Skip to main content
Select Source:

killifish

killifish, any of more than a thousand species of small fish of the several families of the order Cyprinodontiformes (toothed minnows or toothed carps), a group that includes also the topminnows and many popular aquarium fishes (e.g., the guppy or rainbow fish, Poecilia reticulata) among its brightly colored tropical species. Most North American toothed minnows are oviparous, i.e., bearing young hatched from eggs, and some are quite colorful; however, the tropical viviparous species (i.e., bearing live young) are preferred for aquariums, since they are easier to raise. Killifishes average from 2 to 4 in. (5–10 cm) in length and have compressed bodies, small mouths with projecting lower jaws, unforked tails, and large scales. They live in ponds, streams, ditches, and salt marshes throughout the United States and feed on insect larvae, crustaceans, and small water plants. The banded killifish is found in the Mississippi basin; the common killifish (5 in./12.5 cm) is an eastern species. Guppies can survive temperatures of up to 100°F (38°C) as can certain topminnows of the W United States. The greenish-gray female guppy (11/2 in./3.75 cm) produces from 12 to 25 live offspring every few weeks; in captivity they must be separated from the cannibalistic adults. The rainbow colors of the male guppy (1 in./2.5 cm) are marked with black spots and bars. Like the guppy, the 2-in. (5-cm) Gambusia, a topminnow of the S Atlantic and the Gulf, bears live young and is important in controlling mosquitoes, on whose larvae both the guppy and the minnow feed. Killifishes are classified in the phylum Chordata, subphylum Vertebrata, class Actinopterygii, order Cyprinodontiformes.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"killifish." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 17 Sep. 2018 <http://www.encyclopedia.com>.

"killifish." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (September 17, 2018). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/killifish

"killifish." The Columbia Encyclopedia, 6th ed.. . Retrieved September 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/killifish

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

killifish

killifish (Fundulus heteroclitus) See CYPRINODONTIDAE.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"killifish." A Dictionary of Zoology. . Encyclopedia.com. 17 Sep. 2018 <http://www.encyclopedia.com>.

"killifish." A Dictionary of Zoology. . Encyclopedia.com. (September 17, 2018). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/killifish

"killifish." A Dictionary of Zoology. . Retrieved September 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/killifish

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

killifish

killifish •raffish • damselfish •catfish, flatfish •garfish, starfish •redfish •elfish, selfish, shellfish •devilfish •crayfish, waifish •stiffish • kingfish • jellyfish •killifish • filefish • pipefish •white fish •offish, standoffish •codfish • dogfish • rockfish • crawfish •swordfish •blowfish, oafish •goldfish •bonefish, stonefish •wolfish •huffish, roughish, toughish •mudfish • monkfish • cuttlefish •lungfish • lumpfish • spearfish •angelfish • parrotfish • silverfish •haggish, waggish •vaguish •biggish, piggish, priggish, whiggish •doggish, hoggish •roguish, voguish •puggish, sluggish, thuggish •largish

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"killifish." Oxford Dictionary of Rhymes. . Encyclopedia.com. 17 Sep. 2018 <http://www.encyclopedia.com>.

"killifish." Oxford Dictionary of Rhymes. . Encyclopedia.com. (September 17, 2018). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/killifish

"killifish." Oxford Dictionary of Rhymes. . Retrieved September 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/killifish

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Killifish

Killifish

Evolution and taxonomy

Ecology

Biology

Economic importance

Ecological importance

Killifish (Fundulus spp.) are small common intertidal fish tolerant of a wide range of temperatures and salinity, found throughout temperate and tropical waters on every continent except Australia and Antarctica. Not to be confused with the other large group of small fish in the same order (Cyprinodontiformes) known as minnows, killifish have an incomplete lateral line, often not extending past the head, and a protruding lower jaw that allows them to feed from the surface. This characteristic gave rise to their other common name top-minnow. Millions have been removed from the East coast of North America since the 1800s for use in a wide variety of experiments, including a recent trip into outer space to determine the effect of weightlessness on fish development and locomotion. Mummichogs (Fundulus het-eroclitus ) are the best known species of killifish.

