''Xiphophorus pygmaeus'', the pygmy swordtail, is a

poeciliid The Poeciliidae are a family of freshwater fishes of the order Cyprinodontiformes, the tooth-carps, and include well-known live-bearing aquarium fish, such as the guppy, molly, platy, and swordtail. The original distribution of the family was t ...

fish from northeastern Mexico. It is the smallest of the

swordtail ''Xiphophorus'' is a genus of euryhaline and freshwater fishes in the family Poeciliidae of order Cyprinodontiformes, native to Mexico and northern Central America. The many ''Xiphophorus'' species are all known as platyfish (or platies) and s ...

s. The male's sword is barely visible and the species is often called the swordless swordtail. It is sometimes kept in home aquaria, but is a rather delicate species.

Taxonomy and evolution

''Xiphophorus pygmaeus'' is the smallest member of the genus ''

Xiphophorus ''Xiphophorus'' is a genus of euryhaline and freshwater fishes in the family Poeciliidae of order Cyprinodontiformes, native to Mexico and northern Central America. The many ''Xiphophorus'' species are all known as platyfish (or platies) and s ...

''. It is most closely related to '' X. multilineatus'' and '' X. nigrensis'', and males in particular look similar to the small males of the latter two species. The three species form a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

within the larger clade of northern swordtails. ''X. pygmaeus'' lost large-sized males and male traits associated with larger size:

courtship behavior A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate Mate choice, exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ( ...

, body depth, and well-developed swords. Large size eventually reappeared in ''X. pygmaeus'' but the normally accompanying traits did not.

Description

Females grow to , while males attain . The male's sword is only 1-2 mm long. The species was originally thought to consist only of small-sized males (all less than 29 mm long and most less than 24 mm), but in 1994 it was discovered that two populations contain large males. The base color of the fish is gray-brown. Both small and large males can be yellow, but this

color morph In biology, polymorphism is the occurrence of two or more clearly different morphs or forms, also referred to as alternative ''phenotypes'', in the population of a species. To be classified as such, morphs must occupy the same habitat at the s ...

is very rare in nature because such flashy individuals are easy prey. No yellow females exist, and the trait appears to be

sex-linked Sex linked describes the sex-specific patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome ( autosome). In humans, these are termed X-linked recessive, ...

: male descendants of yellow males tend to be yellow too. Males are generally more intensely colored than females.

Distribution and habitat

''Xiphophorus pygmaeus'' discovered in Rio Axtla, a tributary of

Río Pánuco Rio or Río is the Portuguese, Spanish, Italian, and Maltese word for "river". When spoken on its own, the word often means Rio de Janeiro, a major city in Brazil. Rio or Río may also refer to: Geography Brazil * Rio de Janeiro * Rio do Sul, a ...

, by Myron Gordon in 1939. At the type locality, the fish were found in swift water among a dense stand of a ''

Vallisneria ''Vallisneria'' (named in honor of Antonio Vallisneri) is a genus of freshwater aquatic plant, commonly called eelgrass, tape grass or vallis. The genus is widely distributed in tropical and subtropical regions of Asia, Africa, Europe, and North ...

''-like plant along a steep bank. '' X. montezumae'' and '' X. variatus'' are found in the same section of the river, but their preference for shallower and slower water isolated ''X. pygmaeus'' from them. ''Xiphophorus pygmaeus'' also inhabits Rio Axtla's tributaries Rio Tancuilin and Rio Huichihuayan. Its preferred habitat is characterized by soft bottoms and large stands of submerged plants such as ''

Sagittaria ''Sagittaria'' is a genus of about 303. Sagittaria Linnaeus
''Flora of North ...

''.

Reproduction

''Xiphophorus pygmaeus'' is unusual among swordtails in that females are not courted by males, neither small nor large. Instead, the males sneak up on females and chase them to enforce

copulation Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetrat ...

. In most ''Xiphophorus'' species, including the most closely related, ''X. nigrensis'' and ''X. multilineatus'', large males court females, while small males rely on the "sneak-chase" strategy. ''X. pygmaeus'' females prefer to be courted, however, and thus choose males of ''X. nigrensis'' over males of their own species when they can. ''Xiphophorus pygmaeus'' breeds sparsely, with a female rarely giving birth to more than 10 fry at once. The fry at long at birth and usually not preyed upon by the adults.

In aquaria

Caring for ''X. pygmaeus'' in the home

aquarium An aquarium (plural: ''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. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aq ...

is challenging. The species requires excellent filtration and frequent water changes and does not do well in soft water or temperatures exceeding 24°C. The addition of fine-leaved aquatic plants such as ''

Ceratophyllum demersum ''Ceratophyllum demersum'', commonly known as hornwort, rigid hornwort, coontail, or coon's tail, is a species of ''Ceratophyllum''. It is a submerged, free-floating aquatic plant, with a cosmopolitan distribution, native to all continents except ...

'' is recommended because the fish feed on such plants and algae as well as

infusoria Infusoria are minute freshwater life forms including ciliates, euglenoids, protozoa, unicellular algae and small invertebrates. Some authors (e.g., Bütschli) used the term as a synonym for Ciliophora. In modern formal classifications, the term i ...

growing among them.

References

{{Taxonbar, from=Q3768219 pygmaeus Taxa named by Carl Leavitt Hubbs Fish described in 1943 Freshwater fish of Mexico Endemic fish of Mexico