The Cyprinodontiformes (icon) is an order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of ray-finned fish

Actinopterygii

The Actinopterygii or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines , as opposed to the fleshy, lobed fins that characterize...

, comprising mostly small, fresh-water fish. Many popular aquarium fish, such as killifish

Killifish

A killifish is any of various oviparous cyprinodontiform fish . Altogether, there are some 1270 different species of killifish, the biggest family being Rivulidae, containing more than 320 species...

 and live-bearers

Poeciliidae

Poeciliidae is a family of fresh-water fish which are live-bearing aquarium fish . They belong to the order Cyprinodontiformes, tooth-carps, and include well-known aquarium fish such as the guppy, molly, platy, and swordtail...

, are included. They are closely related to the Atheriniformes

Atheriniformes

Atheriniformes, also known as the silversides, is an order of ray-finned fish that includes the Old World silversides and several less-familiar families, including the unusual Phallostethidae...

 and are occasionally included with them. A colloquial term for the order as a whole is toothcarps, though they are not actually close relatives of the true carp

Carp

Carp are various species of oily freshwater fish of the family Cyprinidae, a very large group of fish native to Europe and Asia. The cypriniformes are traditionally grouped with the Characiformes, Siluriformes and Gymnotiformes to create the superorder Ostariophysi, since these groups have certain...

s – the latter belong to the superorder Ostariophysi

Ostariophysi

Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains almost 8,000 species, about 28% of known fish species in the world and 68% of freshwater species, and are present on all major continents except Antarctica...

, while the toothcarps are Acanthopterygii

Acanthopterygii

Acanthopterygii is a superorder of bony fishes in the class Actinopterygii. Members of this superorder are also known as the ray-finned fishes for the characteristic sharp, bony rays in their fins; however this name is also often given to the class Actinopterygii as a whole.Orders:* Order...

.

The families of Cyprinodontiformes can be divided into three groups: viviparous

Vivipary

Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs...

 and ovoviviparous

Ovoviviparity

Ovoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch...

 (all species give live birth

Live birth

In human reproduction, a live birth occurs when a fetus, whatever its gestational age, exits the maternal body and subsequently shows any sign of life, such as voluntary movement, heartbeat, or pulsation of the umbilical cord, for however brief a time and regardless of whether the umbilical cord or...

), and oviparous

Oviparity

Oviparous animals are animals that lay eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, reptiles, all birds, the monotremes, and most insects, some molluscs and arachnids....

 (all species egg-laying). The live-bearing groups differ in whether the young are carried to term within (ovoviviparous) or without (viviparous) an enclosing eggshell. Phylogenetically however, one of the two suborders – the Aplocheiloidei – contains oviparous species exclusively, as do two of the four superfamilies of the other suborder (the Cyprinodontoidea and Valencioidea of the Cyprinodontoidei). Vivipary and ovovivipary have evolved independently from oviparous ancestors, the latter possibly twice.

Description

Members of this order are notable for inhabiting harsh environments, such as saline

Saline water

Saline water is a general term for water that contains a significant concentration of dissolved salts . The concentration is usually expressed in parts per million of salt....

 or very warm waters, water of poor quality, or isolated situations where no other types of fish occur. They are typically omnivore

Omnivore

Omnivores are species that eat both plants and animals as their primary food source...

s, and often live near the surface, where the oxygen-rich water compensates for environmental disadvantages.

They are small to medium fish, with small mouths, large eyes, a single dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

, and a rounded caudal fin. The largest species is the cuatro ojos (Anableps dowi), which measures 34 centimetres (13.4 in) in length, while the smallest, the least killifish (Heterandia formosa), is just 8 millimetre (0.31496062992126 in) long as an adult.

Systematics

ORDER CYPRINODONTIFORMES

• Suborder Aplocheiloidei (all oviparous)
  • Family Aplocheilidae
    Aplocheilidae
    Aplocheilidae are a family of bony fishes containing about 15 species. Formerly, the name "Aplocheilidae" was used for the Aplocheiloidei as a whole, i.e. including the Nothobranchiidae of Africa and the mainly South American Rivulidae as subfamilies. This obsolete system is sometimes still seen,...
    
     – South Asian and Malagasy
    Madagascar
    The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
    
     killifishes (also the West Africa Pseudepiplatys annulatus)
  • Family Nothobranchiidae
    Nothobranchiidae
    Nothobranchiidae are a family of bony fishes containing roughly 300 species. They are small killifish, usually measuring about . They are essentially limited to Africa, and were formerly included in the family Aplocheilidae...
    
     – African killifishes, formerly in Aplocheilidae
  • Family Rivulidae
    Rivulidae
    Rivulidae are a family of killifishes in the order Cyprinodontiformes. They are commonly known as rivulids, South American killifish or New World killifish. The latter names are slightly misleading however, as they are neither restricted to South America – though most are in fact found there...
    
     – rivulids or South American killifishes
• Suborder Cyprinodontoidei
  • Superfamily Funduloidea (oviparous)
    • Family Profundulidae
      Profundulidae
      Profundulidae is a family of teleost fish endemic to Mexico, Guatemala and other countries of Central America. This family is monotypic with only a single genus, Profundulus...
      
       – Central American killifishes (oviparous)
    • Family Goodeidae
      Goodeidae
      Splitfins, are a family, Goodeidae, of teleost fish endemic to Mexico and some areas of the United States. This family contains 40 species within 18 genera. The family is named after ichthyologist George Brown Goode.-Range and Geographic Distribution:...
      
       – splitfins (viviparous)
    • Family Fundulidae
      Fundulidae
      Fundulidae is the family of topminnows and North American killifishes. The 46 species are native to North America as far south as Yucatan, and to the islands of Bermuda and Cuba, occurring in both freshwater and marine environments....
      
       – topminnows and North American killifishes (oviparous)
  • Superfamily Valencioidea (oviparous)
    • Family Valenciidae – Mediterranean killifishes
  • Superfamily Cyprinodontoidea (oviparous)
    • Family Cyprinodontidae – pupfishes
  • Superfamily Poecilioidea
    • Family Anablepidae
      Anablepidae
      Anablepidae is a family of freshwater and brackish water fishes living on river estuaries from southern Mexico to southern South America. There are three genera with sixteen species: the four-eyed fishes , the onesided livebearers and the white-eye, Oxyzygonectes dovii...
      
       – four-eyed fishes and relatives (ovoviviparous)
    • Family Poeciliidae
      Poeciliidae
      Poeciliidae is a family of fresh-water fish which are live-bearing aquarium fish . They belong to the order Cyprinodontiformes, tooth-carps, and include well-known aquarium fish such as the guppy, molly, platy, and swordtail...
      
      – livebearers and relatives (some oviparous, some ovoviviparous)