Evolution and taxonomy

The earliest fossils of the order Cyprinodonti-formes (meaning toothed carp) were found in Europe and date from the early Oligocene (26-37 million years ago [mya]). The most likely ancestor of this order is traced to fossils from the Tethys Sea in the Early Cretaceous (65-136 mya) period. More recent Miocene (12-26 mya) fossils have been located in Kenya. Over time, the species diversified as the continents drifted apart, giving rise to some of the most resilient and tolerant species of fish known on Earth.

Because of their incredible diversification, the class Osteichthyes (bony fishes) has been broken down into several subclasses, including Teleostei, which includes the orders for minnows, killifish, silversides, and others. The killifish taxonomy is quite complex, given the 900 or more different species in the order Cyprinodontiformes and the high degree of endemism resulting from isolated populations. Like other Cyprinodontes, killifish have one dorsal fin that lacks spines. They belong to the suborder Cyprinodontoidei, which includes such interesting families as the cave-fishes (Amblyopsidae) of North America; the live-bearing top minnows, including familiar aquarium fish such as guppies, swordtails, and mollies (Poeciliidae); and the four-eye fishes that can see above and below water (Anablepidae) simultaneously.

The killifish family is known as either Cyprinodontidae or Fundulidae, depending on the taxonomic source. Comprising over 200 species, the Fundulus have been further divided into five subgenera classified by anatomical or geographical characteristics. Among the 34 species in North America, there is a distinct separation of species from East to West, and genetically within species from North to South along the Atlantic coast, as seen in the mummi-chog (F. heteroclitus ). This is probably a result of the combined influences of glaciation and disjunct habitat distribution.

The East Coast species are most numerous in Florida and the Gulf of Mexico, but also include some species from more northern freshwater and brackish regions extending as far north as Newfoundland and the Gulf of St. Lawrence. There are several species found exclusively in the Midwest and West. California has six native species found primarily in the desert, and one introduced species, the rainwater killifish (Lucania parva ), native to the Atlantic and Gulf of Mexico. Due to habitat limitations and disappearance in the wake of development and pumping of groundwater reserves, some of these California species known as pupfish have the most restricted ranges of any known vertebrate. The Devils Hole pupfish (Cyprinodon diabolis) is limited to a 215 square-foot (20 sq m) area of an underwater limestone shelf in a Nevada spring. For at least 10,000 years, a small population (200-700 individuals) has occupied this tiny niche.

Ecology

Characteristic of euryhaline and freshwater habitats, extremes of temperatures, variations in dissolved oxygen and high degree of habitat disturbance are the main parameters shaping the lives of killifishes. The ability to move from fresh to saltwater requires tremendous osmoregulation adjustments. The sheeps-head minnow (Cyprinodon variegatus) is found in marsh ponds from Maine to the West Indies, and can tolerate a wider range of salinity than any other fish. Killifish kidneys have specially adapted renal tubules to process sodium chloride as concentration increases from fresh- to salt water. Special chloride cells in the opercular epithelium of the gills help facilitate this effort for the mummichog (F. heteroclitus).

Metabolic regulation of intercellular activity provides greater hemoglobin-oxygen binding efficiency in mummichogs, allowing them to respond quickly to changes in dissolved oxygen levels, regulate pH, and thus effectively deliver oxygen to muscle tissues. This model may be the same for all killifish, although a genetic component is probably involved, explaining the variety of tolerance levels limiting distribution of certain species.

The pupfishes of the California deserts show the most amazing temperature tolerance, commonly withstanding months of summer water temperatures between 35-40°C (95-104°F), with extremes of 47°C (116.6°F) not uncommon. Very few vertebrates can survive at this thermal level. Their small size (2-3 inches [60-75 mm]), while providing a distinct advantage for survival in shallow, densely populated areas with limited food resources, would seem to present a major thermoregulatory challenge. Desert nights can experience temperatures below freezing as well, at which times the pupfish burrows into detritus on the bottom and remains dormant until temperatures rise again. Since all pupfish species are considered rare or endangered, further study of this remarkable ability awaits better captive rearing techniques.

Another important adaptation necessary for survival in salt marsh habitats is the ability to tolerate high levels of dissolved hydrogen sulfide. This gas is released as a byproduct of anaerobic marsh decomposition and can achieve potentially toxic levels. Experiments with the California killifish (F. parvipinnis ) indicate that metabolic tolerance is achieved by response of cellular mitochondrial oxidation, allowing a tolerance two to three times greater than that of most fishes.

