Livebearers is the common term for a very popular group of aquarium fish that give birth to live free-swimming young rather than laying eggs. It includes the guppy, molly, platy and swordtail, as well as several other fish that are much less often encountered in the hobby. These fish are all within the order Cyprinodontiformes, from the Greek kyprinos [goldfish] and odous [tooth, teeth] plus the Latin forma [shape]. These fish are sometimes referred to as the toothed carps, though they are not closely related to the true carps; the latter belong to the Superorder Ostariophysi, whereas the Cyprinodontiformes (“toothed carps”) belong to the Superorder Acanthopterygii, referred to as ray-finned fish; the name comes from the Greek aktis [= ray, thunderbolt, beam] and pterygion, the diminutive of pteryx [= wing, fin].
The Order Cyprinodontiformes contains the rivulines, killifishes and livebearers. The latter group occur in the Superfamily Peocilioidea; there are two Families, the Anablepidae (Four-eyed Fish and relatives) and the Poeciliidae (commonly, the Poeciliids). Rather recently two subfamilies were transferred from Cyprinodontidae to Poeciliidae, thus changing the livebearers to the subfamily rank of Poeciliinae. The name comes from the Greek poikilos, meaning “with different colours.” Fishbase lists 293 species contained within 30 genera.
The Family Poeciliidae is commonly known as livebearers, but not all species bear live young. Aquarists tend to think of the livebearers as occurring chiefly in Mexico and Central America, but they actually extend from SE United States down into northern Argentina. The majority of species are either true live-bearing species [viviparous] or have internal fertilization with the eggs hatching before being laid so that the female produces live young [ovoviviparous]. Except for one genus, all the American species are live bearing. But there are also some egg scattering species with external fertilization, and these all occur in Africa. The distribution of the Subfamily in Africa and America suggests that the Poeciliidae pre-date the separation of Africa and South America that occurred during the early Cretaceous period some 130 million years ago. Live-bearing subsequently evolved in the American species but not those in Africa.
The degree to which the female supports the developing larvae in many species is lecithotrophic, which means the mother provides the oocyte (the female reproductive cell) with everything it needs prior to fertilization so that the egg develops independent of the female. Some species like the splitfins and halfbeaks are matrotrophic [“mother feeding”], which means the source of nourishment provided by the egg yolk is supplemented directly from the mother after fertilization.
Of interest to almost all aquarists are the genera Poecilia (molly, guppy and endler), Xiphophorus (platy and swordtail) and Gambusia, with Limia (21 species, found on the islands of the Greater Antilles) and Belonesox (pike livebearer) of interest more to advanced aquarists.
· Poecilia contains [at the time of writing] 34 species, of which one is the guppy [P. reticulata], one the Endler [P. wingei], and the remaining 32 are all mollies with one of these, P. vivipara, being the type species* for the genus.
· Xiphophorus hellerii, the green swordtail, is the type species of this genus that includes 28 species native to areas of Belize, Guatemala, Honduras and (primarily) Mexico. Most aquarium fish have been developed from three species, X. hellerii (green swordtail), X. maculatus (southern platyfish) and X. variatus (variable platyfish). The name is derived from the Greek xipho (sword) and phorus (bearer).
· Gambusia [from the Cuban Spanish gambusino for “useless”] currently holds 43 species, two of which are now extinct and several others are critically endangered according to IUCN [International Union for the Conservation of Nature and Natural Resources]. The type species is Gambusia punctata, but the most common species is probably the Mosquito Fish, G. affinis, a very hardy and adaptable fish that was introduced especially in Australia as a supposed means of controlling mosquitoes; it has proven detrimental to native fish species (that were doing better at controlling the mosquitoes) and even amphibian populations (by eating the tadpoles).
All livebearers require basic (alkaline) water and moderate hardness. They are generally hardy fish, and easy to breed which makes them popular with beginning aquarists. However, mollies especially are highly sensitive to ammonia, and should never be introduced to a tank that is not fully cycled.
The male’s anal fin is modified into a gonopodium by which he transfers sperm into the female. This is the most reliable means of determining the sex of the fish; in some species the male is also the more colourful, and in the case of the swordtail the male possess the extended lower lobe of the caudal fin. If both sexes are to be housed in the same aquarium, there should be more females than males to provide some rest for the females from the attention of the males. Maintaining males and females in the same tank will result in regular batches of fry, and once impregnated, a female can deliver several successive batches on her own [this is known as superfetation]. In batches of fry, this impregnation can occur before the males attain their colouration, so fish must be separated out very early if the aquarist wishes to selectively breed them. Fry require hiding places such as thick floating plant cover, or they will be readily eaten by the adults.
Fish readily interbreed within their respective genus, creating the variety of colourful forms especially among the platies and swordtails available today.
* "type species" of a genus refers to the species that has all the physiological characteristics which separate it from all species in other genera. Today, the type species is usually the first species bearing the characteristics to be described by science. All species subsequently included within the genus will share all these characteristics.
June 26, 2010
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