Origin of the name Killifish.

Killifish are secondary freshwater fishes, i.e., primarily living in freshwaters today but originating from marine coastal waters in the remote past.

Their common name, Killi (plural, Killies), stems from an old Dutch name, used by immigrants of USA in the seventeenth century to qualify fishes living in small channels, the "kills", that reminded them their home country.

Progressively, the name has been extended to other groups of related fishes from northern America, Mediterranean countries and finally tropical countries, when aquarists of the twentieth century became passionately interested by these fishes.

Systematics of Killifish.

Scientifically speaking, Killifish are Teleosts (vertebrate fishes), members of the superorder Atherinomorpha, of the order Cyprinodontiformes, with current relatives in related orders Beloniformes (Oryzias), Atheriniformes (Atherinids, such as Bedotia). This upper classification is solidly established by morphology, osteology and genetics (Setiamarga et al., 2008).

Cyprinodontiformes -or shortly Cyprinodonts- are split artificially into oviparous species, properly Killifish (ca. 1100 valid species in 2019), and viviparous groups (ca. 350 valid species in 2019), properly livebearers, featuring Guppies, Xiphos, famous aquarium fishes, or the beautiful Splitfins (Goodeins), or Gambusies, (unfortunately) planted all over the world, for mosquito control, or 4 Eyed-Fish (Anablepsids).


Killifish, the herein aggregated common name for all, oviparous and viviparous cyprinodontiformes fishes, are currently classified into 2 suborders (Aplocheiloidei, i.e., rivulins with hereafter first-listed 3 families, and Cyprinodontoidei, killifish in a strict sense, mostly, but also, pupfishes, toothcarps, lampeyes and livebearers, including splitfins) and in 14 families (full details HERE, constantly updated):
- Aplocheilidae, oviparous, only from India and Madagascar;
- Nothobranchiidae, oviparous, from Africa except Madagascar with 2 subfamilies: Nothobranchiinae and Epiplatinae;
- Rivulidae, oviparous, from Central and South America with 3 subfamilies: Rivulinae, Cynolebiinae and Kryptolebiatinae;
- Anablepsidae, with 3 subfamilies: Anablepsinae, viviparous, the monotypic Oxyzygonectinae, oviparous, and Jenynsiinae, viviparous, from Central and South America;
- Aphaniidae, oviparous, monotypic, from Europe, the Middle East, up to Pakistan and northwestern coastal India;
- Cyprinodontidae, oviparous, with 2 subfamilies: Cubanichthyinae (with 2 tribes, Cubanichthyini, Yssolebiini) and Cyprinodontinae (with 2 tribes, Cyprinodontini and Orestiadini), from North, Central and South America;
- Fluviphylacidae, oviparous, monotypic, from South America.
- Fundulidae, oviparous, from North and Central America;
- Goodeidae, with 2 subfamilies: Empetrichthyinae, oviparous and Goodeinae, viviparous, from North and Central America;
- Pantanodontidae, oviparous, previously monotypic, today with 4 extant genera, from Eastern Africa including Madagascar;
- Poeciliidae, with 3 subfamilies, from North, Central and South America: Poeciliinae viviparous, Tomeurinae, oviparous, monotypic, Xenodexiinae, viviparous, monotypic);
- Procatopodidae, oviparous, from Africa (in 2 subfamilies, Aplocheilichthyinae, monotypic, and Procatopodinae with 2 tribes, Procatopodini and Micropanchini);
- Profundulidae, oviparous, bitypic, from Central America;
- Valenciidae, oviparous, monotypic, from Europe;
These 14 families are also accepted in active international databases on all fishes to ensure coherence and universality, even if the major move from 7 to 14 families is very recent (2018) and is more reflecting the splitting trends in present systematics than a parsimonious analysis (a solid publication by Piller et al. in 2022 even goes to 16 families, likewise another solid and global phylogeny by Morales et al., 2024). The contents of lower levels depend more on authors : a consensus and conservative view is selected herein, fully in line with ICZN recommendations. Future general trend is probably moving to even more divisions at all family-group names levels, even if the pyramid may balance back to fewer families and consequently more lower levels in an un-foreseeable future (notably the first, but preliminary, analysis, by Teimori & Motamedi, in 2019, based on full mitogenome, discloses that family Aphaniidae is probably ambiguous and recent results tend to amalgamate Valenciidae into Aphaniidae).