Biology

In general, female killifish are larger than the males (ranging from 2-6 inches [50-155 mm]), although less brightly colored. As the days lengthen and temperatures rise in the spring/summer reproductive season, hormones cause bright colors to appear on males. These aggressive males fight with each other for the privilege of fertilizing a female, preferably a large one capable of laying up to 50,000 eggs per season. Deposition of eggs is closely tied to the tidal cycle for intertidal fishes like the mummichog. Preference is for placing the sticky mass of eggs above the strand line on stalks of marsh grass (Spartina alterniflora ) or in empty mussel shells (Geulensia demissa ) during a spring tide, allowing them to develop in air until the next spring tide washes them back into the water, stimulating them to hatch. Thus the eggs themselves are tolerant of a wide salinity range.

Young fish fry emerge from the eggs nine to 18 days after laying and are considered more develop-mentally advanced than most other fish species. Mortality among the larva and fry is quite high, despite their tendency to take refuge in the shallow intertidal areas beyond reach of the larger fishes and crabs. Given warm temperatures and adequate food resources, size increases throughout the season. While most species achieve reproductive maturity after the first season, few seem to live for more than two to three years. During the winter, many species take refuge in salt marsh pools which provide slightly warmer temperatures than intertidal creeks.

Killifish are generally carnivorous, but a few species also consume algae and other marine plants. Small invertebrates, especially brine shrimp, insect larvae, worms, and various zooplankton form the bulk of their diet. Killifish in turn become food to many larger fish (such as flounders, perch, eels, and bass); along the upper edges of the water, they are prey for foraging birds and land crabs. Protection from predation varies with species, but includes cryptic coloring, small, elongated bodies to take advantage of hiding places and identification of potential danger by recognizing color contrasts from either above or below. The upturned placement of the mouth indicates their preference for swimming just under the surface and skimming for prey. Competition for a limited food resource between species of killifish may explain the distribution pattern that limits their co-occurrence in any given habitat.

Unlike the majority of killifishes, the small (1.17-3-inch [30-75 mm]) pupfish have a rounder shape and have omnivorous feeding habits. Its mouth is located on the terminus of the body. They also differ in their breeding strategy. Same-age schools forage together until the spring breeding season (temperatures above 20°C [68°F] from April-October), at which time the males establish territories, leaving females and juveniles in the group. When a female is ready to spawn, she leaves the school and ventures into a territory. Here she indicates her intention by biting a piece of the bottom and spitting it out. The male joins her and she begins laying a single egg at a time. Over a season, she may deposit from 50-300 eggs, depending on her size. High temperatures can speed up the life cycle, making it possible for up to 10 generations a year to occur in hot springs.

Economic importance

Many species of Fundulus have been used as bait fish, but perhaps their best known use is in the laboratory. Beginning in the 1800s, the mummichog (F. heteroclitus) was used to study fish embryology. The transparent eggs were stripped of their protective covering, opening a window on the developmental process and allowing easy manipulation with a variety of chemicals to assess endocrinological and biological responses. Capitalizing on the ability of these fish to withstand a wide range of temperatures and salinities, and its natural distribution near pollution sources, the mummichog also became the main study animal for intertidal and nearshore pollution tolerance.

Other experiments have used killifish and mum-michogs to test learning in fish (they are able to navigate a simple maze), examine pigmentation and the function of chromatophores in response to different physical and chemical stimuli, chemical responses allowing survival at subzero temperatures, and most recently in determining population stability in response to environmental stresses.

Other recent efforts have focused on two different extremes: 1) use of the hermaphroditic killifish (Rivulus marmoratus) from Florida as well as other killifish species as a biocontrol agent to reduce populations of mosquito larvae and eggs; and 2) as part of the reestablishment of endangered populations of the desert pupfish (Cyprinodon sp.) species in the California deserts. Both sets of experiments have

KEY TERMS

Anaerobic Describes biological processes that take place in the absence of oxygen.

Brackish Water containing some salt, but not as salty as sea water.

Chromatophores Components of cells responsible for allowing color changes.

Endemic Refers to species with a relatively local distribution, sometimes occurring as small populations confined to a single place, such as an oceanic island. Endemic species are more vulnerable to extinction than are more widespread species.

Endocrinological Refers to the function of hormones in regulating body processes.

Euryhaline Waters that change salinity in response to tides or fresh water influx.

