A killifish is any of various
oviparous (egg-laying)
cyprinodontiform fish,[1] including families
Aplocheilidae,
Cyprinodontidae,
Fundulidae,
Nothobranchiidae,
Profundulidae and
Valenciidae. All together, there are 1,270 species of killifish, the biggest family being
Rivulidae, containing more than 320 species.[2] As an adaptation to living in
ephemeral waters, the eggs of most killifish can survive periods of partial dehydration. Many of the species rely on such a
diapause, since the eggs would not survive more than a few weeks if entirely submerged in water. The adults of some species, such as Kryptolebias marmoratus, can additionally survive out of the water for several weeks.[3] Most killies are small fish, from 2.5 to 5 centimetres (1 to 2 in), with the largest species growing to just under 15 centimetres (6 in).
The word killifish is of uncertain origin, but is likely to have come from the
Dutchkil for a
kill (small stream).[4] Although killifish is sometimes used as an English equivalent to the
taxonomical term Cyprinodontidae, some species belonging to that family have their own common names, such as the
pupfish and the
mummichog.
Range and habitat
Killifish are found mainly in fresh or brackish waters in the Americas, as far south as
Argentina and as far north as southern
Ontario and even
Newfoundland and Labrador.[5] There are also species in southern
Europe, in much of
Africa as far south as
KwaZulu-Natal,
South Africa, in the Middle East and Asia (as far east as
Vietnam), and on several Indian Ocean islands.
The majority of killifish are found in permanent streams, rivers, and lakes, and live between two and three years. Such killifish are common in the Americas (Cyprinodon, Fundulus and Rivulus) as well as in Africa and Asia (including Aphyosemion, Aplocheilus, Epiplatys, Fundulopanchax and Lacustricola) and southern Europe (Aphanius). Some of these habitats can be rather extreme; the only natural habitat of the
Devils Hole pupfish (Cyprinodon diabolis) is
Devils Hole: a cavern at least 91 metres (299 ft) deep, branching out from a small opening at the surface, approximately 1.8 metres (6 ft) by 5.5 metres (18 ft) wide.
Some specialized forms live in temporary ponds and flood plains, and typically have a much shorter lifespan. Such species, known as "annuals", live no longer than nine months, and are used as models for studies on aging. Examples include the African genus Nothobranchius and South American genera ranging from the cold water Austrolebias of
Argentina and
Uruguay to the more tropical Gnatholebias, Pterolebias,[6]Simpsonichthys and Terranatos.
Territorial behaviour
A small number of species will shoal while most are territorial to varying degrees. Populations can be dense and territories can shift quickly, especially for species of the extreme shallows (a few centimetres of water). Many species exist as passive tribes in small streams where dominant males will defend a territory while allowing females and immature males to pass through the area. In the aquarium, territorial behaviour is different for every grouping, and will even vary by individuals. In a large enough aquarium, most species can live in groups as long as there are more than three males.
Diet
Killifish feed primarily on aquatic arthropods such as insect (
mosquito)
larvae, aquatic
crustaceans and worms. Some species of Orestias from
Lake Titicaca are
planktonicfilter feeders. Others, such as Cynolebias and Megalebias species and Nothobranchius ocellatus are predatory and feed mainly on other fish. The American Flagfish (Jordanella floridae) feeds heavily on algae and other plant matter as well as aquatic invertebrates. Nothobranchius furzeri needs much food because it grows quickly, so when food supplied is inadequate, bigger fish will eat the smaller fish.[7]
In lifespan research
Some
strains have a lifespan as short as several months and can thus serve as a model for
biogerontological studies.[8][9][10][11][12] The African turquoise killifish (Nothobranchius furzeri) is the shortest-living vertebrate that can be bred in captivity, having a lifespan of between three and nine months.[9][10] Sexual maturation occurs within 3–4 weeks, with
fecundity peaking in 8–10 weeks.[13]
Transferring the gut
microbiota from young killifish into middle-aged killifish significantly extends the lifespans of the middle-aged killifish.[20][21]
Many killifish are lavishly coloured and most species are easy to keep and breed in an
aquarium. Specimens can be obtained from specialist societies and associations.
Striped panchax (also known as the Golden Wonder killifish) are commonly found in pet shops, but caution must be exercised when considering tank mates, since the mouth of the Striped panchax is as wide as the head, and much smaller fish will be eaten.
Flagfish, native to south Florida, is another species of killifish commonly found in pet stores. They are useful in aquariums for algae control. The
golden topminnow (Fundulus chrysotus) is also native to the United States and often available at auction, but care must be taken with these fish to stop them from jumping out of the tank. A firm cover and a layer of floating plants is best when keeping these fish.
Behaviour-altering infection
Normally, killifish avoid near-surface water to reduce the danger of being eaten by predators. However, when infected with a type of
fluke the fish swim near the surface, and sometimes even swim upside down, exposing their camouflaged bellies. The fluke completes its lifecycle in the digestive tract of birds.[24]
Evolved resistance to extreme levels of toxicity
The large populations of killifish and the
genetic diversity of the species have enabled it to
evolve and survive in areas where other species have died out, including
Superfund sites. Over a few dozen generations of killifish[25][26] in a relatively short period of time (50–60 years), killifish have evolved resistance against levels of
dioxins, PCBs, mercury, and other industrial chemicals up to 8,000 times higher than the previously estimated lethal dose. Sequencing the genomes of the adapted individuals showed a common set of mutations among the pollution-tolerant fish, many of which help to deactivate or turn off a molecular pathway responsible for a large part of the cellular damage caused by the chemicals.[27]
^Graf M, Hartmann N, Reichwald K, Englert C (2013). "Absence of replicative senescence in cultured cells from the short-lived killifish Nothobranchius furzeri". Experimental Gerontology. 48 (1): 17–28.
doi:
10.1016/j.exger.2012.02.012.
PMID22445733.
S2CID11729069.
^McCann, Michael J; Able, Kenneth W; Christian, Robert R; Fodrie, F Joel; Jensen, Olaf P; Johnson, Jessica J; López-Duarte, Paola C; Martin, Charles W; Olin, Jill A; Polito, Michael J; Roberts, Brian J; Ziegler, Shelby L (April 2017). "Key taxa in food web responses to stressors: the Deepwater Horizon oil spill". Frontiers in Ecology and the Environment. 15 (3): 142–149.
doi:
10.1002/fee.1474.