Unlike other fish such as Characoids or Percomorphs which inhabit fresh water and yet show great morphological range, the egg-laying and live-bearing Cyprinodonts (gonopodium apart!) form a remarkably homogeneous entity; alongside this morphological homogeneity has been demonstrated an extreme heterogeneity in genotype and in male color pattern, which can be translated into genera comprising fifty or more species. In addition, here and there, an atypical species, often dwarf, often relict, differs from the homogeneous norm and is given its own genus name (or subgenus, depending on the author). There are at least 32 monotypical genera or subgenera today out of a total of over 105 genera or subgenera deemed valid. These important genera are today very difficult to place in the suprageneric phylogeny of Cyprinodonts.

Biogeography, habitat, and behavior of Killifishes.

Killifishes, or oviparous and viviparous Cyprinodonts, occur all over the world, in tropical and temperate regions of Africa, America and Eurasia, except in northern and eastern Europe (today, but fossils are known) and in Australasia, east of the Wallace line. Then, they can be found all over Africa (except generally in the desert of Sahara and neighbouring regions, and of the extreme southwestern Africa), all over South America (except Patagonia and the south-western Pacific coastal plain), all over Central America, all over North America, including Canada (except its polar regions), all over Mediterranean countries and the Middle East (except deserts), and from Pakistan to Vietnam and Indonesia, up to Lombok (or New Papua if not artificially introduced).

Killifishes, or oviparous and viviparous Cyprinodonts, live in reclusive waters (usually creeks, temporary pools, overfloods, and margins of lakes and rivers), primarily in coastal lowlands, but also in highlands, where they are quasi exclusive (photos in BIOTOPES): they are then very different from the standard ichthyofauna that lives in rivers, such as Cichlids, Barbs, Characids, etc.


Killifishes, or oviparous and viviparous Cyprinodonts, feature some of the oddest fishes, with very adaptative and specialized behaviors, such as:

  • Kryptolebias marmoratus-ocellatus, a selfish hermaphrodite, inhabiting mangroves from the Florida panhandle, U.S.A. to the Sao Paulo latitude in southeastern Brasil;
  • At least 2 gynogenetic species (producing all-females clones), result of natural hybridization in the wild, one named, in genus Poecilia (formosa), the others un-named, in genus Poeciliopsis (because speciation is not finalized), as complex of species, such as lucida-monacha or monacha-occidentalis or monacha-viriosa ;
  • Cyprinodon diabolis, extremely endangered, restricted to a single cave of about 10 meters in Devil's Hole (the smallest range of any fish in the world), in western desert of U.S.A., with the smallest population of a fish species (around 500 specimens up to 2000, but severely declining since… just a few dozen in 2007 and lately, back to 400 specimens… but with safe strains in artificial refuges, not far);
  • More than 2 hundred species, in tropical Africa and America, are annual species, living in seasonal biotopes only filled with water during the rainy season and dry otherwise, and spawning eggs that needs several weeks or months to incubate in dry mud, i.e. without water ;
  • Several hundred species that are livebearers, either truly viviparous (mother feeding embryos and larvae during internal development through sorts of ombilical cords called trophotaeniae) like relict Splitfins (Goodeinae), with about 40 species, living in a dry plateau of central Mexico, or ovoviviparous (mother feeding only embryos, and not always, during internal development), with about 200 species (Poeciliinae) living all over Americas having in male a complete modification of Anal fin into various morpho-types of gonopodium as intromittent copulatory organs;
  • Nothobranchius furzeri, one of these annual species, having the shortest lifespan of all vertebrate animals, in total less than 6 months in the wild from egg hatching to death by aging, and even a bit less for some populations;
  • Divers, which cannot be seen during spawning and bury their eggs well into the substratum : these annual killifishes are famous as "instant fish" in the 1950'ies when a consumer goods company offers to their customers a small bag with eggs that become fish within a few minutes when put in water;
  • Schooling fishes or lampeyes, because they bear a lightning spot on eye, members of the genera Lamprichthys, Procatopus, or Plataplochilus, spawning for example in rock holes, or others in huge lakes in deep waters;
  • Intermediate species between oviparous and viviparous fishes with internal fertilization but external development, like for Campellolebias species or Tomeurus gracilis;
  • Egg-stranding, aestivating and jumping-out-of-the-water species, like most Rivulus species (in a large sense), able to wander several hundred meters or spring several dozens of centimetres to better catch the insects for food or simply look for better biotopes;
  • Cannibalising species, that feed on their smaller congeners in the same biotope, like Cynolebias, Megalebias, Moema or Paranothobranchius species;
  • Adaptative species to extreme conditions in waters with very high sulfuric contents (Aphanius, Jenynsia) or in high altitude waters above 4000 m altitude with very low oxygen dissolved (Orestias), nearly naked, i.e. with strong reduction of the number of scales on sides and head, some being even with no scale at all, such as Aphanius asquamatus, Aphanius furcatus, Orestias minutus, Orestias minimus;
  • A unique parental caring species in a monotypic genus, Jordanella floridae;
  • Adaptative species to extreme conditions, such as hypersaline reefs like some Cyprinodon or Aphanius species, with a trophic speciation similar to that encountered in the Rift African Lake Cichlids, or, as desert permanent creeks or springs, like some other Cyprinodon, with water temperatures above + 55°C as peak (unique, for fish in the world);
  • … and many other oddities that are to be discovered in a small gallery of their BEHAVIORS, within the database for each species and in the answers to each individual QUIZ scattered throughout the website… and most species feeding on insects, mosquitoes and their larvae, hence being the sole natural weapon against malaria disease which kills millions and millions of men and women, notably kids, each year on earth, in tropical countries where killifish live, often single fish to hatch, grow, breed and die in those unique and adverse conditions.