Lateral line A row of sensors found along the sides of fish.

Mitochondria An organ inside cells that releases energy.

Operculum The protective covering over the gills.

Osmoregulation The process regulating the water content of cells in relation to that of the surrounding environment.

Salinity The amount of dissolved salts in water.

important impacts on land use planning and local environmental stability.

While few of the many species of killifish are considered rare or endangered, it was not until the 1960s that a technique for captive rearing was refined and the removal of thousands of fish from the wild discontinued. Even today, sampling of wild populations is still conducted fairly regularly in order to further understanding of their remarkable tolerance ranges.

Ecological importance

Maintaining species diversity in the nearshore, intertidal, and freshwater systems inhabited by the many kinds of killifish has become increasingly important. Because of their incredible ability to withstand a wide range of temperatures, salinities, and pollutants, especially organochloride pesticides and fertilizers, killifish seem to be among the most persistent species. In 1990, five species of killifish were the only fish remaining in Mullet Pond, North Carolina, which had 27 fish species present in 1903 and 1914. In the 1960s, the entire remaining population of the Owens Valley pupfish (Cyprinodon radiosus ) was transferred in a few buckets by Fish and Wildlife agents, as their native pool drained away. Perhaps because of their widespread distribution and ability to adapt to whatever conditions exist, the killifish are among the most important fish species for monitoring the future of our environment.

Rosi Dagit

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Killifish." The Gale Encyclopedia of Science. . Encyclopedia.com. 17 Sep. 2018 <http://www.encyclopedia.com>.

"Killifish." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 17, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/killifish

"Killifish." The Gale Encyclopedia of Science. . Retrieved September 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/killifish

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Killifish

Killifish

Killifish (Fundulus spp.) are small fish tolerant of a wide range of temperatures and salinity, found throughout temperate and tropical waters on every continent except Australia and Antarctica . Not to be confused with the other large group of small fish in the same order (Cyprinodontiformes) known as minnows , killifish differ in having an incomplete lateral line, often not extending past the head, and a protruding lower jaw which allows them to feed from the surface. This characteristic gave rise to their other common name "top-minnow." Mummichogs (Fundulus heteroclitus) are the most well known species of killifish. Millions of these common intertidal fish have been removed from the East coast of North America since the 1800s for use in a wide variety of experiments, including a recent trip into outer space to determine the effect of weightlessness on fish development and locomotion.

Evolution and taxonomy

The earliest fossils of the order Cyprinodontiformes (meaning toothed carp ) were found in Europe and date from the early Oligocene (26-37 mya). The most likely ancestor of this order is traced to fossils from the Tethys Sea in the Early Cretaceous (65-136 mya) time period. More recent Miocene (12-26 mya) fossils have been located in Kenya. Over time, the number of species diversified as the continents drifted apart, giving rise to some of the most resilient and tolerant species of fish known on Earth .

Because of this incredible diversification, the class Osteichthyes (bony fishes) has been broken down into several sub-classes, including Teleostei, which includes the orders for minnows, killifish, silversides, and others. The taxonomy of the killifish is quite complex due to the 900 or more different species in the order Cyprinodontiformes and the high degree of endemism resulting from isolated populations. Like other Cyprinodontes, killifish have one dorsal fin lacking spines. The killifish belongs to the sub-order Cyprinodontoidei, which includes such interesting families as the cavefishes (Amblyopsidae) of North America; the live-bearing top minnows, including familiar aquarium fish such as guppies, swordtails, and mollies (Poeciliidae); and the four-eye fishes that can see simultaneously above and below water (Anablepidae).

The killifish family is known as either Cyprinodontidae or Fundulidae, depending on the taxonomic source. Including over 200 species, the genus Fundulus has been further divided into five sub-genera classified according to anatomical or geographical characteristics. Among the 34 species in North America, there is a distinct separation of species from East to West, and genetically within species from North to South along the Atlantic coast, as seen in the mummichog (F. heteroclitus). This is probably a result of the combined influences of glaciation and disjunct habitat distribution.

