ranks 31st in area and 25 ...

. 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 Hurricane Harvey was a devastating tropical cyclone that made landfall in Texas and Louisiana in August 2017, causing catastrophic flooding and more than 100 deaths. It is tied with 2005's Hurricane Katrina as the costliest tropical cy ...

. It is one of the few aquarium fishes that is

endemic Endemism is the state of a species being found only 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 foun ...

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. The term excludes seawater and brackish water, but it does include non-salty mi ...

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 () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

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 on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

and females also have a dark spot on their

anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...

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

gonopodium Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

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

Diet

''Heterandria formosa'' primarily eats aquatic

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s such as

worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and usually no eyes. Worms vary in size from microscopic to over in length for marine ...

s and

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s. They also eat plant matter.

Breeding

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

livebearer Livebearers are fish that retain their eggs inside the body and give birth to live, free-swimming young. They are especially prized by aquarium owners. Among aquarium fish, livebearers are nearly all members of the family Poeciliidae and include: ...

. 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 genetic distance, related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genet ...

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 caused by loss of genetic diversity as a consequence of inbreeding, the breeding of individuals closely related genetically. This loss of genetic diversity results from small population siz ...

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 fish 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