Interesting Killifishes.

Killifishes, or oviparous and viviparous Cyprinodonts, can be interesting, depending on the view point:

Killifishes, or oviparous and viviparous Cyprinodonts, are among the most beautiful freshwater aquarium fishes: look to the few photos of their BEAUTIES (there are over 3000 photos in the data base!) and you will understand why they are so attractive to aquarists and nature-friends.

Killifishes, or oviparous and viviparous Cyprinodonts, besides combine, for aquarists, the unique qualities of being diverse in their characteristics, of needing at least 2 weeks of incubation (external or internal), and even more for annual species (exchanging eggs by post mail is possible) and of living in small amount of waters (creating a fish room or "Killiarium", holding several dozens of small aquariums at room temperature, is also possible). And aquarists, also nicknamed Killi-hobbyists or Killiphiles or Livebearers-nuts, are grouped in national ASSOCIATIONS to exchange their experiences and fishes.

Killifishes, or oviparous and viviparous Cyprinodonts, are among the most difficult fishes for ichthyological research, because most of their explosive evolution stems from the latest period of Earth history, and biologists, behaviourists, aging specialists, geneticists, molecularists, morphologists, osteologists, palaeontologists combine their efforts and discoveries to ensure scientific progress wherever they are amateurs or professionals in INSTITUTIONS.

This is the whole story of Killi-Data:
to ensure community efforts to gather knowledge on these fishes, 
just as Killifishes, or oviparous and viviparous Cyprinodonts, develop community strategies to face stringent constraints and survive into their highly adverse and fragile environment.

Prospects on Killifishes.

No doubt Killifishes are in every way very different from the standard river fishes and opposite by major traits:
- independent micro-populations of sympatric congeners in reclusive fragile biotopes where they are quasi-exclusive;
- explosive recent genotypes and typified color patterns, together with an amazing basic morphological stability (only 5 basic morphotypes, 3 in Aplocheiloids, 2 in Cyprinodontoids);
- probable recurrent extinction episodes except in refugia, from where a brand new expansion process started again.

By combining data of palaeogeography, palaeoecology, vicariance, endemicity, diversity, internal and external characters of extant phylogenetic lineages, it is possible to hypothesize their centres of origin as well as plausible scenarios of the long term history and migration patterns of Cyprinodonts, since their birth more than 150 Million years ago.

However, the explosive evolution of the tropical Cyprinodonts, is to be found in the near past. Apart from the very old development of the few distinguished morphotypes linked to major Earth events (continental drift), most of the evolution spur is hypothesized to be recent, linked to climatic fluctuations and especially to the late glacial maximums of the Pleistocene-Holocene. The evolution of these fishes, pushed by similar and severe constraints, is playing with a limited number of options. The many similarities between their characteristics in the Old and the New Worlds, or between those of Eurasia and North America plus the Andes, change then somewhat the picture from a purely haphazard convergence into the disclosure of very few options, variably combined as much as possible.

Discover more on Killies? 

  • Their color patterns, their morphs and variability? BEAUTIES of Killies
  • Their typical biotopes and behaviours? ECO-ETHOLOGY of Killies
  • The list of all taxa with their current names, compared to previously USED names
  • The huge knowledge base with thousands of pages, photos and maps and with dozens of lists and statistics on all Cyprinodontiformes is only accessible to K-D-I members, after free REGISTRATION