The East Coast species are most numerous in Florida and the Gulf of Mexico, but do include some species from more northern freshwater and brackish regions extending as far north as Newfoundland and the Gulf of St. Lawrence. There are several species found exclusively in the Midwest and West. California has six native species found primarily in the desert , and one introduced species , the rainwater killifish (Lucania parva), native to the Atlantic and Gulf of Mexico. Due to the geographic limitations of their habitat and its further disappearance in the wake of development and pumping of groundwater reserves, some of these California species known as pupfish have the most restricted ranges of any known vertebrate. The Devil's Hole pupfish (Cyprinodon diabolis) is limited to a 215 sq ft (20 sq m) area of an underwater limestone shelf in a Nevada spring. For at least 10,000 years, a small population (200-700 individuals) has perpetrated itself in this tiny niche .


Ecology

Characteristic of euryhaline and freshwater habitats, extremes of temperatures, variations in dissolved oxygen and high degree of habitat disturbance are the main parameters shaping the lives of killifishes. The ability to move from fresh to saltwater requires tremendous osmoregulation adjustments. The sheepshead minnow (Cyprinodon variegatus) is found in marsh ponds from Maine to the West Indies, and can tolerate a wider range of salinity than any other fish. Killifish kidneys have specially adapted renal tubules to process sodium chloride salts as concentration increases from freshwater to saltwater. Special chloride cells located in the opercular epithelium of the gills help facilitate this effort for the mummichog (F. heteroclitus). Metabolic regulation of intercellular activity provides for greater hemoglobin-oxygen binding efficiency in mummichogs, allowing them to respond quickly to changes in dissolved oxygen levels, regulate pH , and thus effectively deliver oxygen to muscle tissues. This model may be the same for all killifishes, although a genetic component is probably involved, explaining the variety of tolerance levels limiting distribution of certain species.

The pupfishes of the California deserts show the most amazing temperature tolerance, commonly withstanding months of summer water temperatures between 35–40°C (95–104°F), with extremes of 47°C (116.6°F) not uncommon. Very few vertebrates can survive at this thermal level. Their small size (60-75 mm, 2-3 in), while providing a distinct advantage for survival in shallow, densely populated areas with limited food resources, would seem to present a major thermoregulatory challenge. Desert nights can experience temperatures below freezing as well, at which times the pupfish burrows into detritus on the bottom and remains dormant until temperatures rise again. Since all pupfish species are considered rare or endangered, further study of this remarkable ability awaits better captive rearing techniques.

Another important adaptation necessary for survival in salt marsh habitats is the ability to tolerate high levels of dissolved hydrogen sulfide. This gas is released as a by-product of anaerobic marsh decomposition and can achieve potentially toxic levels. Experiments with the California killifish (F. parvipinnis) indicate that metabolic tolerance is achieved by response of cellular mitochondrial oxidation, allowing a tolerance two to three times greater than that of most fishes.


Biology

In general, female killifish are larger than the males (ranging from 50-155 mm, 2-6 in), although less brightly colored. As the days lengthen and temperatures rise during the spring/summer reproductive season, hormones cause bright colors to appear on males. These aggressive males fight with each other for the privilege of fertilizing a female, preferably a large one capable of laying up to 50,000 eggs per season. Deposition of eggs is closely tied to the tidal cycle for intertidal fishes like the mummichog. Preference is for placing the sticky mass of eggs above the strand line on stalks of marsh grass (Spartina alterniflora) or in empty mussel shells (Geulensia demissa) during a spring tide, allowing them to develop in air until the next spring tide washes them back into the water, stimulating them to hatch. Thus the eggs themselves are tolerant of a wide salinity range.

Young fish fry emerge from the eggs nine to 18 days after laying and are considered more developmentally advanced than most other fish species. Mortality among the larva and fry is quite high, despite their tendency to take refuge in the shallow intertidal areas beyond reach of the larger fishes and crabs . Given warm temperatures and adequate food resources, size increases throughout the season. While most species achieve reproductive maturity after the first season, few seem to live for more than two to three years. During the winter, many species take refuge in salt marsh pools which provide slightly warmer temperatures than intertidal creeks.

Killifish are generally carnivorous, but a few species also consume algae and other marine plants. Small invertebrates , especially brine shrimp , insect larvae, worms, and various zooplankton form the bulk of their diet. The killifishes in turn become food to many larger fish (such as flounders, perch , eels, and bass ), and along the upper edges of the water, feed foraging birds and land crabs. Protection from predation varies with species, but includes cryptic coloring, small, elongated bodies to take advantage of hiding places and identification of potential danger by recognizing color contrasts from either above or below. The upturned placement of the mouth indicates their preference for swimming just under the surface and skimming for prey . Competition for a limited food resource between species of killifish may explain the distribution pattern that limits their cooccurrence in any given habitat.

Unlike the majority of killifishes, the small (30-75 mm, 1.17-3 in) pupfish have a more rounded shape and have omnivorous feeding habits. Its mouth is located on the terminus of the body. They also differ in their breeding strategy. Same age class schools forage together until the spring breeding season (temperatures above 20°C [68°F] from April-October), at which time the males establish territories, leaving females and juveniles in the group. When a female is ready to spawn, she leaves the school and ventures into a territory. Here she indicates her intention by biting a piece of the bottom and spitting it out. The male joins her and she begins laying a single egg at a time. Over a season, she may deposit from 50-300 eggs, depending on her size. High temperatures can speed up the life cycle, making it possible for up to 10 generations a year to occur in hot springs.


Economic importance

Many species of Fundulus have been used extensively as bait fish, but perhaps their best known use is in the laboratory. Beginning in the 1800s, the mummichog (F. heteroclitus) was used to study fish embryology . The transparent eggs were stripped of their protective covering, opening a window on the developmental process and allowing easy manipulation with a variety of chemicals to assess endocrinological and biological responses. Capitalizing on the ability of these fish to withstand a wide range of temperatures and salinities, and its natural distribution near pollution sources, the mummichog also became the main study animal for intertidal and nearshore pollution tolerance.

Other experiments have used killifish and mummichogs to test learning in fish (they are able to navigate a simple maze), examine pigmentation and the function of chromatophores in response to different physical and chemical stimuli, chemical responses allowing survival at sub-zero temperatures, and most recently in determining population stability in response to environmental stresses.

Other recent efforts have focused on two different extremes: 1) use of the hermaphroditic killifish (Rivulus marmoratus) from Florida as well as other killifish species as a biocontrol agent for reducing populations of mosquito larvae and eggs; and 2) as part of the reestablishment of endangered populations of the Desert pupfish (Cyprinodon sp.) species in the California deserts. Both sets of experiments have important impacts on land use planning and local environmental stability.

While few of the many species of killifish are considered rare or endangered, it was not until the 1960s that a technique for captive rearing was refined and the removal of thousands of fish from the wild discontinued. Even today, sampling of wild populations is still conducted fairly regularly in order to further understanding of their remarkable tolerance ranges.


Ecological importance

Maintaining species diversity in the nearshore, intertidal, and freshwater systems inhabited by the many kinds of killifish has become increasingly important as the twentieth century draws to a close. Because of their incredible ability to withstand a wide range of temperatures, salinities, and pollutants, especially organochloride pesticides and fertilizers , killifish seem to be among the most persistent species. In 1990, five species of killifish were the only fish remaining in Mullet Pond, North Carolina, which had 27 fish species present in 1903 and 1914. In the 1960s, the entire remaining population of the Owen's Valley pupfish (Cyprinodon radio-sus) was transferred in a few buckets by Fish and Wildlife agents, as their native pool drained away. Perhaps because of their widespread distribution and ability to adapt to whatever conditions exist, the killifish are among the most important fish species for monitoring the future of our environment.

Rosi Dagit

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anaerobic

—Describes biological processes that take place in the absence of oxygen.

Brackish

—Water containing some salt, but not as salty as sea water.

Chromatophores

—Components of cells responsible for allowing color changes.

Endemic

—Refers to species with a relatively local distribution, sometimes occurring as small populations confined to a single place, such as an oceanic island. Endemic species are more vulnerable to extinction than are more widespread species.

Endocrinological

—Refers to the function of hormones in regulating body processes.

Euryhaline

—Waters that change salinity in response to tides or fresh water influx.

Lateral line

—A row of sensors found along the sides of fish.

Mitochondria

—An organ inside cells that releases energy.

Operculum

—The protective covering over the gills.

Osmoregulation

—The process regulating the water content of cells in relation to that of the surrounding environment.

Salinity

—The amount of dissolved salts in water.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Killifish." The Gale Encyclopedia of Science. . Encyclopedia.com. 17 Sep. 2018 <http://www.encyclopedia.com>.

"Killifish." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 17, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/killifish-0

"Killifish." The Gale Encyclopedia of Science. . Retrieved September 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/killifish-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.